The Thermoregulatory Threshold in Humans during Nitrous Oxide-Fentanyl Anesthesia

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Abstract
Narcotics and nitrous oxide (N2O) inhibit thermoregulatory responses in animals. The extent to which N2O/fentanyl anesthesia lowers the thermoregulatory threshold in humans was tested by measuring peripheral cutaneous vasoconstriction using skin-surface temperature gradients (forearm temperature-fingertip temperature) and the laser Doppler perfusion index. Fifteen unpremedicated patients were anesthetized with N2O (70%) and fentanyl (10 .mu.g/kg iv bolus followed by 4 .mu.g .cntdot. kg-1 .cntdot. h-1 infusion) during elective, donor nephrectomy. Patients were randomly assigned to undergo additional warming (humidified respiratory gases, warmed intravenous fluids, and a heating blanket over the legs; n = 5) or standard temperature management (no special warming measures; n = 10). Significant vasoconstriction was prospectively defined as a skin surface temperature gradient between forearm surface and fingertip surface .gtoreq. 4.degree. C, and the thermoregulatory threshold was defined as the esophageal temperature at which such vasoconstriction occurred. Vasoconstriction did not occur in the patients who received additional warming and thus remained nearly normothermic [average minimum esophageal temperature = 35.8 .+-. 0.4.degree. C (SD)] but did in six hypothermic patients at a mean esophageal temperature of 34.2 .+-. 0.5.degree. C. Four hypothermic patients developed a passive thermal steady state without becoming sufficiently cold to trigger vasoconstriction. Thus, active thermoregulation occurs during N2O/fentanyl anesthesia but does not occur until core temperatures are .apprx. 2.5.degree. C lower than normal. The thermoregulatory threshold during N2O/fentanyl anesthesia is similar to that previously determined during halothane (34.4 .+-. 0.2.degree. C). Bupivacaine 0.5% (1.5 ml) injected around the base of the fourth finger prevented vasoconstriction, indicating that intraoperative vasoconstriction can be prevented by local neural blockade (as can normal thermoregulatory vasoconstriction). Decreased skin-surface temperatures occurred primarily in the fingers, whereas the adjacent palms were largely spared. This pattern is similar to that produced by normal thermoregulatory vasoconstriction and is consistent with the known distribution of thermoregulatory arteriovenous shunts.