The measurement of expiratory oxygen as disconnection alarm.

The efficacy of an oxygen-monitoring alarm in the expiratory limb of an anesthetic circuit was evaluated as a method of detecting disconnections of the patient from the anesthetic machine. Oxygen concentrations were determined in the expiratory limb of a semiclosed anesthetic circuit with a ventilator and a 2-1 reservoir bag serving as a simple lung model. Adjustments of ventilation during the different measurements were a PEEP of 5 cmH2O, respiratory rates of 10/min and 5/min, and tidal volumes of 1,000 and 500 ml, respectively. The alarm time measured with a stopwatch was the time that elapsed between disconnection and the acoustical alarm. The circuit was always disconnected at the y-piece when the reservoir bag had been fully expanded after an inspiration. With various fresh flow rates and oxygen concentrations ranging from 33% to 52%, disconnection always was evident within 30 s. Measurement of the oxygen concentration with an alarm time of 30 s after disconnection may be an effective backing system for detecting a disconnection.