Summary
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.
