Revenue generated in Europe was $3.9 million for the year ended December 31, 2021, a decrease of $0.4 million, or 10%, over the year ended December 31, 2020. Total revenue units in Europe decreased to 176 for the year ended December 31, 2021, from 193 for the prior year period. The decrease is due to reduced procedure volumes resulting from the Delta and Omicron variants of COVID-19 in 2021. The number of sales territories in Europe remained consistent at six during the year ended December 31, 2021.
Cost of goods sold and gross margin
Cost of goods sold increased $2.2 million, or 153%, to $3.6 million for the year ended December 31, 2021, compared to the year ended December 31, 2020. This increase was primarily due to higher sales of Barostim.
Gross profit was $9.4 million for the year ended December 31, 2021, an increase of $4.8 million, or 104%, over the year ended December 31, 2020. Gross margin decreased to 72% for the year ended December 31, 2021, compared to 76% for the year ended December 31, 2020. Gross margin for the year ended December 31, 2021 was lower due to a larger percentage of our revenue units coming from full systems versus battery replacements for existing patients. This was partially offset by an increase in the average selling price.
Research and development expenses
R&D expenses increased $1.1 million, or 17%, to $7.5 million for the year ended December 31, 2021, compared to the year ended December 31, 2020. This change was primarily driven by a $0.4 million increase in clinical study expenses, a $0.4 million increase in compensation expenses as a result of increased headcount, and a $0.4 million increase in non-cash stock-based compensation expense.
Selling, general and administrative expenses
SG&A expenses increased $18.1 million, or 187%, to $27.9 million for the year ended December 31, 2021, compared to the year ended December 31, 2020. This change was driven by an increase of $9.0 million in compensation expenses, including salaries and commissions, and other employee-related expenses, mainly as a result of increased headcount, a $3.1 million increase in marketing and advertising expenses, primarily related to the commercialization of Barostim in the U.S., a $1.4 million increase in non-cash stock-based compensation expense, a $1.3 million increase related to D&O insurance costs incurred as a result of becoming a public company, a $1.2 million increase in travel expenses and a $1.0 million increase in consulting expenses.
Interest expense
Interest expense decreased $0.3 million, or 10%, to $2.2 million for the year ended December 31, 2021, compared to the year ended December 31, 2020. This was driven by the repayment of the outstanding debt in November 2021 under the Horizon loan agreement.
Other expense, net
Other expense, net was $14.8 million for the year ended December 31, 2021, compared to other expense, net of $40,000 for the year ended December 31, 2020. The expense in 2021 was primarily driven by a $13.7 million increase in expense related to the increase in fair value of our convertible preferred stock warrants due to the change in the value of our common stock from December 31, 2020 to July 2, 2021, which is the date the warrants converted to common stock warrants. The expense in 2021 was also due to a $1.3 million loss on debt extinguishment in connection with the November 2021, repayment of the outstanding debt under the Horizon loan agreement.
Provision for income taxes
Provision for income taxes was nominal for the years ended December 31, 2021 and 2020.