Potency of halothane-N20 in the horse.

The study examines the potency effects of a mixture of halothane and nitrous oxide (N2O) on horses and compares the cardiovascular effects to those seen with halothane-oxygen (O2) anesthesia. The research found that nitrous oxide concentrations decreased the minimal necessary halothane concentration and the mixture had less cardiovascular depression on spontaneously breathing horses than halothane-oxygen.

Examining the Potency of Halothane-Nitrous Oxide

• The study focused on analyzing the minimal alveolar concentration (MAC) of halothane, a type of anesthetic, in 11 healthy, young horses. MAC is the concentration of an anesthetic at which 50% of subjects fail to respond to a standardized painful stimulus.
• The horses involved in the study were not premedicated and were breathing either pure oxygen (O2) or a mixture of nitrous oxide (N2O) and O2.
• The ventilation of the equine subjects was controlled throughout the study, with their arterial PO2 levels consistently exceeding 90 mm of Hg, and their average PaCO2 ranging between 36 and 40 mm of Hg.
• The MAC for halothane in oxygen without nitrous oxide was established at 0.93 volume percent. However, when nitrous oxide was combined with halothane, the MAC values decreased — alveolar N2O concentrations of 25% and 50% reduced the halothane MAC about 12% and 25%, respectively, suggesting that the presence of nitrous oxide increased the effectiveness or potency of halothane.

Investigating Cardiovascular Effects

• In addition to examining the effects on MAC, the study also investigated how the combination of 50% N2O and halothane (referred to as H50N2O) affected the cardiovascular system in 8 of these horses.
• The cardiovascular effects were studied during both spontaneous and controlled ventilation, and the results were compared with past studies of the effects of halothane with oxygen (without nitrous oxide).
• Just as with halothane-O2 anesthesia, increasing dosages of the H50N2O combination led to a decrease in cardiovascular function. There were some differences depending on the conditions — for example, there was an associated increase in cardiac output and left ventricular work at MAC 1.0 and 1.5 with the presence of N2O, which was not observed with halothane-O2 anesthesia.
• However, when H50N2O was administered to horses breathing spontaneously (without controlled ventilation), most indications of cardiovascular function were found to be depressed less than in cases of similarly administered equipotent level (MAC 1.5) of halothane-O2 anesthesia. This suggests that the halothane-nitrous oxide mixture may present less cardiovascular risk to horses during anesthesia than halothane-oxygen.”