Effect of xylazine and ketamine on the pharmacokinetics of alfentanil during halothane anaesthesia

We measured plasma concentrations of alfentanil in two horses after three different randomly ordered treatments. Each horse received halothane in oxygen by mask followed by a bolus dose of alfentanil 60 μg kg^-1 i. v., halothane in oxygen by mask followed by an i.v. alfentanil infusion for 120 min and xylazine and ketamine followed by halothane and a bolus dose of alfentanil 60 μg kg^-1 i.v. Halothane was maintained at 1.05-1.07% end-tidal concentration with a Pa_co2 of 6-7.3 kPa. The plasma concentration-time curves were similar after bolus and infusion doses of alfentanil with halothane and could be fitted to a two-compartment open model. A plateau was noted in the plasma concentration-time curve in the presence of xylazine-ketamine which could not be modelled adequately, but suggested a three-compartment open model with an additional input component. (Br. J. Anaesth. 1994; 72: 345-347)