5 OXYGEN NITROUS-OXIDE PROPORTIONING SYSTEMS COMPARED

The majority of contemporary gas delivery modules, on anaesthetic work stations, are equipped with oxygen-nitrous oxide proportioning systems which should prevent the delivery of hypoxic gas mixtures. We investi gated five modules, of two different types, using fresh gas flows rang ing from 50 mL min(-1) to 20 L min(-1) with minimally acceptable propo rtions of oxygen and maximally acceptable proportions of nitrous oxide set at the flow control valves. The oxygen concentrations of the resu lting gas mixtures were measured at the fresh gas outlet of the gas de livery modules. All the modules prevented the delivery of hypoxic fres h gas flows both under normal and abnormal working conditions (unequal supply of gas pressures). All systems showed increased O-2 concentrat ions at fresh gas flows below 1 L min(-1). The systems can be used wit h low flow anaesthesia techniques with one exception (Dameca) also wit h minimal flow techniques.