[Hypoxia secondary to the failure of an oxygen-air-nitrous oxide mixer]

A case is reported of hypoxia occurring during anaesthesia in a spontaneously breathing ASA I patient. The patient became cyanotic twice when breathing a gas mixture delivered by a safety mixer. Changing the machine solved the immediate problem. The diagnosis was difficult to make because the rotameters all showed normal delivery of oxygen and nitrous oxide. Oximetry elucidated the cause, which was found to be a defective rapid oxygen control. Because these machines do not appear to be absolutely reliable, the use of gas analysers should become more systematic.