A defective unidirectional dome valve was not discovered during normal testing

An incompetent unidirectional dome valve, that is, a valve which allows a significant amount of retrograde gas flow, was inadvertently discovered on an anesthesia machine that had been in daily clinical service for five years. The defective valve was not previously discovered during daily pre-use machine checkout or quarterly preventive maintenance. A review of the literature revealed that valve incompetence is prevalent and has caused patient morbidity; published daily pre-use checkout and preventive maintenance procedures do not test for valve incompetence. A simple procedure is proposed that positively tests unidirectional dome valves for both obstruction and incompetence.