Reduction in halothane MAC: comparison of morphine and alfentanil.

The anesthetic potency and effectiveness of alfentanil and morphine were established by determining the effects of increasing drug doses on the alveolar anesthetic requirement of halothane to maintain a constant anesthetic (MAC) level. Six selected doses of alfentanil and four of morphine were administered to groups of mechanically ventilated rats anesthetized with halothane. Alfentanil was given as a loading dose followed by an intravenous infusion of 0.01-100 micrograms X kg-1 X min-1, and morphine was administered as a subcutaneous dose of 4-20 mg/kg. The reduction in halothane requirement after morphine was biphasic, with a rapid linear increase occurring up to an 8 mg/kg subcutaneous dose, followed by a further, slower reduction in halothane requirement after doses of 8-20 mg/kg. At a 20 mg/kg dose, the halothane MAC was reduced approximately 84%. With alfentanil, a curvilinear reduction in halothane MAC occurred up to an alfentanil dose of 15 micrograms X kg-1 X min-1, where a 48% reduction was found. Larger doses produced severe truncal, chest wall, and abdominal rigidity, precluding adequate ventilation and the determination of MAC.