Potency of halothane-N20 in the horse.

The minimal alveolar concentration (MAC) of halothane which just prevented purposeful movement in response to electrical stimulation was determined in 11 young, healthy, unpremedicated horses breathing oxygen (O2) or nitrous oxide (N2O) and O2. Ventilation was controlled during these MAC studies. The arterial PO2 was always greater than 90 mm of Hg and the average PaCO2. range was 36 to 40 mm of Hg. The MAC for halothane in O2 was 0.93 vol %. Alveolar N2O concentrations of 25% and 50% reduced the halothane MAC about 12% and 25%, respectively. In 8 of these horses, the cardiovascular effects of halothane-50% N2O-balance O2 (H50N2O) were determined during spontaneous and controlled ventilation and were compared with previously reported results of halothane-O2 studies. Similar to halothane-O2 anesthesia, increasing dosages of H50N2O caused a decrease in cardiovascular function. With the exception of N2O-associated increase in cardiac output and left ventricular work at MAC 1.0 and 1.5, little difference was seen between the 2 forms of general anesthesia during controlled ventilation. However, when H50N20 was administered to spontaneously breathing horses, most indices of cardiovascular function were depressed less than with a similarly administered equipotent level (MAC 1.5) of halothane-O2 anesthesia.