Awareness under anaesthesia due to a defective gas-loaded regulator

Awareness under anaesthesia occurred in two patients due to a defect in an anaesthesic machine, which caused an inadequate supply of nitrous oxide. A defective fail-safe gas-loaded regulator was found to have caused communication between the pipelines for oxygen and nitrous oxide. This resulted in oxygen flow through the nitrous oxide flowmeter. A simple test to detect this type of defect is suggested.