The direction dependence of thermoregulatory vasoconstriction during isoflurane/epidural anesthesia in humans

We tested the hypothesis that once thermoregulatory vasoconstriction is triggered at a given core temperature during isoflurane anesthesia, redilation starts at a substantially higher core temperature. To avoid direct perception of cutaneous cooling and warming, we used epidural anesthesia and limited our thermal manipulations to the blocked area. Seven volunteers were anesthetized with isoflurane/epidural anesthesia (approximately T9 dermatomal level). Core hypothermia was induced by surface cooling restricted to the legs. Cooling was continued until fingertip blood flow suddenly decreased (vasoconstriction threshold). The core was then rewarmed by heating the legs until fingertip flow suddenly increased toward initial values (redilation threshold). The difference between the two thresholds defined the direction-dependent hysteresis. Vasoconstriction occurred at 35.2 +/- 0.6 degrees C and vasodilation at 36.2 +/- 0.5 degrees C (P < 0.01, paired t-test); consequently, the hysteresis was 1.0 +/- 0.6 degrees C. The observed hysteresis suggests that thermoregulatory responses during combined isoflurane/epidural anesthesia are not determined simply by instantaneous thermal input to central controllers, but may also depend on the direction of core temperature change.