NITROUS-OXIDE DECREASES THE THRESHOLD FOR VASOCONSTRICTION LESS-THAN SEVOFLURANE OR ISOFLURANE

The core temperature triggering thermoregulatory arteriovenous shunt c onstriction is designated the threshold for vasoconstriction. High thr esholds are generally desirable because vasoconstriction helps prevent further core hypothermia by decreasing cutaneous heat loss and constr aining metabolic heat to the core thermal compartment. Previous studie s suggest that nitrous oxide (N2O) may inhibit thermoregulatory vasoco nstriction less than comparable doses of volatile anesthetics. To conf irm this impression, we tested the hypothesis that 0.5 minimum alveola r 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 appr oval, we studied 40 patients, aged 20-60 yr, undergoing open abdominal surgery. No premedication was given. Ten patients each were anestheti zed with: 1) N2O (50%) and 0.5 MAC sevoflurane (1%); 2) sevoflurane al one (2%); 3) N2O (60%) and 0.5 MAC isoflurane (0.6%); and, 4) isoflura ne alone (1.2%). A forearm minus fingertip, skin temperature gradient greater than or equal to 0 degrees C was considered significant vasoco nstriction; the esophageal temperature triggering vasoconstriction ide ntified the threshold. Morphemetric characteristics were comparable in each group. The threshold for vasoconstriction was 35.8 +/- 0.3 degre es 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 sevof lurane: 35.1 +/- 0.4 degrees C. Similarly, the threshold for vasoconst riction was 35.9 +/- 0.3 degrees C in the patients given 60% N2O combi ned 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 co nclude that N2O impairs thermoregulation less than sevoflurane or isof lurane.