Hypothermia minimally decreases nitrous oxide anesthetic requirements

Over the 38-28 degrees C range, changes in minimum alveolar anesthetic concentration (MAC) parallel changes in lipid solubility of the anesthetics studied. We hypothesized that there would be minimal change in N2OMAC, since N2O lipid solubility is relatively unaltered by temperature changes. We determined N2OMAC in rats using a hyperbaric chamber. In Group N (normothermia, n = 10) rectal temperature was maintained at 37.5 +/- 1 degrees C (mean +/- SD). In Group H (hypothermia, n = 9) temperature was maintained at 29.7 +/- 1.8 degrees C. The hyperbaric chamber was pressurized with N2O and oxygen partial pressure was 0.4 +/- 0.1 atm. Chamber pressure was adjusted approximately 15% up or down, stabilized for approximately 15 min, and the noxious stimulus (electrical current) was applied. This process was continued until two N2O partial pressures were determined which just prevented and just permitted gross, purposeful movement. Nitrous oxide MAC for Group N and Group H were 1.9 +/- 0.2 atm and 1.6 +/- 0.2 atm, respectively, P < 0.01. Temperature and MAC correlated: r = 0.59, P < 0.01. We conclude that hypothermia minimally decreases N2OMAC, which is consistent with the effects of hypothermia on N2O solubility in lipid membranes.