Thermoregulatory vasoconstriction during isoflurane anesthesia minimally decreases cutaneous heat loss

The authors tested the extent to which thermoregulatory vasoconstriction decreases cutaneous heat loss during isoflurane anesthesia. Thermoregulatory vasoconstriction was provoked by central hypothermia in five nonsurgical volunteers given isoflurane anesthesia. Peripheral arteriovenous shunt flow was quantified using forearm-fingertip skin-surface temperature gradients and volume plethysmography. Capillary blood flow on the chest was evaluated using laser Doppler flowmetry. The central temperature triggering peripheral vasoconstriction (the thermoregulatory threshold) was 34.6 +/- 0.4 degrees C. Central body temperature decreased less than or equal to 0.2 degrees C in the period from 1 h preceding onset of significant vasoconstriction until 1.5 h afterward. Chest skin-surface blood flow decreased 21% during the period from 2 h before to 1 h after significant fingertip vasoconstriction. In contrast, fingertip blood flow decreased approximately 50-fold in the same period. The correlation between fingertip blood flow and skin-temperature gradient was excellent. Total heat loss decreased approximately 26% (25.3 +/- 3.9 W) in the period from 2 h before significant peripheral vasoconstriction to 1 h afterward. Loss from the arms and legs (upper arm, lower arm, thigh, and calf) decreased approximately 24% in the same period. Heat loss from the trunk and head decreased only 14%; in contrast, loss from the hands and feet decreased approximately 57%. There were no clinically important changes in blood pressure or heart rate during vasoconstriction, but oxyhemoglobin saturation (measured by pulse oximetry) increased slightly. These data suggest that thermoregulatory vasoconstriction only minimally decreases cutaneous heat loss.