[Halothane-pentazocine interaction in dogs (author's transl)].

It is known that narcotics reduce the alveolar concentration of inhalation anesthetics in man and animals. However the magnitude and duration of narcotic effect on inhalation anesthesia is not known. Accordingly, I determined in dogs the temporal effect of various doses of pentazocine, a commonly used anesthetic adjuvant, on the minimum alveolar concentration (MAC) of halothane. In addition, I compared plasma pentazocine concentrations in dogs both awake and during halothane anesthesia using an identical intramuscular dose of pentazocine. Intramuscular injection of pentazocine, group II (2.5 mg/kg), group III (5 mg/kg), group IV (10 mg/kg) reduced MAC of halothane required for anesthesia. The magnitude of MAC depression were 19.3% of control halothane MAC in group II, 36.4% of control in group III and 41.7% of control in group IV. Postinjection plasma concentration was fitted by computer with a 2-compartment open pharmacokinetic model. Plasma pentazocine concentration for awake (group I) and anesthetized (group II) dogs given the same dose (2.5 mg/kg) did not differ significantly on biological half-life, total apparent volume of distribution and body clearance. Halothane minimum alveolar concentration (MAC) was correlated to plasma pentazocine concentration (r= -0.60) and cerebrospinal fluid pentazocine concentration (r= -0.74).