Cost of goods sold and gross margin
Cost of goods sold increased $0.1 million, or 9%, to $0.9 million for the three months ended March 31, 2022, compared to the three months ended March 31, 2021. This increase was primarily due to higher sales of Barostim.
Gross profit was $3.1 million for the three months ended March 31, 2022, an increase of $1.1 million, or 57%, over the three months ended March 31, 2021. Gross margin increased to 77% for the three months ended March 31, 2022, compared to 70% for the three months ended March 31, 2021. Gross margin for the three months ended March 31, 2022 was higher due to a decrease in the cost per unit and an increase in the average selling price. This was partially offset by a larger percentage of our revenue units coming from full systems versus battery replacements. New patients receive a full system that includes an IPG and a stimulation lead, which has a lower gross margin than a stand-alone IPG used for a battery replacement.
Research and development expenses
R&D expenses increased $0.5 million, or 29%, to $2.3 million for the three months ended March 31, 2022, compared to the three months ended March 31, 2021. This change was primarily driven by a $0.3 million increase in compensation expenses, mainly as a result of increased headcount, a $0.3 million increase in clinical study expenses, and a $0.1 million increase in non-cash stock-based compensation expense, partially offset by a $0.2 million decrease in consulting expenses.
Selling, general and administrative expenses
SG&A expenses increased $6.3 million, or 142%, to $10.8 million for the three months ended March 31, 2022, compared to the three months ended March 31, 2021. This was primarily driven by a $3.2 million increase in compensation expenses, mainly as a result of increased headcount, a $0.7 million increase in travel expenses, a $0.7 million increase in insurance costs incurred as a result of becoming a public company, a $0.6 million increase in marketing and advertising expenses associated with the commercialization of Barostim in the U.S., a $0.6 million increase in non-cash stock-based compensation expense, a $0.3 million increase in consulting expenses, and a $0.2 million increase in professional fees.
Interest expense
Interest expense decreased $0.6 million for the three months ended March 31, 2022, compared to the three months ended March 31, 2021. This decrease was driven by the repayment of the outstanding debt under the loan agreement with Horizon Technology Finance Corporation (the “Horizon loan agreement”) in November 2021.
Other expense, net
Other expense, net was $0.1 million for the three months ended March 31, 2022, compared to $3.8 million for the three months ended March 31, 2021. The expense in the first quarter of 2021 was primarily driven by the increase in fair value of the convertible preferred stock warrant liability from December 31, 2020 to March 31, 2021. As these preferred stock warrants converted to common stock warrants upon the IPO, there is no longer a change in fair value recorded into other expense, net.
Provision for income taxes
Provision for income taxes was nominal for each of the three months ended March 31, 2022 and March 31, 2021.