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 alveolar concentration (MAC) of halothane in horses was evaluated. Five healthy geldings were anesthetized on 3 occasions, using halothane in oxygen administered through a mask. After induction of anesthesia, horses were instrumented for measurement of blood pressure, airway pressure, and end-tidal halothane concentrations. Blood samples, for measurement of pH and blood gas tensions, were taken from the facial artery. Positive pressure ventilation was begun, maintaining PaCO2 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 concentrations of alfentanil. Starting at 30 minutes after the beginning of the infusion, MAC was redetermined in duplicate. Mean +/- SD measured 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 pressure was increased by infusion of alfentanil and was dose-related, but heart rate did not change. Pharmacokinetic variables of alfentanil were determined after its infusion and were not significantly different among the 3 doses.