Evaluation of 25%, 50%, and 67% nitrous oxide with halothane-oxygen for general anesthesia in horses

Twenty-five percent, 50%, and 67% nitrous oxide was administered to 12 horses anesthetized with halothane and oxygen. Compared to halothane-oxygen alone, there was no significant difference in heart rate, systolic, diastolic, or mean blood pressure values, arterial pH, PaCO2, or plasma bicarbonate values when nitrous oxide was included. A significant linear reduction in PaO2 values could be correlated with N2O:O2 concentrations. The halothane level required to maintain surgical anesthesia was reduced when nitrous oxide was administered, but it was not affected by changing the nitrous oxide concentrations. Nitrous oxide concentrations greater than 25% provide no additional reduction in halothane requirement and may be accompanied by PaO2 values that pose risk to the horse.