Effect of nitrous oxide at sub-MAC concentrations on sevoflurane MAC in adults

The interaction of sevoflurane and nitrous oxide (N2O) on the MAC was studied in the four groups of patients between 30-60 years of age scheduled for laparotomies. Patients received one of four different concentrations of N2O [0% (n = 14), 25% (n = 16), 50% (n = 15), or 70% (n = 18)]. Anesthesia was induced with sevoflurane and N2O using a semiclosed circuit with a carbon dioxide absorber. After endotracheal intubation, sevoflurane and N2O end-tidal concentrations were adjusted to predetermined concentrations until the skin incision was made. The MAC values for sevoflurane in O2 and the presence of 25%, 50%, and 70% N2O were 1.68%, 1.33%, 0.82% and 0.55%, respectively. A regression analysis through all four data points yielded a linear relationship (r = -0.997). The requirement of sevoflurane decreased by approximately 1% for each 1% of N2O administered at three N2O concentrations. The extrapolated MAC value for N2O was 102%. The MAC values of sevoflurane in O2 and N2O were similar to the previously reported values. We conclude that in adults, N2O concentrations in the dose range 0-70% reduce sevoflurane MAC in a linearly additive manner.