The threshold and gain of thermoregulatory vasoconstriction differs during anesthesia in the dependent and upper arms in the lateral position

Increased intraluminal pressure may help maintain vasodilation in a dependent arm even after hypothermia triggers centrally mediated thermoregulatory vasoconstriction. We therefore tested the hypotheses that the threshold (triggering core temperature) and gain (increase in vasoconstriction per degree centigrade) of cold-induced vasoconstriction is reduced in the dependent arm during anesthesia. Anesthesia was maintained with 0.4 minimum alveolar anesthetic concentration of desflurane in 10 volunteers in the left-lateral position. Mean skin temperature was reduced to 31 degrees C to decrease core body temperature. Fingertip blood flow in both arms was measured, as was core body temperature. The vasoconstriction threshold was slightly, but significantly, less in the dependent arm (36.2 degrees C +/- 0.3 degrees C, mean +/- SD) than in the upper arm (36.5 degrees C +/- 0.3 degrees C). However, the gain of vasoconstriction in the dependent arm was 2.3-fold greater than in the upper arm. Consequently, intense vasoconstriction (i.e., a fingertip blood flow of 0.15 mL/min) occurred at similar core temperatures. In the lateral position, the vasoconstriction threshold was reduced in the dependent arm; however, gain was also increased in the dependent arm. The thermoregulatory system may thus recognize that hydrostatic forces reduce the vasoconstriction threshold and may compensate by sufficiently augmenting gain.

The threshold for cold-induced vasoconstriction is reduced in the dependent arm, but the gain of vasoconstriction is increased. Consequently, the core temperature triggering intense vasoconstriction was similar in each arm, suggesting that the thermoregulatory system compensates for the hydrostatic effects of the lateral position.