Nitrous oxide decreases the threshold for vasoconstriction less than sevoflurane or isoflurane

The core temperature triggering thermoregulatory arteriovenous shunt constriction is designated the threshold for vasoconstriction. High thresholds are generally desirable because vasoconstriction helps prevent further core hypothermia by decreasing cutaneous heat loss and constraining metabolic heat to the core thermal compartment. Previous studies suggest that nitrous oxide (N2O) may inhibit thermoregulatory vasoconstriction less than comparable doses of volatile anesthetics. To confirm this impression, we tested the hypothesis that 0.5 minimum alveolar anesthetic concentration (MAC) N2O combined with 0.5 MAC sevoflurane or isoflurane would reduce the vasoconstriction threshold less than 1.0 MAC sevoflurane or isoflurane. With institutional review board approval, we studied 40 patients, aged 20-60 yr, undergoing open abdominal surgery. No premedication was given. Ten patients each were anesthetized with: 1) N2O (50%) and 0.5 MAC sevoflurane (1%); 2) sevoflurane alone (2%); 3) N2O (60%) and 0.5 MAC isoflurane (0.6%); and, 4) isoflurane alone (1.2%). A forearm minus fingertip, skin temperature gradient > or = 0 degree C was considered significant vasoconstriction; the esophageal temperature triggering vasoconstriction identified the threshold. Morphometric characteristics were comparable in each group. The threshold for vasoconstriction was 35.8 +/- 0.3 degrees C in the patients given 50% N2O combined with 0.5 MAC sevoflurane, which was significantly greater than that in those given 1.0 MAC sevoflurane: 35.1 +/- 0.4 degrees C. Similarly, the threshold for vasoconstriction was 35.9 +/- 0.3 degrees C in the patients given 60% N2O combined with 0.5 MAC isoflurane, which was significantly greater than that in those given 1.0 MAC isoflurane: 35.0 +/- 0.5 degrees C. We thus conclude that N2O impairs thermoregulation less than sevoflurane or isoflurane.