Prediction of inspired oxygen concentration within a circle anaesthetic system.

Fresh gas flows of nitrous oxide, oxygen and halothane at 6, 3, 2 and 1 litre/min were introduced into a circle absorber system. Spontaneous respiration and IPPV were studied and a regression of inspired on delivered oxygen concentration % was calculated. The difference between delivered and inspired oxygen concentration % was increased by decreasing the fresh gas flow and by decreasiing the proportion of oxygen in that flow, especially during IPPV. Circuits designed to allow a maximum overflow of alveolar gas provided a greater inspired oxygen concentration. The patients' height and weight were related to the scatter of inspired values observed at 1 litre/min of fresh gas flow with IPPV.