An unusual failure of an oxygen fail-safe device.

OXYGEN FAIL-SAFE SYSTEMS for anaesthetic gas machines have been advocated as standard safety features. 1-4 They are designed to prevent delivery of hypoxic gas mixtures in the event of an interruption or exhaustion of O~ pipeline or tank sources. In such a circumstance the fail-safe device causes all gas flow to cease, so alerting the operator of the apparatus to the problem. Some types of gas machines also incorporate an audible signal which is activated by the failure of 02 sources, 4 An additional feature of some devices is the opening of the anaesthetic circuit to the atmosphere, so avoiding inspiration of an oxygen concentration less than that of air. 4 This report concerns an unusual problem with a fail-safe system which resulted in the delivery of unexpectedly increased 02 concentrations, when N20 was included in the anaesthetic technique. A case of unexplained awareness during a narcotic, relaxant, nitrous oxide (6 L N 2 0 : 2 L 02 per minute) anaesthetic for elective cholecystectomy led to a systematic check of the oxygen deliver}, of various gas machines during clinical use. A Pauling paramagnetic 02 analyzer (Beckman Model D2) was used. One machine, a model 10 Boyle, was found to deliver 80-90 per cent O2 when the flowmeters indicated a 6 : 2 L per minute ratio of N20 : 02, when operated from pipeline gas sources. Re-checking with identical rotameter flows, but using cylinder gases revealed the appropriate 25 per cent of 02 The gas flowing through the N20 rotameter was found to be about 80 per cent 02/20 per cent N20 during operation from pipeline sources, and 100 per cent N20 when operated from cylinders. The cause of this unusual situation was found to be a defect in the O._, fail-safe device serving the N20 pipeline source. This is a gas-loaded regulator in which O2 under pressure displaces a rubber diaphragm which, in turn, moves a pin allowing entry of N20 into the system. It functioned properly as a fail-safe device, but produced dilution of inflowing N20 with 02. A small linear defect in the diaphragm (see Figure 1) was the apparent route of entry of the contaminating O2. Replacement of this regulator solved the problem. The replacement regulator supplied by the manufacturer has been modified so that recurrence of this problem would be unlikely. The separate fail-safe regulator servicing the N,,O cylinder was found to function properly, without 02 leakage, thus explaining the differences encountered in the performance of the apparatus when supplied from different sources. A device to avoid one complication led to a "'second generation" eomplieation, as a product of the primary solution. Detection of this type of special problem before it causes clinical problems may require qualitative analysis of gas eoneen-