Uptake of isoflurane during prolonged clinical anaesthesia

Recent evidence has suggested that the rate of uptake of inhalational anaesthetic is constant during maintenance of anaesthesia, contrary to the predictions of multi-compartment uptake models. We measured isoflurane uptake using a totally closed anaesthetic system during up to 10 h of stable anaesthesia for maxillo-facial surgery on 12 adult patients. Liquid isoflurane was injected into the system under computer control to produce an end tidal concentration of 1.3 MAC of isoflurane. Bench tests demonstrated that the leakage from the system was less than 8 microl min(-1), confirming that the rate of injection of isoflurane into the system was a close upper bound on the patients' uptake. Anaesthetic usage for a 70 kg patient was 0.44e(-0.51t)+0.044e(-0.013t)+0.058e(-0.00098t) ml min(-1) of liquid isoflurane, where t is duration of anaesthesia in minutes. There was a continuing reduction in anaesthetic requirement even at the end of the period of study that was statistically significant. These data do not support the notion that isoflurane uptake is constant during stable maintenance of anaesthesia but is compatible with the conventional multi-compartment model of anaesthetic uptake and distribution.