Inspired oxygen concentrations in semiclosed circle absorber circuits with low flows of nitrous oxide and oxygen.

Fresh gas flows of 1-5 l./min containing oxygen concentrations of 25-50% in nitrous oxide were led into a semiclosed circle absorber circuit. The resulting inspired oxygen concentrations were measured under conditions of spontaneous respiration. The lower the fresh gas flow, the greater was the discrepancy between fresh gas and inspired oxygen concentration. No association between minute volume and inspired oxygen concentration was seen. Measured value of inspired oxygen concentration agreed with those derived from the formulae of Mushin and Galloon (1960), and Fitton (1963), except at very low fresh gas flow. It appears possible to predict inspired oxygen concentration in a circle absorber circuit with low fresh gas flows of nitrous oxide and oxygen after a steady state of anaesthesia has been established.