	Professional Services Agreements		Patents		Total
Cost
Balance as at January 1, 2018	$	215,844,537	$	532,598	$	216,377,135
Additions through asset acquisitions (note 4)		40,646,723		—		40,646,723
Balance as at December 31, 2018	$	256,491,260	$	532,598	$	257,023,858
Additions through asset acquisitions and adjustments (note 4)		18,622,130		—	$	18,622,130
Balance as at December 31, 2019	$	275,113,390	$	532,598	$	275,645,988

	Professional Services Agreements		Patents		Total
Accumulated depreciation
Balance as at January 1, 2018	$	45,752,303	$	497,417	$	46,249,720
Amortization expense		31,387,429		2,446		31,389,875
Balance as at December 31, 2018	$	77,139,732	$	499,863	$	77,639,595
Amortization expense and adjustments (note 4)		34,895,944		2,256		34,898,200
Balance as at December 31, 2019	$	112,035,676	$	502,119	$	112,537,795

	Professional Services Agreements		Patents		Total
Net book value
December 31, 2019	$	163,077,714	$	30,479	$	163,108,193
December 31, 2018	$	179,351,528	$	32,735	$	179,384,263

At December 31, 2019, the Company identified indicators of impairment in respect of six of its professional services agreements. Upon performing undiscounted cash flow models for these assets, the Company identified only two assets that required further review for impairment.

The Company performed discounted cash flow modelling for these assets and compared the resultant discounted cash flows expected over the life of the assets, including a terminal value, to the carrying amounts as at December 31, 2019. The income approach is used for the quantitative assessment to estimate the fair value of the assets, which requires estimating future cash flows and risk-adjusted discount rates in the Company's discounted cash flow model. The overall market outlook and cash flow projections of the reporting unit involves the use of key assumptions, including anesthesia growth rates, revenue rates per case, discount rates and operating cost growth rates. Due to uncertainties in the estimates that are inherent to the Company's industry, actual results could differ significantly from the estimates made. Many key assumptions in the cash flow projections are interdependent on each other. A change in any one or combination of these assumptions could impact the estimated fair value of the reporting unit.

As a result of this test, no write-downs to the intangible assets were required.

At December 31, 2018, the Company identified indicators of impairment in respect of four of its professional services agreements. Upon performing undiscounted cash flow models for these assets, the Company identified only two assets that required further review for impairment. No impairment was indicated.

Various of the Company’s professional services agreements are subject to renewal terms. The weighted average period before the Company’s professional services agreements are up for renewal is 3.28 years. The Company anticipates that it will be able to renew all contract terms under its professional services agreements. The weighted average remaining amortization period for the Company’s professional services agreements is 5.16 years.

Based on the Company’s professional services agreements in place at December 31, 2019, the Company anticipates that the amortization expense to be incurred by the Company over the next five years is as follows: