Relative Contribution of Skin and Core Temperatures to Vasoconstriction and Shivering Thresholds during Isoflurane Anesthesia

Background: Thermoregulatory control is based on both skin and core temperatures. Skin temperature contributes [approximate] 20% to control of vasoconstriction and shivering in unanesthetized humans. However, this value has been used to arithmetically compensate for the cutaneous contribution to thermoregulatory control during anesthesia-although there was little basis for assuming that the relation was unchanged by anesthesia. It even remains unknown whether the relation between skin and core temperatures remains linear during anesthesia. We therefore tested the hypothesis that mean skin temperature contributes [approximate] 20% to control of vasoconstriction and shivering, and that the contribution is linear during general anesthesia.

Methods: Eight healthy male volunteers each participated on 3 separate days. On each day, they were anesthetized with 0.6 minimum alveolar concentrations of isoflurane. They then were assigned in random order to a mean skin temperature of 29, 31.5, or 34 [degree sign]C. Their cores were subsequently cooled by central-venous administration of fluid at [almost equal to] 3 [degree sign]C until vasoconstriction and shivering were detected. The relation between skin and core temperatures at the threshold for each response in each volunteer was determined by linear regression. The proportionality constant was then determined from the slope of this regression. These values were compared with those reported previously in similar but unanesthetized subjects.

Results: There was a linear relation between mean skin and core temperatures at the vasoconstriction and shivering thresholds in each volunteer: r2 = 0.98 +/- 0.02 for vasoconstriction, and 0.96 +/- 0.04 for shivering. The cutaneous contribution to thermoregulatory control, however, differed among the volunteers and was not necessarily the same for vasoconstriction and shivering in individual subjects. Overall, skin temperature contributed 21 +/- 8% to vasoconstriction, and 18 +/- 10% to shivering. These values did not differ significantly from those identified previously in unanesthetized volunteers: 20 +/- 6% and 19 +/- 8%, respectively.