Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Cash Flows from Operating Activities:
Net loss	$ (7,529)	$ (5,497)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	204	139
Share-based compensation	501	144
Non-cash interest expense	177	247
Change in fair value of convertible preferred stock warrant liability		7
Cost of rental units purchased by customers	27	66
Changes in certain assets and liabilities:
Accounts receivable, net	(47)	(15)
Inventory	59	(298)
Net investment in sales-type leases	(360)
Prepaid commission expense	(515)
Prepaid expenses and other assets	504	219
Accounts payable	(1,475)	263
Accrued expenses	(1,236)	(3,106)
Deferred revenue	120	(453)
Deferred rent		(13)
Net Cash Used in Operating Activities	(9,570)	(8,297)	$ (20,600)
Cash Flows from Investing Activities:
Purchases of property and equipment and capitalized software	(274)	(297)
Net Cash Used in Investing Activities	(274)	(297)
Cash Flows from Financing Activities:
Payments of public offering costs		(230)
Proceeds from exercises of stock options	1,407	31
Net Cash Provided by (Used in) Financing Activities	1,407	(199)
Net (Decrease) in Cash and Cash Equivalents	(8,437)	(8,793)
Cash and Cash Equivalents, Beginning of Period	104,583	29,147	29,147
Cash and Cash Equivalents, End of Period	96,146	20,354	$ 104,583
Supplemental disclosure of cash flow information:
Cash paid for interest	737	628
Transfer of inventory to property and equipment	37	169
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment and capitalized software in accounts payable and accrued expenses	$ 44
Deferred initial public offering costs included in accounts payable and accrued expenses		$ 1,339