Pharmacokinetics of inhaled anaesthetics in a clinical setting: description of a novel method based on routine monitoring data.

Pharmacokinetic parameters of inhaled anaesthetics have previously been assessed experimentally in healthy volunteers. In contrast, we developed a method to estimate pharmacokinetic parameters under clinical conditions. We obtained data from the continuous routine monitoring of fractional concentration and ventilation during anaesthesia with desflurane, isoflurane and sevoflurane. By simulation studies, we assessed the effects of several sources of variation, including the noise of measurement, the second gas effect and rounding errors or a limited number of displayed digits. Stable fits to a two-compartment model were obtained for both real and simulated data sets in all cases. The most stable parameter was the intercompartmental clearance, and the most sensitive parameter was the volume of distribution. The bias in pharmacokinetic parameters caused by adding errors to measured concentrations was similar for the different compounds. We conclude that the model allows the estimation of an alternative set of pharmacokinetic parameters that can reliably describe the behaviour of volatile anaesthetics under clinical conditions, and allow comparison between agents.