Shockwave Medical (SWAV) S-1 12 Nov 19 IPO registration Financial data

Cover

Cover	9 Months Ended
	Sep. 30, 2019
Cover page.
Document Type	S-1
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Entity Registrant Name	ShockWave Medical, Inc.
Entity Central Index Key	0001642545
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
CURRENT ASSETS:
Cash and cash equivalents	$ 39,452	$ 39,643	$ 51,923
Short-term investments	74,646		1,806
Accounts receivable, net	5,551	2,850	639
Inventory	9,227	5,131	2,523
Prepaid expenses and other current assets	2,199	1,112	968
Total current assets	131,075	48,736	57,859
Operating lease right-of-use assets	2,155
Property and equipment, net	4,111	2,619	1,372
Other assets	548	2,066	73
TOTAL ASSETS	137,889	53,421	59,304
CURRENT LIABILITIES:
Accounts payable	2,486	1,487	1,148
Term notes, current portion	6,667	1,667
Accrued liabilities	9,496	6,217	3,393
Lease liability, current portion	742
Total current liabilities	19,391	9,371	4,541
Lease liability, noncurrent portion	1,543
Term notes, noncurrent portion	8,705	13,383
Convertible preferred stock warrant liability		313	577
Other liabilities		136	9
TOTAL LIABILITIES	29,639	23,203	5,127
Commitments and contingencies (Note 6)
Convertible preferred stock		152,806	137,469
STOCKHOLDERS' EQUITY (DEFICIT):
Preferred stock
Common stock	28	2	2
Additional paid-incapital	271,394	4,275	2,470
Accumulated other comprehensive income	57		(1)
Accumulated deficit	(163,229)	(126,865)	(85,763)
TOTAL STOCKHOLDERS' EQUITY (DEFICIT)	108,250	(122,588)	(83,292)
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT)	$ 137,889	$ 53,421	$ 59,304

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	229,098,987	216,079,811
Preferred stock, shares issued	18,670,328	17,510,045
Preferred stock, shares outstanding	18,670,328	17,510,045
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	325,000,000	325,000,000
Common stock, shares issued	1,824,852	1,627,032
Common stock, shares outstanding	1,824,852	1,627,032

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Product revenue	$ 28,615	$ 7,201	$ 12,263	$ 1,719
Operating expenses:
Cost of product revenue	11,606	3,946	7,250	2,836
Research and development	22,778	16,579	22,698	17,963
Sales and marketing	21,023	12,611	17,536	6,363
General and administrative	9,684	4,137	5,979	5,422
Total operating expenses	65,091	37,273	53,463	32,584
Loss from operations	(36,476)	(30,072)	(41,200)	(30,865)
Interest expense	(746)	(216)	(401)	(58)
Change in fair value of warrant liability	(609)	(53)	(52)	(32)
Other income, net	1,518	489	589	366
Net loss before taxes	(36,313)	(29,852)	(41,064)	(30,589)
Income tax provision	51	27	38	26
Net loss	(36,364)	(29,879)	(41,102)	(30,615)
Unrealized (loss) gain on available-for-salesecurities	57	1	1	(1)
Total comprehensive loss	$ (36,307)	$ (29,878)	$ (41,101)	$ (30,616)
Net loss per share, basic and diluted	$ (1.66)	$ (17.15)	$ (23.39)	$ (19.71)
Shares used in computing net loss per share, basic and diluted	21,886,396	1,742,572	1,757,102	1,553,365

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders Deficit - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ (53,831)		$ 2	$ 1,315		$ (55,148)
Beginning balance, shares at Dec. 31, 2016			1,555,510
Temporary equity, Beginning balance at Dec. 31, 2016		$ 102,180
Temporary equity, Beginning balance, shares at Dec. 31, 2016		14,605,589
Issuance of Series C and D convertible preferred stock, net of issuance costs		$ 34,907
Issuance of Series C and D convertible preferred stock, net of issuance costs of,shares		2,840,504
Exercise of Series A-1 warrants		$ 382
Temporary equity, Exercise of Series A-1 warrants, Shares		63,952
Exercise of stock options	$ 139			139
Exercise of stock options, Shares	71,522		71,522
Vesting of early exercised options	$ 51			51
Stock-based compensation	965			965
Unrealized gain (loss) on available-for-sale securities	(1)				$ (1)
Net loss	(30,615)					(30,615)
Ending balance at Dec. 31, 2017	(83,292)		$ 2	2,470	(1)	(85,763)
Ending balance at Dec. 31, 2017			1,627,032
Temporary equity, Ending balance at Dec. 31, 2017	$ 137,469	$ 137,469
Temporary equity, Ending balance, shares at Dec. 31, 2017	17,510,045	17,510,045
Exercise of Series A-1 warrants		$ 417
Temporary equity, Exercise of Series A-1 warrants, Shares		69,675
Issuance of common stock warrants	$ 104			104
Exercise of stock options	300			300
Exercise of stock options, Shares			183,471
Vesting of early exercised options	60			60
Stock-based compensation	874			874
Unrealized gain (loss) on available-for-sale securities	1				1
Net loss	(29,879)					(29,879)
Ending balance at Sep. 30, 2018	(111,832)		$ 2	3,808		(115,642)
Ending balance at Sep. 30, 2018			1,810,503
Temporary equity, Ending balance at Sep. 30, 2018		$ 137,886
Temporary equity, Ending balance, shares at Sep. 30, 2018		17,579,720
Beginning Balance at Dec. 31, 2017	(83,292)		$ 2	2,470	(1)	(85,763)
Beginning balance, shares at Dec. 31, 2017			1,627,032
Temporary equity, Beginning balance at Dec. 31, 2017	$ 137,469	$ 137,469
Temporary equity, Beginning balance, shares at Dec. 31, 2017	17,510,045	17,510,045
Issuance of Series C and D convertible preferred stock, net of issuance costs		$ 14,920
Issuance of Series C and D convertible preferred stock, net of issuance costs of,shares		1,090,608
Exercise of Series A-1 warrants		$ 417
Temporary equity, Exercise of Series A-1 warrants, Shares	69,675	69,675
Issuance of common stock warrants	$ 104			104
Exercise of stock options	$ 326			326
Exercise of stock options, Shares	197,820		197,820
Vesting of early exercised options	$ 78			78
Stock-based compensation	1,297			1,297
Unrealized gain (loss) on available-for-sale securities	1				1
Net loss	(41,102)					(41,102)
Ending balance at Dec. 31, 2018	(122,588)		$ 2	4,275		(126,865)
Ending balance at Dec. 31, 2018			1,824,852
Temporary equity, Ending balance at Dec. 31, 2018	$ 152,806	$ 152,806
Temporary equity, Ending balance, shares at Dec. 31, 2018	18,670,328	18,670,328
Exercise of common stock warrants for cash	$ 110			110
Exercise of common stock warrants for cash, shares			50,331
Issuance of common stock upon net exercise of warrants	133			133
Issuance of common stock upon net exercise of warrants, shares			180,952
Temporary equity conversion of preferred stock shares to common stock upon initial public offering, shares		(18,670,328)
Temporary equity conversion of preferred stock value to common stock upon initial public offering		$ (152,806)
Conversion of preferred stock to common stock upon initial public offering	152,806		$ 18	152,788
Conversion of preferred to common stock upon initial public offering, Shares			18,670,328
Conversion of Series A-1warrants to common stock warrants upon initial public offering	789			789
Issuance of common stock in connection with initial public offering, net of issuance costs of $11.5 million	99,924		$ 7	99,917
Issuance of common stock in connection with initial public offering, net of issuance costs, Shares			6,555,000
Issuance of common stock in connection with private placement	10,000		$ 1	9,999
Issuance of common stock in connection with private placement, Shares			588,235
Exercise of stock options	$ 1,023			1,023
Exercise of stock options, Shares	386,309		386,309
Vesting of early exercised options	$ 27			27
Stock-based compensation	2,336			2,336
Adjustment for fractional shares resulting from reverse stock split	(3)			(3)
Adjustment for fractional shares resulting from reverse stock split, Shares			(114)
Unrealized gain (loss) on available-for-sale securities	57				57
Net loss	(36,364)					(36,364)
Ending balance at Sep. 30, 2019	$ 108,250		$ 28	$ 271,394	$ 57	$ (163,229)
Ending balance at Sep. 30, 2019			28,255,893

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders Deficit (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Stockholders' Equity [Abstract]
Issuance cost		$ 80	$ 93
Issuance costs, initial public offering	$ 11,500

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (36,364)	$ (29,879)	$ (41,102)	$ (30,615)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	854	466	700	468
Stock-based compensation	2,336	874	1,297	965
Amortization of right-of-use assets	739
Accretion of discount on available-for-sale securities	(542)
Loss on write down of fixed assets	90	25	31	38
Change in fair value of warrant liability	609	53	52	32
Amortization of debt issuance costs	322	107	206	18
Changes in operating assets and liabilities:
Accounts receivable	(2,701)	(2,016)	(2,211)	(594)
Inventory	(4,096)	(2,687)	(2,608)	(1,863)
Prepaid expenses and other current assets	(1,087)	(143)	(144)	(373)
Other assets	(1)	(9)	(917)
Accounts payable	884	(187)	360	249
Accrued and other current liabilities	4,169	1,720	2,773	1,328
Lease liabilities	(749)
Other liabilities		25	98
Net cash used in operating activities	(35,537)	(31,651)	(41,465)	(30,347)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of available-for-sale securities	(106,347)			(17,707)
Proceeds from maturities of available-for-sale securities	32,300	1,807	1,807	15,900
Purchase of property and equipment	(2,287)	(1,130)	(1,981)	(425)
Net cash provided by (used in) investing activities	(76,334)	677	(174)	(2,232)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs paid	100,547		14,920	34,907
Proceeds from issuance of common stock in private placement	10,000
Proceeds from term loans		9,988	14,988
Payments of deferred offering costs			(626)
Proceeds from stock option exercises	1,023	400	426	139
Proceeds from warrant exercises	110	101	101	198
Principal payment of term loan				(1,557)
Net cash provided by financing activities	111,680	10,489	29,809	33,687
Net decrease in cash, cash equivalents and restricted cash	(191)	(20,485)	(11,830)	1,108
Cash, cash equivalents and restricted cash at beginning of period	40,093	51,923	51,923	50,815
Cash, cash equivalents and restricted cash equivalents at end of period	39,902	31,438	40,093	51,923
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Interest paid	420	68	156	40
Income tax paid	120	5	5
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued on conversion of convertible preferred stock	152,806
Issuance of Series A-1 convertible preferred stock on net exercise of warrants		316	316
Common stock issued upon net exercise of warrants	133
Common stock warrants issued on conversion of preferred stock warrants and the reclassification of the warrant liability	789
Right-of-use asset obtained in exchange for lease liability	73
Property and equipment purchases included in accounts payable	$ 204	180	55	$ 58
Issuance of common stock warrants in connection with debt financing		$ 104	104
Deferred offering costs included in accounts payable and accrued liabilities			$ 893

Organization and Basis of Prese

Organization and Basis of Presentation	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation ShockWave Medical, Inc. (the “Company”) was incorporated on June 17, 2009. The Company is primarily engaged in the development of Intravascular Lithotripsy (“IVL”) technology for the treatment of calcified plaque in patients with peripheral vascular, coronary vascular and heart valve disease. Built on a balloon catheter platform, the IVL technology uses lithotripsy to disrupt both superficial and deep vascular calcium, while minimizing soft tissue injury, and an integrated angioplasty balloon to dilate blockages at low pressures, restoring blood flow. In 2016, the Company began commercial and manufacturing operations, and began selling catheters based on the IVL technology. The Company’s headquarters are in Santa Clara, California. The Company is located and operates primarily in the United States and has a subsidiary in Germany. Initial Public Offering On March 11, 2019, the Company completed an initial public offering (“IPO”) of its common stock. As part of the IPO, the Company issued and sold 6,555,000 shares of its common stock, which included 855,000 shares sold pursuant to the exercise of the underwriters’ over-allotment option, at a public offering price of $17.00 per share. The Company received net proceeds of approximately $99.9 million from the IPO, after deducting underwriters’ discounts and commissions of $7.1 million and offering costs of $4.4 million, of which $1.5 million was incurred as of December 31, 2018. Prior to the completion of the IPO, all shares of Series A, A-1, one-to-one In addition, on the completion of the IPO, all the Company’s outstanding preferred stock warrants were converted into 54,903 common stock warrants, which resulted in the reclassification of the convertible preferred stock warrant liability to additional paid-in capital. Concurrent with the IPO, the Company issued 588,235 shares of its common stock in a private placement for net proceeds of $10.0 million. Reverse Stock Split In February 2019, the Company’s board of directors approved an amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock and convertible preferred stock on a 12.2-for-one Need for Additional Capital The Company has incurred significant losses and has negative cash flows from operations. As of September 30, 2019, the Company had an accumulated deficit of $163.2 million. Management expects to continue to incur additional substantial losses for the foreseeable future. As of September 30, 2019, the Company had cash, cash equivalents and short-term investments of $114.1 million, which are available to fund future operations. The Company believes that its cash, cash equivalents and short-term investments as of September 30, 2019, will be sufficient for the Company to continue as a going concern for at least 12 months from the date the unaudited condensed consolidated financial statements are filed with the Securities and Exchange Commission (“SEC”). The Company’s future capital requirements will depend on many factors, including its growth rate, the timing and extent of its spending to support research and development activities and the timing and cost of establishing additional sales and marketing capabilities.	1. Organization and Basis of Presentation ShockWave Medical, Inc. (the “Company”) was incorporated on June 17, 2009. The Company is primarily engaged in the development of Intravascular Lithotripsy (“IVL”) Technology for the treatment of calcified plaque in patients with peripheral vascular, coronary vascular and heart valve disease. Built on a balloon catheter platform, the IVL technology uses lithotripsy to disrupt both superficial and deep vascular calcium, while minimizing soft tissue injury, and an integrated angioplasty balloon to dilate blockages at low pressures, restoring blood flow. In 2016, the Company began commercial and manufacturing operations, and began selling catheters based on the IVL Technology. The Company’s headquarters are in Santa Clara, California. The Company is located and operates primarily in the United States and has a subsidiary in Germany. Going Concern The Company has incurred significant losses and has negative cash flows from operations. As of December 31, 2018, the Company had an accumulated deficit of $126.9 million. Management expects to continue to incur additional substantial losses in the foreseeable future. As of December 31, 2018, the Company had cash and cash equivalents of $39.6 million, which are available to fund future operations. The Company will need to raise additional capital to support the commercialization of its products and research and development activities. The Company’s activities are subject to significant risks and uncertainties, including the market acceptance of the Company’s products and the timing and extent of spending on research and development. The Company believes that its cash and cash equivalents as of December 31, 2018, together with available borrowings under a revolving line of credit, will not be sufficient for the Company to continue as a going concern for at least one year from the issuance date of its consolidated financial statements. The Company believes that this raises substantial doubt about its ability to continue as a going concern. As a result, the Company will be required to raise additional capital from the sale of common stock or convertible preferred stock or the issuance of debt. However, no assurance can be given as to whether additional needed financing will be available on terms acceptable to the Company, if at all. If sufficient funds on acceptable terms are not available when needed, the Company may be required to curtail planned activities to significantly reduce its operating expenses. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives and have an adverse effect on its results of operations and future prospects. The accompanying consolidated financial statements have been prepared assuming that the Company will continue as a going concern, which contemplates the realization of assets and the settlement of liabilities and commitments in the normal course of business. The accompanying consolidated financial statements do not reflect any adjustments relating to the recoverability and reclassifications of assets and liabilities that might be necessary if the Company is unable to continue as a going concern.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and applicable rules and regulations of SEC regarding interim financial reporting. The interim condensed consolidated financial statements are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows. The results of operations for the nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated March 6, 2019 (“Prospectus”) that forms a part of the Company’s Registration Statements on Form S-1 No. 333-229590), Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. Restricted cash as of September 30, 2019 and December 31, 2018 relates to a letter of credit established for a lease entered into in May 2018 and is recorded as other assets on the condensed consolidated balance sheets. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows: December 31, 2018 September 30, 2019 (in thousands) Cash and cash equivalents $ 39,643 $ 39,452 Restricted cash 450 450 Total cash, cash equivalents, and restricted cash $ 40,093 $ 39,902 Short-Term Investments Short-term investments have been classified as available-for-sale and purchase. Available-for-sale securities Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income, net. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net. Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, accounts receivable, accounts payable and accrued liabilities approximate their fair value due to their short maturities. Management believes that its term notes bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument approximates its fair value. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3 Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) Upon adoption of Topic 842, on January 1, 2019, the Company recorded operating right-of-use For its long-term operating lease, the Company recognized a right-of-use right-of-use Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the condensed consolidated statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company elected to exclude from its balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease Deferred Offering Costs Offering costs, consisting of legal, accounting, printer and filing fees related to the IPO, were deferred until the completion of the IPO. As of December 31, 2018, $1.5 million of deferred offering costs were recorded as other assets on the condensed consolidated balance sheet. In March 2019, upon the closing of the IPO, the deferred costs were offset against the Company’s IPO proceeds. Convertible Preferred Stock Warrant Liability The Company has accounted for its freestanding warrants to purchase shares of the Company’s convertible preferred stock as liabilities at fair value upon issuance primarily because the shares underlying the warrants contain contingent redemption features outside the control of the Company. The warrants are subject to re-measurement at additional paid-in capital, Revenue The Company records product revenue primarily from the sale of its IVL catheters. The Company sells its products to hospitals, primarily through direct sales representatives, as well as through distributors in selected international markets. Additionally, a significant portion of the Company’s revenue is generated through a consignment model under which inventory is maintained at hospitals. Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. For products sold through direct sales representatives, control is transferred upon delivery to customers. For products sold to distributors internationally and certain customers that purchase stocking orders in the United States, control is transferred upon shipment or delivery to the customer’s named location, based on the contractual shipping terms. For consignment inventory, control is transferred at the time the IVL catheters are consumed in a procedure. The Company generally provides for the use of an IVL generator and connector cable under an agreement to customers at no charge to facilitate use of the IVL catheters. These agreements do not contain contractually enforceable minimum commitments and are generally cancellable by either party with 30 days’ notice.	2. Summary of Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the valuation of inventory, the fair value of common stock, the fair value of preferred stock warrant liabilities, the fair value of stock options, recoverability of the Company’s net deferred tax assets, and related valuation allowance and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates . Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: December 31, 2017 2018 (in thousands) Cash and cash equivalents $ 51,923 $ 39,643 Restricted cash — 450 Total cash, cash equivalents, and restricted cash $ 51,923 $ 40,093 Restricted cash as of December 31, 2018 relates to a letter of credit established for a lease entered into in May 2018 and is recorded as other assets on the consolidated balance sheets. Short-Term Investments Short-term investments have been classified as available-for-sale Available-for-sale Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income, net. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net. Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, investments and trade receivables. Risks associated with cash, cash equivalents and restricted cash are mitigated by banking with creditworthy institutions and the Company’s investments have investment grade ratings when purchased. The Company performs ongoing evaluations of its customers and generally does not require collateral. Concentration of Customers For the year ended December 31, 2017, one customer accounted for 19% of the Company’s revenue. For the year ended December 31, 2108, there were no customers which accounted for more than 10% of the Company’s revenue. There were no customers which accounted for more than 10% of the Company’s accounts receivable as of December 31, 2017 and 2018. Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, accounts receivable, accounts payable and accrued liabilities approximate their fair value due to their short maturities. Management believes that its term notes bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument approximates its fair value. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3 Accounts Receivable and Allowance for Doubtful Accounts Accounts receivable are recorded at invoice value, net of any allowance for doubtful accounts. Estimates of the allowance for doubtful accounts are determined based on existing contractual payment terms, historical payment patterns of customers and individual customer circumstances. The allowance for doubtful accounts was $76,000 as of December 31, 2018 and the Company recognized accounts receivable write-offs in the amount of $1,000 for the year ended December 31, 2018. There was no allowance for doubtful accounts recorded and no accounts receivable write-offs recognized as of and for the year ended December 31, 2017. Inventory Inventory is stated at the lower of standard cost (which approximates actual cost on a first-in, first-out Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the lesser of their useful life or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized. Maintenance and repairs are charged to operations as incurred. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. The Company has not identified any such impairment losses to date. Convertible Preferred Stock Warrant Liability The Company has accounted for its freestanding warrants to purchase shares of the Company’s convertible preferred stock as liabilities at fair value upon issuance primarily because the shares underlying the warrants contain contingent redemption features outside the control of the Company. The warrants are subject to re-measurement paid-in Revenue The Company adopted Accounting Standards Codification (“ASC”) 606, Revenue from Contracts with Customers Revenue Recognition The Company records product revenue primarily from the sale of its IVL catheters. The Company sells its products to hospitals, primarily through direct sales representatives, as well as through distributors in selected international markets. Additionally, a significant portion of the Company’s revenue is generated through a consignment model under which inventory is maintained at hospitals. Under ASC 605, revenue is recognized when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price to the customer is fixed or determinable, and collectability is reasonably assured. For products sold through direct sales representatives, revenue is recognized upon delivery to customers. For products sold to distributors, revenue is recognized upon transfer of title and risk of loss to the distributor. For consignment inventory, revenue is recognized at the time the catheters are consumed in a procedure. Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. For products sold through direct sales representatives, control is transferred upon delivery to customers. For products sold to distributors internationally and certain customers that purchase stocking orders in the United States, control is transferred upon shipment or delivery to the customer’s named location, based on the contractual shipping terms. For consignment inventory, control is transferred at the time the catheters are consumed in a procedure. The Company generally provides for the use of an IVL generator and connector cable under an agreement to customers at no charge to facilitate use of the IVL catheters. These agreements do not contain contractually enforceable minimum commitments and are generally cancellable by either party with 30 days’ notice. Research and Development Costs Research and development costs, including new product development, regulatory compliance, and clinical research are expensed as incurred. Accrued Research and Development Costs The Company accrues liabilities for estimated costs of research and development activities conducted by its third-party service providers, which include the conduct of preclinical and clinical studies. The estimated costs of research and development activities are recorded based upon the estimated amount of services provided but not yet invoiced, and these costs are included in accrued liabilities on the consolidated balance sheets and within research and development expense on the consolidated statements of operations and comprehensive loss. These costs are accrued for based on factors such as estimates of the work completed and budget provided and in accordance with agreements established with third-party service providers. Significant judgments and estimates are made in determining the accrued liabilities balance in each reporting period. Accrued liabilities are adjusted as actual costs become known. There have not been any material differences between accrued costs and actual costs incurred since the Company’s inception. Stock-Based Compensation The Company accounts for share-based payments at fair value. The fair value of stock options is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis, over the vesting period. The Company accounts for forfeitures as they occur. Deferred Offering Costs Offering costs, consisting of legal, accounting, printer and filing fees related to the IPO, are deferred and will be offset against proceeds from the IPO upon the effectiveness of the offering. In the event the offering is terminated, all deferred offering costs will be expensed. As of December 31, 2018, $1.5 million of deferred offering costs were recorded as other assets on the consolidated balance sheet. There were no deferred offering costs recorded as of December 31, 2017. Defined Contribution Plan The Company has a defined contribution retirement savings plan under Section 401(k) of the Internal Revenue Code. This plan allows eligible employees to defer a portion of their annual compensation on a pre-tax Comprehensive Loss Comprehensive loss is comprised of net loss and changes in unrealized gains and losses on the Company’s available-for-sale Foreign Currency The functional currency of the Company’s foreign subsidiary is the U.S. Dollar. Accordingly, all monetary assets and liabilities of the subsidiary are remeasured at the current exchange rate at the end of the period, nonmonetary assets and liabilities are remeasured at historical rates, and revenue and expenses are remeasured at average exchange rates during the period. There were net foreign currency transaction gains of $36,000 for the year ended December 31, 2017 and net foreign currency transaction losses of $46,000 for the year ended December 31, 2018. Net Loss Per Share Basic net loss per share is calculated by dividing net loss by the weighted-average number of shares of common stock outstanding for the period, without consideration of potential dilutive shares of common stock. The unvested portion of early exercised stock options are excluded from the computation of weighted-average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Since the Company was in a loss position for the period presented, basic net loss per share is the same as diluted net loss per share since the effects of potentially dilutive securities are antidilutive. Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net deferred tax assets have been fully offset by a valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of provision for income taxes. Segment Reporting Operating segments are defined as components of an entity for which separate financial information is available and that is regularly reviewed by the Chief Operating Decision Maker (“CODM”) in deciding how to allocate resources to an individual segment and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company has determined it operates in one segment. Recently Issued and Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting ASU 2018-07 In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases right-of-use No. 2018-11, Leases (Topic 842): Targeted Improvements non-lease a right-of-use asset

Financial Instruments and Fair

Financial Instruments and Fair Value Measurements	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Financial Instruments and Fair Value Measurements	3. Financial Instruments and Fair Value Measurements The following tables summarize the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 21,680 $ — $ — $ 21,680 Total assets $ 21,680 $ — $ — $ 21,680 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 313 $ 313 Total liabilities $ — $ — $ 313 $ 313 September 30, 2019 Level 1 Level 2 Level 3 Total (in thousands) Assets: U.S. Treasury securities $ 58,006 $ — $ — $ 58,006 Money market funds 19,987 — — 19,987 Reverse repurchase agreements — 10,601 — 10,601 Commercial paper — 8,562 — 8,562 Corporate bonds — 8,078 — 8,078 Total assets $ 77,993 $ 27,241 $ — $ 105,234 There were no transfers between Levels 1, 2 or 3 for the periods presented. The change in the fair value of the warrant liability for the nine months ended September 30, 2018 is summarized below (in thousands): Balance at December 31, 2017 $ 577 Exercise of warrants (316 ) Expiration of warrants, included in change in fair value of warrant liability (133 ) Change in fair value of warrant liability 186 Balance at September 30, 2018 $ 314 The change in the fair value of the warrant liability for the nine months ended September 30, 2019 is summarized below (in thousands): Balance at December 31, 2018 $ 313 Change in fair value of warrant liability 609 Net exercise of warrants (133 ) Conversion of Series A preferred stock warrants to common stock warrants upon the closing of the IPO (789 ) Balance at September 30, 2019 $ — The valuation of the Company’s convertible preferred stock warrant liability contains unobservable inputs that reflect the Company’s own assumptions for which there is little, if any, market activity for at the measurement date. Accordingly, the Company’s convertible preferred stock warrant liability is measured at fair value on a recurring basis using unobservable inputs and are classified as Level 3 inputs, and any change in fair value is recognized as change in fair value of warrant liability in the condensed consolidated statements of operations and comprehensive loss. The fair value of the warrants, which were converted to common stock warrants upon the closing of the IPO in March 2019, was determined using the Black-Scholes option pricing model and the following assumptions: Nine Months Ended September 30, 2018 2019 Expected term (in years) 5.71-6.21 5.28 Expected volatility 42.8% 43.9% Risk-free interest rate 2.62-2.85% 2.49% Expected dividend yield 0% 0%	3. Financial Instruments and Fair Value Measurements The following tables summarize the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2017 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 26,379 $ — $ — $ 26,379 U.S. treasury note 1,806 — — 1,806 Total assets $ 28,185 $ — $ — $ 28,185 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 577 $ 577 Total liabilities $ — $ — $ 577 $ 577 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 21,680 $ — $ — $ 21,680 Total assets $ 21,680 $ — $ — $ 21,680 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 313 $ 313 Total liabilities $ — $ — $ 313 $ 313 The change in the fair value of the warrant liability is summarized below: Year Ended 2017 2018 (in thousands) Beginning balance $ 729 $ 577 Exercise of warrants (184 ) (316 ) Expiration of warrants, included in change in fair value of warrant liability — (133 ) Change in fair value of warrant liability 32 185 Ending balance $ 577 $ 313 The valuation of the Company’s convertible preferred stock warrant liability contains unobservable inputs that reflect the Company’s own assumptions for which there is little, if any, market activity for at the measurement date. Accordingly, the Company’s convertible preferred stock warrant liability is measured at fair value on a recurring basis using unobservable inputs and are classified as Level 3 inputs, and any change in fair value is recognized as change in fair value of warrant liability in the consolidated statements of operations and comprehensive loss. Refer to Note 9 for the valuation technique and assumptions used in estimating the fair value of the warrants. There were no transfers between Levels 1, 2 or 3 for the periods presented.

Cash Equivalents and Short-Term

Cash Equivalents and Short-Term Investments	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Short-Term Investments	4. Cash Equivalents and Short-Term Investments The following is a summary of the Company’s cash equivalents and short-term investments: December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 21,680 $ — $ — $ 21,680 Total $ 21,680 $ — $ — $ 21,680 Reported as: Cash equivalents $ 21,680 Total $ 21,680 September 30, 2019 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) U.S. Treasury securities $ 57,959 $ 47 $ — $ 58,006 Money market funds 19,987 — — 19,987 Reverse repurchase agreements 10,601 — — 10,601 Commercial paper 8,562 — — 8,562 Corporate bonds 8,068 10 — 8,078 Total $ 105,177 $ 57 $ — $ 105,234 Reported as: Cash equivalents $ 30,588 Short-term investments 74,646 Total $ 105,234 The Company recognized no material gains or losses on its cash equivalents and short-term investments in the periods presented.	4. Cash Equivalents and Short-Term Investments The following is a summary of the Company’s cash equivalents and short-term investments: December 31, 2017 Amortized Unrealized Unrealized Fair Value (in thousands) Money market funds $ 26,379 $ — $ — $ 26,379 U.S. treasury note 1,807 — (1 ) 1,806 Total $ 28,186 $ — $ (1 ) $ 28,185 Reported as: Cash equivalents $ 26,379 Short-term investments 1,806 Total $ 28,185 December 31, 2018 Amortized Unrealized Unrealized Fair Value (in thousands) Money market funds $ 21,680 $ — $ — $ 21,680 Total $ 21,680 $ — $ — $ 21,680 Reported as: Cash equivalents $ 21,680 Total $ 21,680 For the years ended December 31, 2017 and 2018, the Company recognized no material realized gains or losses on cash equivalents and short-term investments.

Balance Sheet Components

Balance Sheet Components	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Inventory Inventory consists of the following: December 31, September 30, (in thousands) Raw material $ 1,084 $ 2,169 Work in progress 634 850 Finished goods 2,313 4,451 Consigned inventory 1,100 1,757 Total inventory $ 5,131 $ 9,227 Accrued Liabilities Accrued liabilities consist of the following: December 31, September 30, (in thousands) Accrued employee compensation $ 3,135 $ 5,984 Accrued research and development costs 1,115 1,844 Accrued professional services 1,391 856 Other 576 812 Total accrued liabilities $ 6,217 $ 9,496	5. Balance Sheet Components Inventory Inventory consists of the following: December 31, 2017 2018 (in thousands) Raw material $ 478 $ 1,084 Work in progress 198 634 Finished goods 1,041 2,313 Consigned inventory 806 1,100 Total inventory $ 2,523 $ 5,131 Property and Equipment, Net Property and equipment, net consists of the following: December 31, 2017 2018 (in thousands) Equipment $ 1,298 $ 2,321 Equipment on loan to customers 263 786 Office furniture 68 90 Software 76 76 Leasehold improvements 366 764 Construction in progress 258 236 Property and equipment, gross 2,329 4,273 Less accumulated depreciation and amortization (957 ) (1,654 ) Total property and equipment, net $ 1,372 $ 2,619 Depreciation and amortization expense was $0.5 million and $0.7 million for the years ended December 31, 2017 and 2018, respectively. Other Assets Other assets consist of the following: December 31, 2017 2018 (in thousands) Deferred offering costs $ — $ 1,519 Restricted cash — 450 Other 73 97 Total other assets $ 73 $ 2,066 Accrued Liabilities Accrued liabilities consist of the following: December 31, 2017 2018 (in thousands) Accrued employee compensation $ 2,072 $ 3,135 Accrued research and development costs 507 1,115 Accrued professional services 348 1,391 Other 466 576 Total accrued liabilities $ 3,393 $ 6,217

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Operating Leases In August 2012, the Company entered into a lease for office space located in Fremont, California. In October 2018, the Company extended the term of the lease to June 30, 2019 and in February 2019, the Company exercised the option to extend the lease further until September 30, 2019. The lease expired on September 30, 2019. The Company was using the facility for office, manufacturing and research and development purposes. In May 2018, the Company entered into a new lease agreement for office and laboratory space which consist of approximately 35,000 square feet located in Santa Clara, California. The lease term commenced in September 2018 and ends in August 2022. In connection with the lease, the Company maintains a letter of credit in the amount of $0.5 million, which is secured by restricted cash recorded as other assets on the condensed consolidated balance sheets. In connection with the lease, the Company has an operating lease right-of-use The Company also leases vehicles for use by employees. In connection with the vehicle leases, the Company has an operating lease right-of-use The weighted average incremental borrowing rate used to measure the operating lease liability is 6.93%. Rent expense for the nine months ended September 30, 2018 and 2019 was $0.5 million and $0.9 million, respectively. Short-term leases are leases having a term of 12 months or less. The Company recognizes short-term leases on a straight-line basis and does not record a related lease asset or liability for such leases. The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of September 30, 2019: (in thousands) Remainder of 2019 $ 222 2020 861 2021 855 2022 581 Total minimum lease payments $ 2,519 Less: imputed interest (234 ) Total lease liability $ 2,285	6. Commitments and Contingencies Operating Leases In August 2012, the Company entered into a lease for office space located in Fremont, California. In October 2018, the Company extended the term of the lease to June 30, 2019. The Company is using the facility for office, manufacturing and research and development purposes. In May 2018, the Company entered into a new lease agreement for office and laboratory space which consist of approximately 35,000 square feet located in Santa Clara, California. The lease term commenced in September 2018 and ends in August 2022. In connection with the lease, the Company maintains a letter of credit for the benefit of the landlord in the amount of $0.5 million, which is secured by restricted cash recorded as other assets on the consolidated balance sheets. The following are minimum future rental payments owed under these agreements: Year ending December 31: (in thousands) 2019 $ 940 2020 830 2021 855 2022 582 Total $ 3,207 Rent expense for the years ended December 31, 2017 and 2018 was $0.4 million and $0.8 million, respectively.

Term Notes

Term Notes	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Term Notes	7. Term Notes 2014 Loan and Security Agreement In June 2014, the Company entered into a Loan and Security Agreement with Silicon Valley Bank (the “2014 Loan and Security Agreement”), under which a total of $4.0 million was borrowed. The Company made monthly payments of principal and interest through the maturity date of October 1, 2017, and a one-time In connection with the 2014 Loan and Security Agreement, the Company issued warrants to purchase shares of the Company’s Series A-1 2018 Loan and Security Agreement In February 2018, the Company entered into a Loan and Security Agreement with Silicon Valley Bank (the “2018 Loan and Security Agreement”). The terms of the 2018 Loan and Security Agreement include a term loan of $15.0 million and a revolving line of credit of $2.0 million. The term loan was available in two tranches, of which the first tranche of $10.0 million was funded in June 2018 and the second tranche of $5.0 million was funded in December 2018. The Company has not drawn down on its revolving line of credit as of December 31, 2018. The term loan matures in December 2021, with interest-only monthly payments until September 2019. The interest-only period will extend through December 2019 if certain financing milestones are met. The term loan accrues interest at a floating per annum rate equal to the greater of the Wall Street Journal prime rate minus 1.75% and 2.75% (3.75% as of December 31, 2018). There is a final payment equal to 6.75% of the original aggregate principal amount, or $1.0 million, of the term loan advances, which will be accrued over the term of the loan using the effective-interest method. The line of credit matures in February 2021 and accrues interest at the Wall Street Journal prime rate. In connection with the execution of the 2018 Loan and Security Agreement, the Company issued warrants to purchase 34,440 shares of the Company’s common stock. Upon issuance, the fair value of the warrants of $0.1 million was recorded as a debt issuance cost. The debt issuance cost will be amortized to interest expense, net over the repayment period of the loan. During the year ended December 31, 2018, the Company recorded interest expense related to the 2018 Loan and Security Agreement and amortization of debt discount of $0.2 million and $0.2 million, respectively. The term loan is secured by all of the Company’s assets, excluding intellectual property and certain other assets. The loan contains customary affirmative and restrictive covenants, including with respect to the Company’s ability to enter into fundamental transactions, incur additional indebtedness, grant liens, pay any dividend or make any distributions to its holders, make investments, merge or consolidate with any other person or engage in transactions with the Company’s affiliates, but does not include any financial covenants. Long-term debt and net premium balances are as follows: December 31, (in thousands) Face value of term note $ 15,000 Net premium associated with accretion of final payment, issuance of common stock warrants, and other debt issuance costs 50 Term note, current and noncurrent 15,050 Less term note, current portion (1,667 ) Term note, noncurrent portion $ 13,383 Future minimum payments of principal and estimated payments of interest on the Company’s outstanding variable rate borrowings as of December 31, 2018 are as follows: Year ending December 31: (in thousands) 2019 $ 2,243 2020 7,079 2021 7,816 Total future payments 17,138 Less amounts representing interest (1,125 ) Less final payment (1,013 ) Total principal amount of term note payments $ 15,000

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock In September 2017, the Company issued 2,840,504 shares of its Series C convertible preferred stock at a price per share of $12.32176 for net proceeds of $34.9 million. In December 2018, the Company issued 1,090,608 shares of its Series D convertible preferred stock at a price per share of $13.75379 for gross proceeds of $15.0 million. The Series D investor has the option to purchase up to $10.0 million in common stock in a concurrent private placement at a price per share equal to the price per share of the common stock to the public in an initial public offering. During 2018, the Company issued 69,675 shares of its Series A-1 Series A-1 Series A-1 Convertible preferred stock consists of the following: December 31, 2017 Shares Shares Net Carrying Aggregate (in thousands, except share amounts) Series A 19,280,722 1,580,387 $ 4,226 $ 4,473 Series A-1 52,589,632 4,127,463 13,637 12,780 Series B 65,000,000 5,309,617 39,877 40,000 Series C 79,209,457 6,492,578 79,729 80,000 216,079,811 17,510,045 $ 137,469 $ 137,253 December 31, 2018 Shares Shares Net Carrying Aggregate (in thousands, except share amounts) Series A 19,280,722 1,580,387 $ 4,226 $ 4,473 Series A-1 51,874,893 4,197,138 14,054 12,996 Series B 64,777,331 5,309,617 39,877 40,000 Series C 79,209,457 6,492,578 79,729 80,000 Series D 13,956,584 1,090,608 14,920 15,000 229,098,987 18,670,328 $ 152,806 $ 152,469 The Company recorded its convertible preferred stock at fair value on the dates of issuance, net of issuance costs. As of December 31, 2017 and 2018, the Company classified its Series A, Series A-1, A-1, The holders of the Convertible Preferred Stock have the following rights, privileges and preferences: Optional Conversion Rights Each share of Series A, Series A-1, Series A-1, A-1, one-for-one A-1, Automatic Conversion Rights Each share of convertible preferred stock shall automatically be converted into shares of common stock at the then effective conversion rate for such share (i) immediately prior to the closing of a firm commitment underwritten initial public offering pursuant to an effective registration statement filed under the Securities Act of 1933, as amended (the “Securities Act”), covering the offer and sale of the Company’s common stock, provided that the offering price per share is not less than $15.067 (as adjusted for recapitalizations) and the aggregate gross proceeds to the Company are not less than $30.0 million, or (ii) upon the receipt by the Company of a written request for such conversion from the preferred requisite majority, or, if later, the effective date for conversion specified in such requests. The conversion prices and rates for each series of convertible preferred stock are the same in the event of an automatic conversion as they would be in the event of an optional conversion. Voting Rights Each share of convertible preferred stock has a number of votes equal to the number of shares of common stock into which it is convertible. The holders of the Series A, Series A-1 as-converted Liquidation Rights In the event of any liquidation, dissolution or winding-up A-1, A-1, A-1, If upon the liquidation, dissolution or winding up of the Company the assets of the Company legally available for distribution to the holders of the Series A, Series A-1, Series A-1 A Liquidation Event is defined as including (i) the acquisition of the corporation by another entity by means of any transaction or series of related transactions to which the corporation is party other than a transaction or series of transactions in which the holders of the voting securing of the corporation outstanding immediately prior to such transaction retain, immediately after such transaction or series of transactions, as a result of shares in the corporation held by such holders prior to such transaction, at least a majority of the total voting power represented by the outstanding voting securities of the corporation or such other surviving or resulting entity; (ii) a sale, lease or other disposition (excluding by exclusive license) of all or substantially all of the assets of the corporation; or (iii) any liquidation, dissolution or winding up of the corporation whether voluntary or involuntary. Dividend Rights The convertible preferred stockholders are entitled to receive dividends at an annual rate of $0.22643 per share of Series A, $0.24766 per share of Series A-1, A-1, as-converted Redemption Rights There are no redemption rights afforded to the holders of convertible preferred stock. Upon certain change in control events that are outside of the Company’s control, including liquidation, sale or transfer of control of the Company, the convertible preferred stock is contingently redeemable.

Warrant Liability

Warrant Liability	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Warrant Liability	9. Warrant Liability The key terms of the outstanding convertible preferred stock warrants are summarized in the following table: Warrants Outstanding Warrants Warrants Exercise Expiration Series A-1 128,259 — $ 3.09636 Various dates in 2018 Series A-1 54,903 54,903 $ 3.09636 June 2024 Total convertible preferred stock warrants 183,162 54,903 As discussed in Note 8, during 2018, the Company issued 69,675 shares of its Series A-1 Series A-1 The fair value of the warrants was determined using the Black-Scholes option pricing model and the following assumptions: December 31, 2017 2018 Expected term (in years) 0-6.5 5.5 Expected volatility 19.8%-42.3% 42.8% Risk-free interest rate 1.1%-1.8% 2.9% Expected dividend yield 0% 0%

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Total stock-based compensation was as follows: Nine Months September 30, 2018 2019 (in thousands) Cost of product revenue $ 38 $ 156 Research and development 146 582 Sales and marketing 185 597 General and administrative 505 1,001 Total stock-based compensation $ 874 $ 2,336 Determination of Fair Value The Company estimates the grant-date fair value of the Company’s option awards using the Black-Scholes option pricing model. The assumptions for the Black-Scholes model for the nine months ended September 30, 2018 and 2019 were as follows: Nine Months Ended September 30, 2018 2019 Expected term (in years) 6.08 6.08 Expected volatility 40.8%-45.9% 42.4%-42.9% Risk-free interest rate 1.9%-2.9% 2.4%-2.6% Expected dividend yield 0 0% 2009 Equity Incentive Plan and 2019 Equity Incentive Plan On June 17, 2009, the Company adopted the 2009 Equity Incentive Plan (the “2009 Plan”) under which the Board had the authority to issue stock options to employees, directors and consultants. In February 2019, the Company adopted the 2019 Stock Option and Incentive Plan (the “2019 Plan”), which became effective in connection with the IPO. As a result, effective as of March 6, 2019, the Company may not grant any additional awards under the 2009 Plan. The 2009 Plan will continue to govern outstanding equity awards granted thereunder. The Company has initially reserved 2,000,430 shares of common stock for the issuance of a variety of awards under the 2019 Plan, including stock options, stock appreciation rights, awards of restricted stock and awards of restricted stock units. In addition, the number of shares of common stock reserved for issuance under the 2019 Plan will automatically increase on the first day of January for a period of up to ten years, commencing on January 1, 2020, in an amount equal to 3% of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors. As of September 30, 2019, there were 1,470,192 shares available for issuance under the 2019 Plan. Stock Options Option activity under the 2009 Plan and 2019 Plan is set forth below: Number of Weighted- Average Exercise Price Per Share Weighted- Average Remaining Term Aggregate Intrinsic Value (in years) (in thousands) Balance, December 31, 2018 3,636,358 $ 3.54 7.79 $ 11,267 Options granted 442,858 14.69 Options exercised (386,309 ) 2.65 Options forfeited (34,471 ) 3.49 Balance, September 30, 2019 3,658,436 $ 4.95 7.52 $ 91,399 Vested and exercisable, September 30, 2019 1,831,368 $ 3.14 6.59 $ 49,068 Restricted Stock Units Restricted stock units (“RSUs”) are share awards that entitle the holder to receive freely tradable shares of the Company’s common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s employment terminates prior to the release of the vesting restrictions. The RSUs generally vest over a four-year period with straight-line vesting and a 25% one year cliff or over a three year period in equal amounts on a semi-annual basis, provided the employee remains continuously employed with the Company. The fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date. RSU activity under the 2019 Plan is set forth below: Number of Shares Weighted- Average Grant Date Fair Value Per Share Balance, December 31, 2018 — $ — RSUs granted 229,450 38.26 RSUs forfeited (2,900 ) 37.51 Balance, September 30, 2019 226,550 $ 38.27 Employee Share Purchase Plan (ESPP) In February 2019, the Company adopted the 2019 Employee Stock Purchase Plan (“ESPP”), which became effective as of March 6, 2019. The Company has initially reserved 300,650 shares of common stock for purchase under the ESPP. Each offering to the employees to purchase stock under the ESPP will begin on each September 1 and March 1 and will end on the following February 28 or 29 and August 30, respectively. The first offering period began on September 1, 2019 and will end on February 29, 2020. On each purchase date, which falls on the last date of each offering period, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date. The occurrence and duration of offering periods under the ESPP are subject to the determinations of the Company’s Compensation Committee, in its sole discretion. The fair value of the ESPP shares is estimated using the Black-Scholes option pricing model. The Company recorded $64,000 of stock-based compensation expense related to the ESPP for the nine months ended September 30, 2019.	10. Stock-Based Compensation Total stock-based compensation was as follows: Year Ended 2017 2018 (in thousands) Cost of product revenue $ 46 $ 67 Research and development 185 235 Sales and marketing 130 294 General and administrative 604 701 Total stock-based compensation $ 965 $ 1,297 Determination of Fair Value The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: December 31, 2017 2018 Expected term (in years) 6.08 6.08 Expected volatility 45.6% 40.8%-41.9% Risk-free interest rate 1.9%-2.2% 2.5%-3.1% Expected dividend yield 0% 0% The fair value of each stock option grant was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. Expected Term— Expected Volatility— Risk-Free Interest Rate— zero-coupon Expected Dividend Yield— The Company has elected to recognize forfeitures of share-based payment awards as they occur. Equity Incentive Plan On June 17, 2009, the Company adopted the 2009 Equity Incentive Plan (the “Plan”) under which the Board may issue stock options to employees, directors and consultants. The Board has the authority to determine to whom options will be granted, the number of shares, the term and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the Board. Options granted under the Plan have a term of up to 10 years and generally vest over a 4 year period with a straight-line vesting and a 25% one year cliff. As of December 31, 2018, the Company had reserved 5,057,744 shares of common stock for issuance under the Plan. Activity under the 2009 Plan is set forth below: Shares Number of Weighted- Weighted- Aggregate (in years) (in thousands) Balance, December 31, 2016 887,885 1,365,934 $ 1.95 $ 2,011 Awards authorized 1,352,677 — Options granted (1,974,589 ) 1,974,589 3.42 Options exercised — (71,522 ) 1.95 Options cancelled 160,397 (160,397 ) 2.44 Balance, December 31, 2017 426,370 3,108,604 $ 2.81 8.03 $ 3,647 Awards authorized 691,503 — Options granted (1,015,963 ) 1,015,963 5.25 Options exercised — (197,820 ) 2.20 Options cancelled 290,389 (290,389 ) 3.42 Balance, December 31, 2018 392,299 3,636,358 $ 3.54 7.79 $ 11,267 Vested and exercisable, December 31, 2018 1,585,273 $ 2.68 6.50 $ 6,249 Vested and expected to vest, December 31, 2018 3,636,358 $ 3.54 7.79 $ 11,267 The weighted-average grant date fair value of options granted during the years ended December 31, 2017 and 2018 was $1.59 and $2.56 per share, respectively. The total grant date fair value of options vested was $0.9 million and $1.6 million for the years ended December 31, 2017 and 2018, respectively. The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock, as determined by the Board, as of December 31, 2018. The aggregate intrinsic value of options exercised was $0.1 million and $0.5 million for the years ended December 31, 2017 and 2018, respectively. As of December 31, 2018, total unrecognized stock-based compensation related to unvested stock options was $3.8 million, which the Company expects to recognize over a remaining weighted-average period of 2.72 years. Early Exercise of Options Certain stock options granted under the Company’s stock option Plan provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. A summary of the restricted stock shares issued under the Company’s Plan is as follows: Number of Weighted- Outstanding and unvested at December 31, 2016 94,610 $ 0.61 Vested 83,007 $ 0.61 Outstanding and unvested at December 31, 2017 11,603 $ 0.61 Exercised 45,287 $ 2.20 Vested (43,468 ) $ 1.83 Outstanding and unvested at December 31, 2018 13,422 $ 2.20 Terms of the Plan permit certain option holders to exercise options before their options are vested, subject to certain limitations. Upon early exercise, the awards become subject to a restricted stock agreement. The shares of restricted stock granted upon early exercise of the options are subject to the same vesting provisions in the original stock option awards. Common stock outstanding in these consolidated financial statements includes restricted stock subject to repurchase. Shares issued as a result of early exercise that have not vested are subject to repurchase by the Company upon termination of the purchaser’s employment, at the price paid by the purchaser, and are not deemed to be issued for accounting purposes until those related shares vest. The liability is reclassified into common stock and additional paid-in Stock-Based Awards Granted Outside of Equity Incentive Plans Common Stock Warrants In May 2015, the Company issued warrants to purchase shares of its common stock to the three founders of the Company. In February 2018, in connection with the execution of the 2018 Loan and Security Agreement, the Company issued warrants to purchase shares of the Company’s common stock. The key terms of the outstanding common stock warrants are summarized in the following table: Warrants Outstanding Warrants Warrants Exercise Expiration Related party common stock warrants 141,778 141,778 $ 2.196 May 2025 Common stock warrants issued in connection with the 2018 Loan and Security Agreement — 34,440 $ 4.026 February 2028 Total common stock warrants 141,778 176,218

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes Current income tax provision consists of the following: December 31, 2017 2018 (in thousands) Domestic $ 1 $ 3 Foreign 25 35 Total current income tax provision $ 26 $ 38 The components of the deferred tax assets are as follows: December 31, 2017 2018 (in thousands) Deferred tax assets: Net operating loss carryovers $ 19,251 $ 28,834 Fixed and intangible assets 664 837 Accruals and reserves 465 761 Stock-based compensation 31 132 Research and development credits 1,858 1,716 Contributions 13 14 Total deferred tax assets 22,282 32,294 Less valuation allowance (22,282 ) (32,294 ) Total net deferred tax assets $ — $ — Reconciliation of the statutory federal income tax to the Company’s effective tax is as follows: December 31, 2017 2018 (in thousands) Income tax benefit at federal statutory rate $ (10,404 ) $ (8,624 ) State and local income taxes net of federal tax benefit 1 3 Foreign tax rate differential (3 ) 11 Change in valuation allowance (522 ) 8,497 Stock-based compensation 309 123 R&D tax credits (222 ) (313 ) Other 109 341 Federal rate change (pursuant to the Tax Cuts and Jobs Act of 2017) 10,758 — Total current income tax provision $ 26 $ 38 Due to the uncertainties surrounding the realization of deferred assets through future taxable income, the Company has provided a full valuation allowance and, therefore, no benefit has been recognized for the net operating loss and other deferred tax assets. The valuation allowance increased by $0.6 million and $10.0 million during the years ended December 31, 2017 and 2018, respectively. As of December 31, 2018, the Company had net operating loss carryforwards available to reduce future federal, California and other state income of $119.5 million, $37.2 million and $19.5 million, respectively. The federal net operating loss carryforwards begin expiring in 2030, the California net operating loss carryforwards begin expiring in 2031 and other state net operating loss carryforwards begin expiring in various years, starting in 2029. The net operating loss related deferred tax assets do not include excess tax benefits from employee stock option exercises. As of December 31, 2018, the Company had research and development credit carryforwards of $1.0 million for federal income tax purposes and $0.9 million for California state income tax purposes available to reduce future taxable income, if any. The federal research and development credit carryforwards expire beginning 2032 and California credits can be carried forward indefinitely. Utilization of the net operating loss carryforward may be subject to an annual limitation due to the ownership percentage change limitations provided by the Internal Revenue Code of 1986, as amended, and similar state provisions. The annual limitation may result in the expiration of the net operating loss before utilization. Legislation enacted in 2017, informally known as the Tax Cuts and Jobs Act (“TCJA”), reduces the top U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time one-time Subsequent to the enactment of the TCJA, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which allowed companies to record provisional amounts related to the effects of the TCJA during a measurement period not to extend beyond one year of the enactment date. The accounting for the tax effects of the TCJA has been completed as of December 31, 2018 and was not material to income tax expense for the year then ended. The Company has adopted the approach of recording the consequences of the global intangible low-taxed Uncertain Tax Positions The activity related to the gross amount of unrecognized tax benefits is as follows: December 31, 2017 2018 (in thousands) Beginning balance $ 688 $ 893 Additions based on tax positions related to prior years — 394 Additions based on tax positions related to current years 205 609 Balance at end of year $ 893 $ 1,896 If recognized, gross unrecognized tax benefits would not have an impact on the Company’s effective tax rate due to the Company’s full valuation allowance position. While it is often difficult to predict the final outcome of any particular uncertain tax position, the Company does not believe that the amount of gross unrecognized tax benefits will change significantly in the next twelve months. The Company is subject to taxation in the United States and in Germany. The Company files federal, California, and various other state income tax returns. The Company is not currently under examination by any income tax authorities. The federal and California statute of limitations remains open for three and four years, respectively, from the date of utilization of any net operating loss or credits. It is the Company’s policy to include penalties and interest expense related to income taxes as a component of the income tax provision as necessary. The Company determined that no accrual for interest and penalties was required as of December 31, 2018.

Net Loss Per Share

Net Loss Per Share	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	9. Net Loss Per Share The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: September 30, 2018 2019 (in thousands) Convertible preferred stock on an as-converted 17,579,720 — Common stock options issued and outstanding 3,313,257 3,658,436 Restricted stock units — 226,550 Early exercised options subject to future vesting 21,620 — Convertible preferred stock warrants 54,903 — Common stock warrants 176,218 — Total 21,145,718 3,884,986	12. Net Loss Per Share The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the period presented due to their anti-dilutive effect: Year Ended 2017 2018 (in thousands) Convertible preferred stock 17,510,045 18,670,328 Common stock options issued and outstanding 3,108,604 3,636,358 Early exercised options subject to future vesting 11,603 13,422 Convertible preferred stock warrants 183,162 54,903 Common stock warrants 141,778 176,218 Total 20,955,192 22,551,229

Segment and Geographic Informat

Segment and Geographic Information	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Segment and Geographic Information	10. Segment and Geographic Information The following table represents the Company’s product revenue based on the location to which the product is shipped: Nine Months Ended September 30, 2018 2019 (in thousands) United States $ 4,476 $ 15,054 Germany 958 2,389 Rest of Europe 1,597 9,473 All other countries 170 1,699 Product revenue $ 7,201 $ 28,615 As of September 30, 2019 and December 31, 2018, the Company’s long-lived assets are all held in the United States with the exception of certain equipment on loan to customers held internationally, which was not material as of each period end.	13. Segment and Geographic Information The following table represents the Company’s product revenue based on the location to which the product is shipped: December 31, 2017 2018 (in thousands) United States $ 969 $ 7,022 Germany 597 1,393 Rest of Europe 143 3,516 All other countries 10 332 Product revenue $ 1,719 $ 12,263 As of December 31, 2017 and 2018, the Company’s long-lived assets are all held in the United States with exception of certain equipment on loan to customers held internationally, which was not material as of each period end.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events Except as noted below, subsequent events have been evaluated through February 8, 2019, which is the date that the consolidated financial statements were available to be issued. Reverse Stock Split The Company’s board of directors approved a reverse split of shares of the Company’s common stock and convertible preferred stock on a 12.2-for-one

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Equity (Deficit)	9 Months Ended
	Sep. 30, 2019
Federal Home Loan Banks [Abstract]
Convertible Preferred Stock and Stockholders' Equity (Deficit)	7. Convertible Preferred Stock and Stockholders’ Equity (Deficit) Convertible Preferred Stock Immediately prior to the closing of the Company’s IPO, 18,670,259 shares of outstanding convertible preferred stock converted into 18,670,259 shares of common stock. As discussed in Note 1, the fractional shares resulting from the Reverse Stock Split, aggregating to 45 whole shares of common stock and 69 whole shares of convertible preferred stock were settled in cash in fiscal 2019. Preferred Stock The Company’s amended and restated certificate of incorporation, which became effective upon the completion of the IPO, authorizes 5,000,000 shares of preferred stock, of which no shares were issued or outstanding as of September 30, 2019. Preferred Stock Warrants Upon the closing of the IPO, all of the outstanding convertible preferred stock warrants were converted into 54,903 common stock warrants, which resulted in the reclassification of the convertible preferred stock warrant liability of $0.8 million to additional paid-in capital. Common Stock Warrants Upon the IPO, 91,446 common stock warrants held by related parties were net exercised based on the IPO price of $17.00 per share into 79,632 shares of common stock. In February 2018, in connection with the execution of a Loan and Security Agreement with Silicon Valley Bank for a term loan and revolving line of credit (the “2018 Loan and Security Agreement”), the Company issued warrants to purchase shares of the Company’s common stock. In April 2019, all of these common stock warrants were net exercised into 29,887 shares of common stock. The key terms of the outstanding common stock warrants are summarized in the following table: Warrants Outstanding December 31, 2018 Warrants Outstanding September 30, 2019 Exercise Price Expiration Related party common stock warrants 141,778 — $ 2.196 May 2025 Common stock warrants issued in connection with the 2018 Loan and Security Agreement 34,440 — $ 4.026 February 2028 Total common stock warrants 176,218 —

Organization and Basis of Pre_2

Organization and Basis of Presentation (Policies)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Going Concern		Going Concern The Company has incurred significant losses and has negative cash flows from operations. As of December 31, 2018, the Company had an accumulated deficit of $126.9 million. Management expects to continue to incur additional substantial losses in the foreseeable future. As of December 31, 2018, the Company had cash and cash equivalents of $39.6 million, which are available to fund future operations. The Company will need to raise additional capital to support the commercialization of its products and research and development activities. The Company’s activities are subject to significant risks and uncertainties, including the market acceptance of the Company’s products and the timing and extent of spending on research and development. The Company believes that its cash and cash equivalents as of December 31, 2018, together with available borrowings under a revolving line of credit, will not be sufficient for the Company to continue as a going concern for at least one year from the issuance date of its consolidated financial statements. The Company believes that this raises substantial doubt about its ability to continue as a going concern. As a result, the Company will be required to raise additional capital from the sale of common stock or convertible preferred stock or the issuance of debt. However, no assurance can be given as to whether additional needed financing will be available on terms acceptable to the Company, if at all. If sufficient funds on acceptable terms are not available when needed, the Company may be required to curtail planned activities to significantly reduce its operating expenses. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives and have an adverse effect on its results of operations and future prospects. The accompanying consolidated financial statements have been prepared assuming that the Company will continue as a going concern, which contemplates the realization of assets and the settlement of liabilities and commitments in the normal course of business. The accompanying consolidated financial statements do not reflect any adjustments relating to the recoverability and reclassifications of assets and liabilities that might be necessary if the Company is unable to continue as a going concern.
Principles of Consolidation		Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates		Use of Estimates The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the valuation of inventory, the fair value of common stock, the fair value of preferred stock warrant liabilities, the fair value of stock options, recoverability of the Company’s net deferred tax assets, and related valuation allowance and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates .
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. Restricted cash as of September 30, 2019 and December 31, 2018 relates to a letter of credit established for a lease entered into in May 2018 and is recorded as other assets on the condensed consolidated balance sheets. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows: December 31, 2018 September 30, 2019 (in thousands) Cash and cash equivalents $ 39,643 $ 39,452 Restricted cash 450 450 Total cash, cash equivalents, and restricted cash $ 40,093 $ 39,902	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments purchased with original maturities of three months or less from the purchase date to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: December 31, 2017 2018 (in thousands) Cash and cash equivalents $ 51,923 $ 39,643 Restricted cash — 450 Total cash, cash equivalents, and restricted cash $ 51,923 $ 40,093 Restricted cash as of December 31, 2018 relates to a letter of credit established for a lease entered into in May 2018 and is recorded as other assets on the consolidated balance sheets.
Short-Term Investments	Short-Term Investments Short-term investments have been classified as available-for-sale and purchase. Available-for-sale securities Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income, net. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net.	Short-Term Investments Short-term investments have been classified as available-for-sale Available-for-sale Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on marketable securities are included in other income, net. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net.
Concentration of Credit Risk		Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, investments and trade receivables. Risks associated with cash, cash equivalents and restricted cash are mitigated by banking with creditworthy institutions and the Company’s investments have investment grade ratings when purchased. The Company performs ongoing evaluations of its customers and generally does not require collateral.
Concentration of Customers		Concentration of Customers For the year ended December 31, 2017, one customer accounted for 19% of the Company’s revenue. For the year ended December 31, 2108, there were no customers which accounted for more than 10% of the Company’s revenue. There were no customers which accounted for more than 10% of the Company’s accounts receivable as of December 31, 2017 and 2018.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, accounts receivable, accounts payable and accrued liabilities approximate their fair value due to their short maturities. Management believes that its term notes bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument approximates its fair value. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, short-term investments, accounts receivable, accounts payable and accrued liabilities approximate their fair value due to their short maturities. Management believes that its term notes bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument approximates its fair value. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 Level 2 Level 3
Accounts Receivable and Allowance for Doubtful Accounts		Accounts Receivable and Allowance for Doubtful Accounts Accounts receivable are recorded at invoice value, net of any allowance for doubtful accounts. Estimates of the allowance for doubtful accounts are determined based on existing contractual payment terms, historical payment patterns of customers and individual customer circumstances. The allowance for doubtful accounts was $76,000 as of December 31, 2018 and the Company recognized accounts receivable write-offs in the amount of $1,000 for the year ended December 31, 2018. There was no allowance for doubtful accounts recorded and no accounts receivable write-offs recognized as of and for the year ended December 31, 2017.
Inventory		Inventory Inventory is stated at the lower of standard cost (which approximates actual cost on a first-in, first-out
Property and Equipment		Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the lesser of their useful life or the remaining life of the lease. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation and amortization are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized. Maintenance and repairs are charged to operations as incurred.
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparing the carrying amount to the future net undiscounted cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. The Company has not identified any such impairment losses to date.
Convertible Preferred Stock Warrant Liability	Convertible Preferred Stock Warrant Liability The Company has accounted for its freestanding warrants to purchase shares of the Company’s convertible preferred stock as liabilities at fair value upon issuance primarily because the shares underlying the warrants contain contingent redemption features outside the control of the Company. The warrants are subject to re-measurement at additional paid-in capital,	Convertible Preferred Stock Warrant Liability The Company has accounted for its freestanding warrants to purchase shares of the Company’s convertible preferred stock as liabilities at fair value upon issuance primarily because the shares underlying the warrants contain contingent redemption features outside the control of the Company. The warrants are subject to re-measurement paid-in
Revenue	Revenue The Company records product revenue primarily from the sale of its IVL catheters. The Company sells its products to hospitals, primarily through direct sales representatives, as well as through distributors in selected international markets. Additionally, a significant portion of the Company’s revenue is generated through a consignment model under which inventory is maintained at hospitals. Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. For products sold through direct sales representatives, control is transferred upon delivery to customers. For products sold to distributors internationally and certain customers that purchase stocking orders in the United States, control is transferred upon shipment or delivery to the customer’s named location, based on the contractual shipping terms. For consignment inventory, control is transferred at the time the IVL catheters are consumed in a procedure. The Company generally provides for the use of an IVL generator and connector cable under an agreement to customers at no charge to facilitate use of the IVL catheters. These agreements do not contain contractually enforceable minimum commitments and are generally cancellable by either party with 30 days’ notice.	Revenue The Company adopted Accounting Standards Codification (“ASC”) 606, Revenue from Contracts with Customers Revenue Recognition The Company records product revenue primarily from the sale of its IVL catheters. The Company sells its products to hospitals, primarily through direct sales representatives, as well as through distributors in selected international markets. Additionally, a significant portion of the Company’s revenue is generated through a consignment model under which inventory is maintained at hospitals. Under ASC 605, revenue is recognized when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the price to the customer is fixed or determinable, and collectability is reasonably assured. For products sold through direct sales representatives, revenue is recognized upon delivery to customers. For products sold to distributors, revenue is recognized upon transfer of title and risk of loss to the distributor. For consignment inventory, revenue is recognized at the time the catheters are consumed in a procedure. Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. For products sold through direct sales representatives, control is transferred upon delivery to customers. For products sold to distributors internationally and certain customers that purchase stocking orders in the United States, control is transferred upon shipment or delivery to the customer’s named location, based on the contractual shipping terms. For consignment inventory, control is transferred at the time the catheters are consumed in a procedure. The Company generally provides for the use of an IVL generator and connector cable under an agreement to customers at no charge to facilitate use of the IVL catheters. These agreements do not contain contractually enforceable minimum commitments and are generally cancellable by either party with 30 days’ notice.
Research and Development Costs		Research and Development Costs Research and development costs, including new product development, regulatory compliance, and clinical research are expensed as incurred.
Accrued Research and Development Costs		Accrued Research and Development Costs The Company accrues liabilities for estimated costs of research and development activities conducted by its third-party service providers, which include the conduct of preclinical and clinical studies. The estimated costs of research and development activities are recorded based upon the estimated amount of services provided but not yet invoiced, and these costs are included in accrued liabilities on the consolidated balance sheets and within research and development expense on the consolidated statements of operations and comprehensive loss. These costs are accrued for based on factors such as estimates of the work completed and budget provided and in accordance with agreements established with third-party service providers. Significant judgments and estimates are made in determining the accrued liabilities balance in each reporting period. Accrued liabilities are adjusted as actual costs become known. There have not been any material differences between accrued costs and actual costs incurred since the Company’s inception.
Stock-Based Compensation		Stock-Based Compensation The Company accounts for share-based payments at fair value. The fair value of stock options is measured using the Black-Scholes option-pricing model. For share-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis, over the vesting period. The Company accounts for forfeitures as they occur.
Deferred Offering Costs		Deferred Offering Costs Offering costs, consisting of legal, accounting, printer and filing fees related to the IPO, are deferred and will be offset against proceeds from the IPO upon the effectiveness of the offering. In the event the offering is terminated, all deferred offering costs will be expensed. As of December 31, 2018, $1.5 million of deferred offering costs were recorded as other assets on the consolidated balance sheet. There were no deferred offering costs recorded as of December 31, 2017.
Defined Contribution Plan		Defined Contribution Plan The Company has a defined contribution retirement savings plan under Section 401(k) of the Internal Revenue Code. This plan allows eligible employees to defer a portion of their annual compensation on a pre-tax
Comprehensive Loss		Comprehensive Loss Comprehensive loss is comprised of net loss and changes in unrealized gains and losses on the Company’s available-for-sale
Foreign Currency		Foreign Currency The functional currency of the Company’s foreign subsidiary is the U.S. Dollar. Accordingly, all monetary assets and liabilities of the subsidiary are remeasured at the current exchange rate at the end of the period, nonmonetary assets and liabilities are remeasured at historical rates, and revenue and expenses are remeasured at average exchange rates during the period. There were net foreign currency transaction gains of $36,000 for the year ended December 31, 2017 and net foreign currency transaction losses of $46,000 for the year ended December 31, 2018.
Net Loss Per Share		Net Loss Per Share Basic net loss per share is calculated by dividing net loss by the weighted-average number of shares of common stock outstanding for the period, without consideration of potential dilutive shares of common stock. The unvested portion of early exercised stock options are excluded from the computation of weighted-average shares as the continuing vesting of such shares is contingent on the holders’ continued service to the Company. Since the Company was in a loss position for the period presented, basic net loss per share is the same as diluted net loss per share since the effects of potentially dilutive securities are antidilutive.
Income Taxes		Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net deferred tax assets have been fully offset by a valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of provision for income taxes.
Segment Reporting		Segment Reporting Operating segments are defined as components of an entity for which separate financial information is available and that is regularly reviewed by the Chief Operating Decision Maker (“CODM”) in deciding how to allocate resources to an individual segment and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company has determined it operates in one segment.
Recently Issued and Adopted Accounting Pronouncements		Recently Issued and Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting ASU 2018-07 In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash
Recent Accounting Pronouncements		Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases right-of-use No. 2018-11, Leases (Topic 842): Targeted Improvements non-lease a right-of-use asset
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and applicable rules and regulations of SEC regarding interim financial reporting. The interim condensed consolidated financial statements are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows. The results of operations for the nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements included in the prospectus dated March 6, 2019 (“Prospectus”) that forms a part of the Company’s Registration Statements on Form S-1 No. 333-229590),
Leases	Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842) Upon adoption of Topic 842, on January 1, 2019, the Company recorded operating right-of-use For its long-term operating lease, the Company recognized a right-of-use right-of-use Rent expense for the operating lease is recognized on a straight-line basis over the lease term and is included in operating expenses on the condensed consolidated statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The Company elected to exclude from its balance sheets recognition of leases having a term of 12 months or less (short-term leases) and elected to not separate lease components and non-lease
Deferred Offering Costs	Deferred Offering Costs Offering costs, consisting of legal, accounting, printer and filing fees related to the IPO, were deferred until the completion of the IPO. As of December 31, 2018, $1.5 million of deferred offering costs were recorded as other assets on the condensed consolidated balance sheet. In March 2019, upon the closing of the IPO, the deferred costs were offset against the Company’s IPO proceeds.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows: December 31, 2018 September 30, 2019 (in thousands) Cash and cash equivalents $ 39,643 $ 39,452 Restricted cash 450 450 Total cash, cash equivalents, and restricted cash $ 40,093 $ 39,902	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows: December 31, 2017 2018 (in thousands) Cash and cash equivalents $ 51,923 $ 39,643 Restricted cash — 450 Total cash, cash equivalents, and restricted cash $ 51,923 $ 40,093

Financial Instruments and Fai_2

Financial Instruments and Fair Value Measurements (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value	The following tables summarize the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 21,680 $ — $ — $ 21,680 Total assets $ 21,680 $ — $ — $ 21,680 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 313 $ 313 Total liabilities $ — $ — $ 313 $ 313 September 30, 2019 Level 1 Level 2 Level 3 Total (in thousands) Assets: U.S. Treasury securities $ 58,006 $ — $ — $ 58,006 Money market funds 19,987 — — 19,987 Reverse repurchase agreements — 10,601 — 10,601 Commercial paper — 8,562 — 8,562 Corporate bonds — 8,078 — 8,078 Total assets $ 77,993 $ 27,241 $ — $ 105,234	The following tables summarize the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy: December 31, 2017 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 26,379 $ — $ — $ 26,379 U.S. treasury note 1,806 — — 1,806 Total assets $ 28,185 $ — $ — $ 28,185 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 577 $ 577 Total liabilities $ — $ — $ 577 $ 577 December 31, 2018 Level 1 Level 2 Level 3 Total (in thousands) Assets: Money market funds $ 21,680 $ — $ — $ 21,680 Total assets $ 21,680 $ — $ — $ 21,680 Liabilities: Convertible preferred stock warrant liability $ — $ — $ 313 $ 313 Total liabilities $ — $ — $ 313 $ 313
Schedule of Change in the Fair Value of the Warrant Liability	There were no transfers between Levels 1, 2 or 3 for the periods presented. The change in the fair value of the warrant liability for the nine months ended September 30, 2018 is summarized below (in thousands): Balance at December 31, 2017 $ 577 Exercise of warrants (316 ) Expiration of warrants, included in change in fair value of warrant liability (133 ) Change in fair value of warrant liability 186 Balance at September 30, 2018 $ 314 The change in the fair value of the warrant liability for the nine months ended September 30, 2019 is summarized below (in thousands): Balance at December 31, 2018 $ 313 Change in fair value of warrant liability 609 Net exercise of warrants (133 ) Conversion of Series A preferred stock warrants to common stock warrants upon the closing of the IPO (789 ) Balance at September 30, 2019 $ —	The change in the fair value of the warrant liability is summarized below: Year Ended 2017 2018 (in thousands) Beginning balance $ 729 $ 577 Exercise of warrants (184 ) (316 ) Expiration of warrants, included in change in fair value of warrant liability — (133 ) Change in fair value of warrant liability 32 185 Ending balance $ 577 $ 313
Schedule of Fair Value of Warrants Determined Using the Black-Scholes Option Pricing Model	The fair value of the warrants, which were converted to common stock warrants upon the closing of the IPO in March 2019, was determined using the Black-Scholes option pricing model and the following assumptions: Nine Months Ended September 30, 2018 2019 Expected term (in years) 5.71-6.21 5.28 Expected volatility 42.8% 43.9% Risk-free interest rate 2.62-2.85% 2.49% Expected dividend yield 0% 0%

Cash Equivalents and Short-Te_2

Cash Equivalents and Short-Term Investments (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Summary of Cash Equivalents and Short-Term Investments	The following is a summary of the Company’s cash equivalents and short-term investments: December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) Money market funds $ 21,680 $ — $ — $ 21,680 Total $ 21,680 $ — $ — $ 21,680 Reported as: Cash equivalents $ 21,680 Total $ 21,680 September 30, 2019 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value (in thousands) U.S. Treasury securities $ 57,959 $ 47 $ — $ 58,006 Money market funds 19,987 — — 19,987 Reverse repurchase agreements 10,601 — — 10,601 Commercial paper 8,562 — — 8,562 Corporate bonds 8,068 10 — 8,078 Total $ 105,177 $ 57 $ — $ 105,234 Reported as: Cash equivalents $ 30,588 Short-term investments 74,646 Total $ 105,234	The following is a summary of the Company’s cash equivalents and short-term investments: December 31, 2017 Amortized Unrealized Unrealized Fair Value (in thousands) Money market funds $ 26,379 $ — $ — $ 26,379 U.S. treasury note 1,807 — (1 ) 1,806 Total $ 28,186 $ — $ (1 ) $ 28,185 Reported as: Cash equivalents $ 26,379 Short-term investments 1,806 Total $ 28,185 December 31, 2018 Amortized Unrealized Unrealized Fair Value (in thousands) Money market funds $ 21,680 $ — $ — $ 21,680 Total $ 21,680 $ — $ — $ 21,680 Reported as: Cash equivalents $ 21,680 Total $ 21,680

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Inventory	Inventory consists of the following: December 31, September 30, (in thousands) Raw material $ 1,084 $ 2,169 Work in progress 634 850 Finished goods 2,313 4,451 Consigned inventory 1,100 1,757 Total inventory $ 5,131 $ 9,227	Inventory consists of the following: December 31, 2017 2018 (in thousands) Raw material $ 478 $ 1,084 Work in progress 198 634 Finished goods 1,041 2,313 Consigned inventory 806 1,100 Total inventory $ 2,523 $ 5,131
Schedule of Property and Equipment		Property and equipment, net consists of the following: December 31, 2017 2018 (in thousands) Equipment $ 1,298 $ 2,321 Equipment on loan to customers 263 786 Office furniture 68 90 Software 76 76 Leasehold improvements 366 764 Construction in progress 258 236 Property and equipment, gross 2,329 4,273 Less accumulated depreciation and amortization (957 ) (1,654 ) Total property and equipment, net $ 1,372 $ 2,619
Schedule of Other Assets		Other assets consist of the following: December 31, 2017 2018 (in thousands) Deferred offering costs $ — $ 1,519 Restricted cash — 450 Other 73 97 Total other assets $ 73 $ 2,066
Schedule of Accrued Liabilities	Accrued liabilities consist of the following: December 31, September 30, (in thousands) Accrued employee compensation $ 3,135 $ 5,984 Accrued research and development costs 1,115 1,844 Accrued professional services 1,391 856 Other 576 812 Total accrued liabilities $ 6,217 $ 9,496	Accrued liabilities consist of the following: December 31, 2017 2018 (in thousands) Accrued employee compensation $ 2,072 $ 3,135 Accrued research and development costs 507 1,115 Accrued professional services 348 1,391 Other 466 576 Total accrued liabilities $ 3,393 $ 6,217

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Future Rental Payments		The following are minimum future rental payments owed under these agreements: Year ending December 31: (in thousands) 2019 $ 940 2020 830 2021 855 2022 582 Total $ 3,207
Schedule of Carrying Value of Operating Lease Liabilities	The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of September 30, 2019: (in thousands) Remainder of 2019 $ 222 2020 861 2021 855 2022 581 Total minimum lease payments $ 2,519 Less: imputed interest (234 ) Total lease liability $ 2,285

Term Notes (Tables)

Term Notes (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of long term debt	Long-term debt and net premium balances are as follows: December 31, (in thousands) Face value of term note $ 15,000 Net premium associated with accretion of final payment, issuance of common stock warrants, and other debt issuance costs 50 Term note, current and noncurrent 15,050 Less term note, current portion (1,667 ) Term note, noncurrent portion $ 13,383
Schedule of future minimum payments for principal and interest	Future minimum payments of principal and estimated payments of interest on the Company’s outstanding variable rate borrowings as of December 31, 2018 are as follows: Year ending December 31: (in thousands) 2019 $ 2,243 2020 7,079 2021 7,816 Total future payments 17,138 Less amounts representing interest (1,125 ) Less final payment (1,013 ) Total principal amount of term note payments $ 15,000

Convertible Preferred Stock a_2

Convertible Preferred Stock and Stockholders' Equity (Deficit) (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Federal Home Loan Banks [Abstract]
Summary of Key Terms of Outstanding Common Stock Warrants	The key terms of the outstanding common stock warrants are summarized in the following table: Warrants Outstanding December 31, 2018 Warrants Outstanding September 30, 2019 Exercise Price Expiration Related party common stock warrants 141,778 — $ 2.196 May 2025 Common stock warrants issued in connection with the 2018 Loan and Security Agreement 34,440 — $ 4.026 February 2028 Total common stock warrants 176,218 —	Convertible preferred stock consists of the following: December 31, 2017 Shares Shares Net Carrying Aggregate (in thousands, except share amounts) Series A 19,280,722 1,580,387 $ 4,226 $ 4,473 Series A-1 52,589,632 4,127,463 13,637 12,780 Series B 65,000,000 5,309,617 39,877 40,000 Series C 79,209,457 6,492,578 79,729 80,000 216,079,811 17,510,045 $ 137,469 $ 137,253 December 31, 2018 Shares Shares Net Carrying Aggregate (in thousands, except share amounts) Series A 19,280,722 1,580,387 $ 4,226 $ 4,473 Series A-1 51,874,893 4,197,138 14,054 12,996 Series B 64,777,331 5,309,617 39,877 40,000 Series C 79,209,457 6,492,578 79,729 80,000 Series D 13,956,584 1,090,608 14,920 15,000 229,098,987 18,670,328 $ 152,806 $ 152,469

Warrant Liability (Tables)

Warrant Liability (Tables)	12 Months Ended
	Dec. 31, 2018
Text Block [Abstract]
Summary of Outstanding Convertible Preferred Stock Warrants	The key terms of the outstanding convertible preferred stock warrants are summarized in the following table: Warrants Outstanding Warrants Warrants Exercise Expiration Series A-1 128,259 — $ 3.09636 Various dates in 2018 Series A-1 54,903 54,903 $ 3.09636 June 2024 Total convertible preferred stock warrants 183,162 54,903
Fair Value Warrants Using Black-Scholes Option Pricing Model	The fair value of the warrants was determined using the Black-Scholes option pricing model and the following assumptions: December 31, 2017 2018 Expected term (in years) 0-6.5 5.5 Expected volatility 19.8%-42.3% 42.8% Risk-free interest rate 1.1%-1.8% 2.9% Expected dividend yield 0% 0%

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Schedule of Total Stock-Based Compensation	Total stock-based compensation was as follows: Nine Months September 30, 2018 2019 (in thousands) Cost of product revenue $ 38 $ 156 Research and development 146 582 Sales and marketing 185 597 General and administrative 505 1,001 Total stock-based compensation $ 874 $ 2,336	Total stock-based compensation was as follows: Year Ended 2017 2018 (in thousands) Cost of product revenue $ 46 $ 67 Research and development 185 235 Sales and marketing 130 294 General and administrative 604 701 Total stock-based compensation $ 965 $ 1,297
Schedule of Estimates Grant-Date Fair Value of Option Awards Using Black-Scholes Option Pricing Model with Assumptions	The Company estimates the grant-date fair value of the Company’s option awards using the Black-Scholes option pricing model. The assumptions for the Black-Scholes model for the nine months ended September 30, 2018 and 2019 were as follows: Nine Months Ended September 30, 2018 2019 Expected term (in years) 6.08 6.08 Expected volatility 40.8%-45.9% 42.4%-42.9% Risk-free interest rate 1.9%-2.9% 2.4%-2.6% Expected dividend yield 0 0%	The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: December 31, 2017 2018 Expected term (in years) 6.08 6.08 Expected volatility 45.6% 40.8%-41.9% Risk-free interest rate 1.9%-2.2% 2.5%-3.1% Expected dividend yield 0% 0%
Schedule of Activity under 2009 Plan and 2019 Plan	Option activity under the 2009 Plan and 2019 Plan is set forth below: Number of Weighted- Average Exercise Price Per Share Weighted- Average Remaining Term Aggregate Intrinsic Value (in years) (in thousands) Balance, December 31, 2018 3,636,358 $ 3.54 7.79 $ 11,267 Options granted 442,858 14.69 Options exercised (386,309 ) 2.65 Options forfeited (34,471 ) 3.49 Balance, September 30, 2019 3,658,436 $ 4.95 7.52 $ 91,399 Vested and exercisable, September 30, 2019 1,831,368 $ 3.14 6.59 $ 49,068	Activity under the 2009 Plan is set forth below: Shares Number of Weighted- Weighted- Aggregate (in years) (in thousands) Balance, December 31, 2016 887,885 1,365,934 $ 1.95 $ 2,011 Awards authorized 1,352,677 — Options granted (1,974,589 ) 1,974,589 3.42 Options exercised — (71,522 ) 1.95 Options cancelled 160,397 (160,397 ) 2.44 Balance, December 31, 2017 426,370 3,108,604 $ 2.81 8.03 $ 3,647 Awards authorized 691,503 — Options granted (1,015,963 ) 1,015,963 5.25 Options exercised — (197,820 ) 2.20 Options cancelled 290,389 (290,389 ) 3.42 Balance, December 31, 2018 392,299 3,636,358 $ 3.54 7.79 $ 11,267 Vested and exercisable, December 31, 2018 1,585,273 $ 2.68 6.50 $ 6,249 Vested and expected to vest, December 31, 2018 3,636,358 $ 3.54 7.79 $ 11,267
Summary of Restricted Stock Shares Under Company Plan		A summary of the restricted stock shares issued under the Company’s Plan is as follows: Number of Weighted- Outstanding and unvested at December 31, 2016 94,610 $ 0.61 Vested 83,007 $ 0.61 Outstanding and unvested at December 31, 2017 11,603 $ 0.61 Exercised 45,287 $ 2.20 Vested (43,468 ) $ 1.83 Outstanding and unvested at December 31, 2018 13,422 $ 2.20
Summary of Key Terms of Outstanding Common Stock Warrants		The key terms of the outstanding common stock warrants are summarized in the following table: Warrants Outstanding Warrants Warrants Exercise Expiration Related party common stock warrants 141,778 141,778 $ 2.196 May 2025 Common stock warrants issued in connection with the 2018 Loan and Security Agreement — 34,440 $ 4.026 February 2028 Total common stock warrants 141,778 176,218
Schedule of RSU Activity under 2019 Plan	RSU activity under the 2019 Plan is set forth below: Number of Shares Weighted- Average Grant Date Fair Value Per Share Balance, December 31, 2018 — $ — RSUs granted 229,450 38.26 RSUs forfeited (2,900 ) 37.51 Balance, September 30, 2019 226,550 $ 38.27

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Current Income Tax Provision	Current income tax provision consists of the following: December 31, 2017 2018 (in thousands) Domestic $ 1 $ 3 Foreign 25 35 Total current income tax provision $ 26 $ 38
Components of Deferred Tax Assets	The components of the deferred tax assets are as follows: December 31, 2017 2018 (in thousands) Deferred tax assets: Net operating loss carryovers $ 19,251 $ 28,834 Fixed and intangible assets 664 837 Accruals and reserves 465 761 Stock-based compensation 31 132 Research and development credits 1,858 1,716 Contributions 13 14 Total deferred tax assets 22,282 32,294 Less valuation allowance (22,282 ) (32,294 ) Total net deferred tax assets $ — $ —
Reconciliation of Statutory Federal Income Tax	Reconciliation of the statutory federal income tax to the Company’s effective tax is as follows: December 31, 2017 2018 (in thousands) Income tax benefit at federal statutory rate $ (10,404 ) $ (8,624 ) State and local income taxes net of federal tax benefit 1 3 Foreign tax rate differential (3 ) 11 Change in valuation allowance (522 ) 8,497 Stock-based compensation 309 123 R&D tax credits (222 ) (313 ) Other 109 341 Federal rate change (pursuant to the Tax Cuts and Jobs Act of 2017) 10,758 — Total current income tax provision $ 26 $ 38
Schedule of Unrecognized Tax Benefits	The activity related to the gross amount of unrecognized tax benefits is as follows: December 31, 2017 2018 (in thousands) Beginning balance $ 688 $ 893 Additions based on tax positions related to prior years — 394 Additions based on tax positions related to current years 205 609 Balance at end of year $ 893 $ 1,896

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: September 30, 2018 2019 (in thousands) Convertible preferred stock on an as-converted 17,579,720 — Common stock options issued and outstanding 3,313,257 3,658,436 Restricted stock units — 226,550 Early exercised options subject to future vesting 21,620 — Convertible preferred stock warrants 54,903 — Common stock warrants 176,218 — Total 21,145,718 3,884,986	The following outstanding potentially dilutive common stock equivalents have been excluded from the calculation of diluted net loss per share for the period presented due to their anti-dilutive effect: Year Ended 2017 2018 (in thousands) Convertible preferred stock 17,510,045 18,670,328 Common stock options issued and outstanding 3,108,604 3,636,358 Early exercised options subject to future vesting 11,603 13,422 Convertible preferred stock warrants 183,162 54,903 Common stock warrants 141,778 176,218 Total 20,955,192 22,551,229

Segment and Geographic Inform_2

Segment and Geographic Information (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Schedule of Product Revenue Based on Location	The following table represents the Company’s product revenue based on the location to which the product is shipped: Nine Months Ended September 30, 2018 2019 (in thousands) United States $ 4,476 $ 15,054 Germany 958 2,389 Rest of Europe 1,597 9,473 All other countries 170 1,699 Product revenue $ 7,201 $ 28,615	The following table represents the Company’s product revenue based on the location to which the product is shipped: December 31, 2017 2018 (in thousands) United States $ 969 $ 7,022 Germany 597 1,393 Rest of Europe 143 3,516 All other countries 10 332 Product revenue $ 1,719 $ 12,263

Organization and Basis of Pre_3

Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Mar. 11, 2019USD ($)$ / sharesshares	Apr. 30, 2019shares	Sep. 30, 2019USD ($)shares	Dec. 31, 2018USD ($)shares	Mar. 05, 2019shares	Dec. 31, 2017USD ($)
Organization And Basis Of Presentation [Line Items]
Accumulated deficit | $			$ (163,229)	$ (126,865)		$ (85,763)
Cash, cash equivalents and short-term investment | $			$ 114,100	$ 39,600		$ 28,185
Entity incorporation, date of incorporation			Jun. 17, 2009
Net proceeds from IPO, after deducting underwriters' discounts and commissions | $	$ 7,100
Common stock, conversion basis			One-to-one basis
Stock split, conversion ratio			12.2
Stockholders' equity, reverse stock split			The Company's common stock and convertible preferred stock on a 12.2-for-one basis (the "Reverse Stock Split")
Common Stock
Organization And Basis Of Presentation [Line Items]
Common stock issued			6,555,000
Proceeds from initial public offering | $	$ 99,900
Issuance of common stock upon net exercise of warrants, shares	101,744	29,887	180,952
Fractional shares issued in reverse stock split			45	45
Warrant [Member]
Organization And Basis Of Presentation [Line Items]
Convertible preferred stock, shares issued upon conversion	54,903
Preferred Stock
Organization And Basis Of Presentation [Line Items]
Fractional shares issued in reverse stock split				69
Initial Public Offering
Organization And Basis Of Presentation [Line Items]
Common stock issued	6,555,000
Common stock, offering price per share | $ / shares	$ 17
Offering costs | $	$ 4,400
Stock split, conversion ratio	1
Initial Public Offering | Common Stock
Organization And Basis Of Presentation [Line Items]
Convertible preferred stock, shares issued upon conversion					18,670,259
Over-Allotment Option [Member]
Organization And Basis Of Presentation [Line Items]
Common stock issued	855,000
Offering costs | $				$ 1,500
Private Placement [Member]
Organization And Basis Of Presentation [Line Items]
Common stock issued	588,235
Proceeds from issuance of common stock | $	$ 10,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 39,452	$ 39,643		$ 51,923
Restricted cash	450	450
Total cash, cash equivalents, and restricted cash	$ 39,902	$ 40,093	$ 31,438	$ 51,923	$ 50,815

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	Jan. 01, 2019USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Allowance for doubtful accounts			$ 76,000	$ 0
Write-off of accounts receivable			1,000	0
Deferred offering costs			1,519,000
Foreign currency transaction realized and unrealized gain (loss)			$ (46,000)	36,000
Number of Operating segments			1
Operating lease right-of-use assets		$ 2,155,000
Operating lease liabilities		$ 2,285,000
Notice period for cancellation of agreement		30 days
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life			5 years
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life			3 years
Adoption of Topic 842
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets	$ 2,900,000
Operating lease liabilities	3,000,000
Derecognized the deferred rent liability	$ 100,000
Other Assets
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs			$ 1,500,000	$ 0
Customer [Member] | Revenue Benchmark [Member] | Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage				19.00%
Customer [Member] | Revenue Benchmark [Member] | Customer One [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage			10.00%
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Customer One [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage			10.00%	10.00%

Financial Instruments and Fai_3

Financial Instruments and Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Liabilities:
Convertible preferred stock warrant liability		$ 313	$ 577
Assets:
Money market funds	$ 105,234	21,680	28,185
Money Market Funds
Assets:
Money market funds	19,987	21,680	26,379
U.S. Treasury securities
Assets:
Money market funds	58,006		1,806
Reverse Repurchase Agreements
Assets:
Money market funds	10,601
Commercial paper
Assets:
Money market funds	8,562
Corporate bonds
Assets:
Money market funds	8,078
Convertible Preferred Stock Warrants Liability
Liabilities:
Convertible preferred stock warrant liability		313	577
Fair Value, Inputs, Level 1 [Member]
Assets:
Money market funds	77,993	21,680	28,185
Fair Value, Inputs, Level 1 [Member] | Money Market Funds
Assets:
Money market funds	19,987	21,680	26,379
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury securities
Assets:
Money market funds	58,006		1,806
Fair Value, Inputs, Level 2 [Member]
Assets:
Money market funds	27,241
Fair Value, Inputs, Level 2 [Member] | Reverse Repurchase Agreements
Assets:
Money market funds	10,601
Fair Value, Inputs, Level 2 [Member] | Commercial paper
Assets:
Money market funds	8,562
Fair Value, Inputs, Level 2 [Member] | Corporate bonds
Assets:
Money market funds	$ 8,078
Fair Value, Inputs, Level 3 [Member]
Liabilities:
Convertible preferred stock warrant liability		313	577
Fair Value, Inputs, Level 3 [Member] | Convertible Preferred Stock Warrants Liability
Liabilities:
Convertible preferred stock warrant liability		$ 313	$ 577

Financial Instruments and Fai_4

Financial Instruments and Fair Value Measurements - Schedule of Change in the Fair Value of the Warrant Liability (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Beginning balance	$ 313	$ 577	$ 577	$ 729
Exercise of warrants	(133)	(316)	(316)	(184)
Expiration of warrants, included in change in fair value of warrant liability		(133)	(133)
Change in fair value of warrant liability	609	186	185	32
Conversion of Series A preferred stock warrants to common stock warrants upon the closing of the IPO	$ (789)
Ending balance		$ 314	$ 313	$ 577

Cash Equivalents and Short-Te_3

Cash Equivalents and Short-Term Investments - Summary of Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Line Items]
Cash equivalents Amortized Cost Basis		$ 21,680
Cash equivalents and short-term investments, Unrealized Gains	$ 57
Cash equivalents and short-term investments, Fair Value	105,234
Cash equivalents and short-term investments, Cost Basis	105,177		$ 28,186
Cash equivalents and short-term investments, Unrealized Losses			(1)
Cash equivalents and short-term investments, Cost Basis	114,100	39,600	28,185
Cash equivalents, Fair Value	30,588	21,680	26,379
Short-term investments	74,646		1,806
U.S. Treasury securities
Cash And Cash Equivalents [Line Items]
Short-term investments Amortized Cost Basis	57,959		1,807
Short-term investments Unrealized Losses			(1)
Short-term investments Unrealized Gains	47
Short-term investments, Fair Value	58,006		1,806
Reverse Repurchase Agreements
Cash And Cash Equivalents [Line Items]
Short-term investments Amortized Cost Basis	10,601
Short-term investments, Fair Value	10,601
Commercial paper
Cash And Cash Equivalents [Line Items]
Short-term investments Amortized Cost Basis	8,562
Short-term investments, Fair Value	8,562
Corporate bonds
Cash And Cash Equivalents [Line Items]
Short-term investments Amortized Cost Basis	8,068
Short-term investments Unrealized Gains	10
Short-term investments, Fair Value	8,078
Money Market Funds
Cash And Cash Equivalents [Line Items]
Cash equivalents Amortized Cost Basis	19,987	21,680	26,379
Cash equivalents, Fair Value	$ 19,987	$ 21,680	$ 26,379

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Inventory (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw material	$ 2,169	$ 1,084	$ 478
Work in progress	850	634	198
Finished goods	4,451	2,313	1,041
Consigned inventory	1,757	1,100	806
Total inventory	$ 9,227	$ 5,131	$ 2,523

Balance Sheet Components - Sc_2

Balance Sheet Components - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		$ 4,273	$ 2,329
Less accumulated depreciation and amortization		(1,654)	(957)
Total property and equipment, net	$ 4,111	2,619	1,372
Equipment
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		2,321	1,298
Equipment Loan To Customer
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		786	263
Office Furniture
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		90	68
Software
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		76	76
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		764	366
Construction in Progress
Property, Plant and Equipment [Line Items]
Equipment on loan to customers		$ 236	$ 258

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization expense	$ 854	$ 466	$ 700	$ 468

Balance Sheet Components - Sc_3

Balance Sheet Components - Schedule of Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Assets [Abstract]
Deferred offering costs		$ 1,519
Restricted cash	$ 450	450
Other		97	$ 73
Total other assets		$ 2,066	$ 73

Balance Sheet Components - Sc_4

Balance Sheet Components - Schedule of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued employee compensation	$ 5,984	$ 3,135	$ 2,072
Accrued research and development costs	1,844	1,115	507
Accrued professional services	856	1,391	348
Other	812	576	466
Total accrued liabilities	$ 9,496	$ 6,217	$ 3,393

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			9 Months Ended		12 Months Ended
	Feb. 28, 2019	Oct. 31, 2018	May 31, 2018ft²	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Area of office and laboratory space | ft²			35,000
Letter of credit secured by restricted cash						$ 500,000
Rent expense				$ 900,000	$ 500,000	$ 800,000	$ 400,000
Operating lease right-of-use asset				2,155,000
Operating lease liability				$ 2,285,000
Fremont, California
Loss Contingencies [Line Items]
Lease term expiration date	Sep. 30, 2019	Jun. 30, 2019		Sep. 30, 2019
Santa Clara, California
Loss Contingencies [Line Items]
Area of office and laboratory space | ft²			35,000
Letter of credit secured by restricted cash				$ 500,000
Lease term expiration date			Aug. 31, 2022
Lease commencement date			2018-09
Operating lease right-of-use asset				2,200,000
Operating lease liability				$ 2,200,000
Operating lease, remaining lease term				2 years 10 months
Germany
Loss Contingencies [Line Items]
Operating lease right-of-use asset				$ 47,000
Operating lease liability				$ 47,000
Operating lease, weighted average remaining lease term				8 months
Operating lease liability, weighted average incremental borrowing rate				6.93%

Commitments and Contingencies_2

Commitments and Contingencies - Minimum Future Rental payments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 940
2020	830
2021	855
2022	582
Total	$ 3,207

Term Notes - Additional Informa

Term Notes - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2018	Feb. 28, 2018	Jun. 30, 2014
Debt Instrument [Line Items]
Debt Amount			$ 15,000
Debt, final payment amount			1,013
Interest expense	$ 746	$ 216	401	$ 58
2014 Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Amortization of debt discount				100
2014 Loan and Security Agreement [Member] | Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Debt Amount							$ 4,000
Debt repaid amount				$ 200
2018 Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Amortization of debt discount			$ 200
Debt maturity date			Dec. 31, 2021
Debt interest rate			3.75%
Debt, final payment as percentage of original aggregate principal amount			6.75%
Debt, final payment amount			$ 1,000
Debt, warrants issued			34,440
Debt, fair value of warrants issued			$ 100
Interest expense			$ 200
2018 Loan and Security Agreement [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt interest rate			1.75%
2018 Loan and Security Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt interest rate basis			2.75%
2018 Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Amount						$ 2,000
2018 Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Short Term Notes [Member]
Debt Instrument [Line Items]
Debt Amount						$ 15,000
2018 Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Short Term Notes [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Debt Amount					$ 10,000
2018 Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Short Term Notes [Member] | Tranche Two [Member]
Debt Instrument [Line Items]
Debt Amount			$ 5,000

Term Note - Long-term Debt and

Term Note - Long-term Debt and Net Premium Balance (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Debt Instrument, Periodic Payment [Abstract]
Face value of term note	$ 15,000
Net premium associated with accretion of final payment, issuance of common stock warrants, and other debt issuance costs	50
Term note, current and noncurrent	15,050
Less term note, current portion	(1,667)
Term note, noncurrent portion	$ 13,383

Term Note - Schedule of Future

Term Note - Schedule of Future Minimum and Estimated Payments (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Debt Disclosure [Abstract]
2019	$ 2,243
2020	7,079
2021	7,816
Total future payments	17,138
Less amounts representing interest	(1,125)
Less final payment	(1,013)
Total principal amount of term note payments	$ 15,000

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	Mar. 11, 2019	Dec. 31, 2018	Sep. 30, 2017	Dec. 31, 2018
Preferred Stock | Minimum [Member]
Temporary Equity And Stockholders Equity [Line Items]
Convertible preferred stock issued, price per share		$ 15.067		$ 15.067
Aggregate gross proceeds from issuance of common stock				$ 30,000,000
Private Placement [Member]
Temporary Equity And Stockholders Equity [Line Items]
Share of Convertible preferred stock issued	588,235
Private Placement [Member] | Maximum [Member]
Temporary Equity And Stockholders Equity [Line Items]
Common stock available for purchase for Series D investor		10,000,000		10,000,000
Series C Convertible Preferred Stock [Member]
Temporary Equity And Stockholders Equity [Line Items]
Net proceed from issuance of convertible preferred stock			$ 34,900,000
Convertible preferred stock issued, price per share		$ 12.32176	$ 12.32176	$ 12.32176
Share of Convertible preferred stock issued			2,840,504
Convertible preferred stock issued, conversion price per share				12.32176
Preferred stock, liquidation preference per share		12.32176		12.32176
Dividends payable, amount per share		$ 0.98576		0.98576
Series D Convertible Preferred Stock [Member]
Temporary Equity And Stockholders Equity [Line Items]
Net proceed from issuance of convertible preferred stock		$ 15,000,000
Convertible preferred stock issued, price per share		$ 13.75379		13.75379
Share of Convertible preferred stock issued		1,090,608
Convertible preferred stock issued, conversion price per share				13.75379
Preferred stock, liquidation preference per share		$ 13.75379		13.75379
Dividends payable, amount per share		1.10044		1.10044
Series A- Convertible Preferred Stock [Member]
Temporary Equity And Stockholders Equity [Line Items]
Convertible preferred stock issued, price per share		2.8304		2.8304
Convertible preferred stock issued, conversion price per share				2.8304
Preferred stock, liquidation preference per share		2.8304		2.8304
Dividends payable, amount per share		$ 0.22643		0.22643
Series A- 1 Convertible Preferred Stock [Member]
Temporary Equity And Stockholders Equity [Line Items]
Net proceed from issuance of convertible preferred stock		$ 100,000
Share of Convertible preferred stock issued		69,675
Convertible preferred stock issued, fair value per share		$ 5.978
Convertible preferred stock issued, price per share		3.09636		3.09636
Convertible preferred stock issued, conversion price per share				3.09636
Preferred stock, liquidation preference per share		3.09636		3.09636
Dividends payable, amount per share		$ 0.24766		0.24766
Series A- 1 Convertible Preferred Stock [Member] | Payment In Cash [Member]
Temporary Equity And Stockholders Equity [Line Items]
Share of Convertible preferred stock issued		32,498
Series A- 1 Convertible Preferred Stock [Member] | Warrant [Member]
Temporary Equity And Stockholders Equity [Line Items]
Share of Convertible preferred stock issued		37,177
Share of Convertible preferred stock issued up on conversion of warrants		49,655
Series B Convertible Preferred Stock [Member]
Temporary Equity And Stockholders Equity [Line Items]
Convertible preferred stock issued, price per share		$ 7.5335		7.5335
Convertible preferred stock issued, conversion price per share				7.5335
Preferred stock, liquidation preference per share		7.5335		7.5335
Dividends payable, amount per share		$ 0.60268		$ 0.60268

Convertible Preferred Stock - S

Convertible Preferred Stock - Summary of Convertible Preferred Series Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	229,098,987	216,079,811
Convertible preferred stock,shares issued	18,670,328	17,510,045
Convertible preferred stock,shares outstanding	18,670,328	17,510,045
Convertible preferred stock, net carrying value	$ 152,806	$ 137,469
Convertible preferred stock, aggregate liquidation preference	$ 152,469	$ 137,253
Series A- Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	19,280,722	19,280,722
Convertible preferred stock,shares issued	1,580,387	1,580,387
Convertible preferred stock,shares outstanding	1,580,387	1,580,387
Convertible preferred stock, net carrying value	$ 4,226	$ 4,226
Convertible preferred stock, aggregate liquidation preference	$ 4,473	$ 4,473
Series A- 1 Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	51,874,893	52,589,632
Convertible preferred stock,shares issued	4,197,138	4,127,463
Convertible preferred stock,shares outstanding	4,197,138	4,127,463
Convertible preferred stock, net carrying value	$ 14,054	$ 13,637
Convertible preferred stock, aggregate liquidation preference	$ 12,996	$ 12,780
Series B Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	64,777,331	65,000,000
Convertible preferred stock,shares issued	5,309,617	5,309,617
Convertible preferred stock,shares outstanding	5,309,617	5,309,617
Convertible preferred stock, net carrying value	$ 39,877	$ 39,877
Convertible preferred stock, aggregate liquidation preference	$ 40,000	$ 40,000
Series C Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	79,209,457	79,209,457
Convertible preferred stock,shares issued	6,492,578	6,492,578
Convertible preferred stock,shares outstanding	6,492,578	6,492,578
Convertible preferred stock, net carrying value	$ 79,729	$ 79,729
Convertible preferred stock, aggregate liquidation preference	$ 80,000	$ 80,000
Series D Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	13,956,584
Convertible preferred stock,shares issued	1,090,608
Convertible preferred stock,shares outstanding	1,090,608
Convertible preferred stock, net carrying value	$ 14,920
Convertible preferred stock, aggregate liquidation preference	$ 15,000

Warrant Liability - Summary of

Warrant Liability - Summary of Outstanding Convertible Preferred Stock Warrants (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Series A One Preferred Stock Convertible Warrants One [Member]
Class of Warrant or Right [Line Items]
Common stock warrants outstanding		128,259
Warrants Outstanding Exercise Price	$ 3.09636
Warrants Outstanding Expiration	2018-12
Series A-1 convertible preferred stock warrants two [Member]
Class of Warrant or Right [Line Items]
Common stock warrants outstanding	54,903	54,903
Warrants Outstanding Exercise Price	$ 3.09636
Warrants Outstanding Expiration	2024-06
Convertible Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Common stock warrants outstanding	54,903	183,162

Warrant Liability - Additional

Warrant Liability - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Class of Warrant or Right [Line Items]
Exercise of Series A-1 warrants, shares	69,675
Warrants cancelled, as payment for net exercise of convertible preferred stock	12,478
Series A- 1 Convertible Preferred Stock [Member]
Class of Warrant or Right [Line Items]
Issuance of Series A-1 convertible preferred stock, per share | $ / shares	$ 5.978
Convertible preferred stock warrants expired due to unexercised	46,106

Warrants Liabilities - Fair Val

Warrants Liabilities - Fair Value Warrants Using Black-Scholes Option Pricing Model (Detail)	Dec. 31, 2018	Dec. 31, 2017
Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.423
Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.198
Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term (in years)	5 years 6 months
Measurement Input, Expected Term [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term (in years)		6 years 6 months
Measurement Input, Expected Term [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term (in years)		0 years
Measurement Input, Option Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield	0.428
Risk-free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield	0.0290
Risk-free Interest Rate | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.018
Risk-free Interest Rate | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.011
Measurement Input, Expected Dividend Payment [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield	0	0

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Total Stock-Based Compensation (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 2,336	$ 874	$ 1,297	$ 965
Cost of Product Revenue
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	156	38	67	46
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	582	146	235	185
Sales and Marketing
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	597	185	294	130
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,001	$ 505	$ 701	$ 604

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Estimates Grant-Date Fair Value of Option Awards Using Black-Scholes Option Pricing Model with Assumptions (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days	6 years 29 days
Expected volatility				45.60%
Expected volatility, minimum	42.40%	40.80%	40.80%
Expected volatility, maximum	42.90%	45.90%	41.90%
Risk-free interest rate, minimum	2.40%	1.90%	2.50%	1.90%
Risk-free interest rate, maximum	2.60%	2.90%	3.10%	2.20%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	1 Months Ended		9 Months Ended		12 Months Ended
	Mar. 01, 2019	Feb. 28, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of options granted					$ 2.56	$ 1.59
Grant date fair value of options vested					$ 1,600,000	$ 900,000
Aggregate intrinsic value of options exercised					500,000	100,000
Unvested stock options					$ 3,800,000
Unvested stock options, remaining weighted-average period					2 years 8 months 19 days
Unvested portion of the exercise proceeds					$ 29,000	$ 7,000
Shares available for issuance					392,299	426,370	887,885
Stock-based compensation			$ 2,336,000	$ 874,000	$ 1,297,000	$ 965,000
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation vesting rights, terms			The RSUs generally vest over a four-year period with straight-line vesting and a 25% one year cliff or over a three year period in equal amounts on a semi-annual basis,
Vesting period			4 years
Cliff vesting, percentage			25.00%
2009 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation vesting rights, terms					If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the Board. Options granted under the Plan have a term of up to 10 years and generally vest over a 4 year period with a straight-line vesting and a 25% one year cliff.
Options granted term					10 years
Vesting period					4 years
Cliff vesting, percentage					25.00%
Common stock reserved for issuance					5,057,744
2019 Stock Option and Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance		2,000,430
Maximum period of automatic annual increase in common stock reserved for issuance		10 years
Automatic annual increase in common stock reserved for issuance		3.00%
Shares available for issuance			1,470,192
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance			300,650
Purchase shares of common stock, price per share, percentage of fair market value	85.00%
Stock-based compensation			$ 64,000

Stock-Based Compensation - Sc_3

Stock-Based Compensation - Schedule of Activity under 2009 Plan and 2019 Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares Available for Grant
Shares Available for Grant, Beginning balance	392,299	426,370	887,885
Shares Available for Grant, Awards authorized		691,503	1,352,677
Shares Available for Grant, Options granted		(1,015,963)	(1,974,589)
Shares Available for Grant, Options cancelled		290,389	160,397
Shares Available for Grant, Ending balance		392,299	426,370
Number of Shares
Number of Shares, Beginning balance	3,636,358	3,108,604	1,365,934
Number of Shares, Options granted	442,858	1,015,963	1,974,589
Number of Shares, Options exercised	(386,309)	(197,820)	(71,522)
Number of Shares, Options cancelled	(34,471)	(290,389)	(160,397)
Number of Shares, Ending balance	3,658,436	3,636,358	3,108,604
Number of Shares, Vested and exercisable	1,831,368	1,585,273
Number of Shares, Vested and expected to vest		3,636,358
Weighted-Average Exercise Price Per Share
"Weighted-Average Exercise Price Per Share, Beginning balance, shares	$ 3.54	$ 2.81	$ 1.95
"Weighted-Average Exercise Price Per Share, Options granted	14.69	5.25	3.42
Weighted-Average Exercise Price Per Share, Options exercised	2.65	2.20	1.95
Weighted-Average Exercise Price Per Share, Options cancelled	3.49	3.42	2.44
Weighted-Average Exercise Price Per Share, Ending balance	4.95	3.54	$ 2.81
Weighted-Average Exercise Price Per Share, Vested and exercisable	$ 3.14	2.68
Weighted-Average Exercise Price Per Share, Vested and expected to vest		$ 3.54
Weighted-Average Remaining Term (in years)
Weighted-Average Remaining Term (in years), Balance	7 years 6 months 7 days	7 years 9 months 14 days	8 years 10 days
Weighted-Average Remaining Term (in years), vested and exercisable	6 years 7 months 2 days	6 years 6 months
Weighted-Average Remaining Term (in years), Vested and expected to vest		7 years 9 months 14 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Balance	$ 91,339	$ 11,267	$ 3,647	$ 2,011
Aggregate Intrinsic Value, Vested and exercisable	$ 49,068	6,249
Aggregate Intrinsic Value, Vested and expected to vest		$ 11,267

Stock-Based Compensation - Sc_4

Stock-Based Compensation - Schedule of RSU Activity (Detail) - Restricted Stock Units - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Number of Shares, Beginning balance	13,422	11,603	94,610
Number of Shares, Exercised		45,287
Number of Shares, Vested		43,468	83,007
Number of Shares, Ending balance		13,422	11,603
Weighted-Average Grant Date Fair Value Per Share
Weighted-Average Grant Date Fair Value Per Share, Beginning balance			$ 0.61
Exercised		$ 2.20
Vested		$ 1.83	$ 0.61
Weighted-Average Grant Date Fair Value Per Share, Ending balance		$ 2.20	$ 0.61
2019 Stock Option and Incentive Plan
Number of Shares
Number of Shares, RSUs granted	229,450
Number of Shares, RSUs forfeited	(2,900)
Number of Shares, Ending balance	226,550
Weighted-Average Grant Date Fair Value Per Share
Weighted-Average Grant Date Fair Value Per Share, RSUs granted	$ 38.26
Weighted-Average Grant Date Fair Value Per Share, RSUs forfeited	37.51
Weighted-Average Grant Date Fair Value Per Share, Ending balance	$ 38.27

Stock-Based Compensation - Sc_5

Stock-Based Compensation - Schedule of Common Stock Warrants (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Common Stock Warrants
Common stock warrants outstanding		141,778	141,778
Warrants Outstanding Exercise Price	$ 2.196	$ 2.196
Common stock warrants issued in connection with the 2018 Loan and Security Agreement	2025-05	2025-05
Common Stock Warrants Related to 2018 Loan and Security Agreement
Common stock warrants outstanding		34,440
Warrants Outstanding Exercise Price	$ 4.026	$ 4.026
Common stock warrants issued in connection with the 2018 Loan and Security Agreement	2028-02	2028-02
Common Stock Warrants
Common stock warrants outstanding		176,218	141,778

Income Taxes - Schedule of Curr

Income Taxes - Schedule of Current Income Tax Provision (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
Total current income tax provision	$ 38	$ 26
Domestic Federal
Income Taxes Disclosure [Line Items]
Total current income tax provision	3	1
Foreign Tax Authority [Member]
Income Taxes Disclosure [Line Items]
Total current income tax provision	$ 35	$ 25

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryovers	$ 28,834	$ 19,251
Fixed and intangible assets	837	664
Accruals and reserves	761	465
Stock-based compensation	132	31
Research and development credits	1,716	1,858
Contributions	14	13
Total deferred tax assets	32,294	22,282
Less valuation allowance	(32,294)	(22,282)
Total net deferred tax assets	$ 0	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Federal Income Tax (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation
Income tax benefit at federal statutory rate			$ (8,624)	$ (10,404)
State and local income taxes net of federal tax benefit			3	1
Foreign tax rate differential			11	(3)
Change in valuation allowance			8,497	(522)
Stock-based compensation			123	309
R&D tax credits			(313)	(222)
Other			341	109
Federal rate change (pursuant to the Tax Cuts and Jobs Act of 2017)				10,758
Income tax provision	$ 51	$ 27	$ 38	$ 26

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
Increase in valuation allowance	$ 10	$ 0.6
Federal Corporate tax rate	21.00%	35.00%
Effect of remeasurement of deferred tax assets due to TCJA		$ 10.1
Domestic Federal
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 119.5
Net operating loss carryforwards	2030
Research and development credit carryforwards, california	$ 1
California | State and Local Jurisdiction
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 37.2
Net operating loss carryforwards	2031
Research and development credit carryforwards, california	$ 0.9
All Other States | State and Local Jurisdiction
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	$ 19.5
Net operating loss carryforwards	2029

Income Taxes - Schedule of Unre

Income Taxes - Schedule of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unrecognized tax benefits [Abstract]
Beginning balance	$ 893	$ 688
Additions based on tax positions related to prior years	394
Additions based on tax positions related to current years	609	205
Balance at end of year	$ 1,896	$ 893

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	3,884,986	21,145,718	22,551,229	20,955,192
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share		17,579,720	18,670,328	17,510,045
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	3,658,436	3,313,257	3,636,358	3,108,604
Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	226,550
Early Exercised Options Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share		21,620	13,422	11,603
Convertible Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share		54,903	54,903	183,162
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share		176,218	176,218	141,778

Segment and Geographic Inform_3

Segment and Geographic Information - Schedule of Product Revenue Based on Location (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Product revenue	$ 28,615	$ 7,201	$ 12,263	$ 1,719
United States
Revenues From External Customers And Long Lived Assets [Line Items]
Product revenue	15,054	4,476	7,022	969
Germany
Revenues From External Customers And Long Lived Assets [Line Items]
Product revenue	2,389	958	1,393	597
Rest of Europe
Revenues From External Customers And Long Lived Assets [Line Items]
Product revenue	9,473	1,597	3,516	143
All Other Countries
Revenues From External Customers And Long Lived Assets [Line Items]
Product revenue	$ 1,699	$ 170	$ 332	$ 10

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail)	Feb. 22, 2019shares	Sep. 30, 2019shares	Dec. 31, 2018shares
Subsequent Event [Line Items]
Stockholders' equity, reverse stock split		The Company's common stock and convertible preferred stock on a 12.2-for-one basis (the "Reverse Stock Split")
Stock split, conversion ratio		12.2
Subsequent Event
Subsequent Event [Line Items]
Stockholders' equity, reverse stock split	The Company's common stock and convertible preferred stock on a 12.2-for-one basis (the "Reverse Stock Split")
Stock split, conversion ratio	12.2
Common Stock
Subsequent Event [Line Items]
Fractional shares issued in reverse stock split		45	45
Common Stock | Subsequent Event
Subsequent Event [Line Items]
Fractional shares issued in reverse stock split	45
Preferred Stock
Subsequent Event [Line Items]
Fractional shares issued in reverse stock split			69
Preferred Stock | Subsequent Event
Subsequent Event [Line Items]
Fractional shares issued in reverse stock split	69

Financial Instruments and Fai_5

Financial Instruments and Fair Value Measurements - Schedule of Fair Value of Warrants Determined Using the Black-Scholes Option Pricing Model (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Expected Volatility
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value assumption measurement input	0.439	0.428
Measurement Input, Expected Dividend Rate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value assumption measurement input	0	0
Minimum [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Expected term (in years)	5 years 3 months 10 days	5 years 8 months 15 days
Minimum [Member] | Risk-free Interest Rate
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value assumption measurement input	0.0249	0.0262
Maximum [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Expected term (in years)		6 years 2 months 15 days
Maximum [Member] | Risk-free Interest Rate
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value assumption measurement input		2.85

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Carrying Value of Operating Lease Liabilities (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Operating Lease Liabilities Payments Due [Abstract]
Remainder of 2019	$ 222
2020	861
2021	855
2022	581
Total minimum lease payments	2,519
Less: imputed interest	(234)
Total lease liability	$ 2,285

Convertible Preferred Stock a_3

Convertible Preferred Stock and Stockholder's Equity (Deficit) - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Mar. 11, 2019	Mar. 06, 2019	Mar. 05, 2019	Apr. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity And Stockholders Equity [Line Items]
Convertible preferred stock, shares outstanding						18,670,328		17,510,045
Preferred stock, shares authorized					5,000,000
Preferred stock, shares issued					0
Preferred stock, shares outstanding					0
Common Stock
Temporary Equity And Stockholders Equity [Line Items]
Fractional shares issued in reverse stock split					45	45
Conversion of stock, common stock issued			18,670,259
Issuance of common stock upon net exercise of warrants	101,744			29,887	180,952
Convertible Preferred Stock
Temporary Equity And Stockholders Equity [Line Items]
Convertible preferred stock, shares outstanding			18,670,259			18,670,328	17,579,720	17,510,045	14,605,589
Fractional shares issued in reverse stock split					69
Related Party Common Stock Warrants
Temporary Equity And Stockholders Equity [Line Items]
Warrants exercised price					$ 2.196	$ 2.196
Initial Public Offering | Common Stock Warrants Related to Convertible Preferred Stock Warrants
Temporary Equity And Stockholders Equity [Line Items]
Common stock warrants issued		54,903
Adjustments to additional paid in capital		$ 0.8
Issuance of common stock upon net exercise of warrants				49,321
Initial Public Offering | Related Party Common Stock Warrants
Temporary Equity And Stockholders Equity [Line Items]
Common stock warrants exercised		91,446
Warrants exercised price		$ 17
Common stock issued upon exercise of common stock warrants		79,632

Convertible Preferred Stock a_4

Convertible Preferred Stock and Stockholder's Equity (Deficit) - Summary of Key Terms of Outstanding Common Stock Warrants (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock Warrants
Class of Warrant or Right [Line Items]
Common stock warrants outstanding		176,218	141,778
Related Party Common Stock Warrants
Class of Warrant or Right [Line Items]
Common stock warrants outstanding		141,778	141,778
Warrants Outstanding Exercise Price	$ 2.196	$ 2.196
Warrants Outstanding Expiration	2025-05	2025-05
Common Stock Warrants Related to 2018 Loan and Security Agreement
Class of Warrant or Right [Line Items]
Common stock warrants outstanding		34,440
Warrants Outstanding Exercise Price	$ 4.026	$ 4.026
Warrants Outstanding Expiration	2028-02	2028-02