EFFECT OF XYLAZINE AND KETAMINE ON THE PHARMACOKINETICS OF ALFENTANILDURING HALOTHANE ANESTHESIA

We measured plasma concentrations of alfentanil in two horses after th ree different randomly ordered treatments. Each horse received halotha ne in oxygen by mask followed by a bolus dose of alfentanil 60 mu g kg (-1) iv., halothane in oxygen by mask followed by an iv. alfentanil in fusion for 120 min and xylazine and ketamine followed by halothane and a bolus dose of alfentanil 60 mu g kg(-1) iv. Halothane was maintaine d at 1.05-1.07% end-tidal concentration with a Pa-CO2 of 6-7.3 kPa. Th e plasma concentration-time curves were similar after bolus and infusi on doses of alfentanil with halothane and could be fitted to a two-com partment open model. A plateau was noted in the plasma concentration-t ime curve in the presence of xylazine-ketamine which could nor be mode lled adequately, but suggested a three-compartment open model with an additional input component.