Uptake of enflurane: a study of the variability between patients

Variability in the uptake of enflurane has been studied using a computer-controlled "overpressure" induction technique in 23 paediatric patients. The administration of enflurane was feedback-controlled and its uptake measured using a closed anaesthesia delivery system with the end-tidal concentration held constant at 2.0%. The standard deviation in uptake was greater than 28% of the average uptake during the first 60 min of enflurane anaesthesia. The uptake at 60 min ranged from 5.7 to 16.2 ml min-1 m-2. The variability in enflurane uptake between patients was best minimized by normalizing the patient's uptake according to body surface area. Although the average total volume of enflurane calculated according to a model compared well with the average total volume given in our study, the variation in uptake between patients indicated that a single model cannot be used to deliver anaesthesia to a varied patient population. The system described in this paper demonstrates the feasibility of using feedback control of fresh gas delivery for the automatic control of an "overpressure" induction technique.