Uptake of enflurane and isoflurane during spontaneous and controlled ventilation

The uptake of enflurane and of isoflurane were studied in forty patients during anaesthesia with nitrous oxide using either spontaneous or controlled ventilation. A Douglas bag method was used in combination with low fresh gas flows to a circle system and constant end-tidal anaesthetic concentration. The mean enflurane uptake rates were between 24 and 14 ml.70kg-1.min-1 between 10 and 60 minutes. Corresponding isoflurane uptake rates were between 15 and 8 ml. 70 kg-1.min-1. The initial uptake rates were lower than expected from "the square root of time concept". During spontaneous ventilation, the anaesthetic uptake rates were similar or even higher than corresponding rates during controlled ventilation in spite of lower minute ventilation volumes.