Uptake of isoflurane during prolonged clinical anaesthesia

Recent evidence has suggested that the rate of uptake of inhalational anaes thetic is constant during maintenance of anaesthesia, contrary to the predi ctions of multi-compartment uptake models. We measured isoflurane uptake us ing a totally closed anaesthetic system during up to 10 h of stable anaesth esia for maxillo-facial surgery on 12 adult patients. Liquid isoflurane was injected into the system under computer control to produce an end tidal co ncentration of 1.3 MAC of isoflurane. Bench tests demonstrated that the lea kage from the system was less than 8 mul min(-1), confirming that the rate of injection of isoflurane into the system was a close upper bound on the p atients' uptake. Anaesthetic usage for a 70 kg patient was 0.44e(-0.51t+) 0 .044e(-0.013t)+0.058e(-0.00098t) ml(-1) of liquid isoflurane, where t is du ration of anaesthesia in minutes. There was a continuing reduction in anaes thetic requirement even at the end of the period of study that was statisti cally significant. These data do not support the notion that isoflurane upt ake is constant during stable maintenance of anaesthesia but is compatible with the conventional multi-compartment model of anaesthetic uptake and dis tribution.