EVALUATION OF THE EFFECT OF ALFENTANIL ON THE MINIMUM ALVEOLAR CONCENTRATION OF HALOTHANE IN HORSES

The effect of 3 plasma concentrations of alfentanil on the minimum alv eolar concentration (MAC) of halothane in horses was evaluated. Five h ealthy geldings were anesthetized on 3 occasions, using halothane in o xygen administered through a mask. After induction of anesthesia, hors es were instrumented for measurement of blood pressure, airway pressur e, and end-tidal halothane concentrations. Blood samples, for measurem ent of pH and blood gas tensions, were taken from the facial artery. P ositive pressure ventilation was begun, maintaining Pa(CO2) at 49.1 +/ - 3.3 mm of Hg and airway pressure at 20 +/- 2 cm of H2O. The MAC was determined in triplicate, using a supramaximal electrical stimulus of the oral mucous membranes. Alfentanil infusion was then begun, using a computer-driven infusion pump to achieve and maintain 1 of 3 plasma c oncentrations of alfentanil. Starting at 30 minutes after the beginnin g of the infusion, MAC was redetermined in duplicate. Mean +/- SD meas ured plasma alfentanil concentration during the infusions were 94.8 +/ - 29.0, 170.7 +/- 29.2 and 390.9 +/- 107.4 ng/ml. Significant changes in MAC were not observed for any concentration of alfentanil. Blood pr essure was increased by infusion of alfentanil and was dose-related, b ut heart rate did not change. Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly differen t among the 3 doses.