HEAT-BALANCE AND DISTRIBUTION DURING THE CORE-TEMPERATURE PLATEAU IN ANESTHETIZED HUMANS

Background: Once triggered, intraoperative thermoregulatory vasoconstr iction is remarkably effective in preventing further hypothermia. Prot ection results from both vasocanstriction-induced decrease in cutaneou s heat loss and altered distribution of body heat. However, the indepe ndent contributions of each mechanism have not been quantified. Accord ingly, we evaluated overall heat balance and distribution of heat with in the body during the core-temperature plateau.Methods: Nine minimall y clothed male volunteers were anesthetized with propofol and isoflura ne and maintained in an approximate to 22 degrees C environment, They were monitored for approximate to 2 h before vasoconstriction and for 3 h subsequently, Overall heat balance was determined from the differe nce between cutaneous heat loss (thermal nux transducers) and metaboli c heat production (oxygen consumption). Arm and leg tissue heat conten ts were determined from 19 intramuscular temperatures, ten skin temper atures, and ''deep'' foot temperature. Heat constrained by vasoconstri ction to the trunk and head was calculated by subtracting the expected change in that region (overall heat balance multiplied by the fractio nal weight of the trunk and head) from the actual change (change in di stal esophageal temperature multiplied by the specific heat of human t issue and the weight of the trunk and head); the result represents the amount by which core heat exceeded that which would be expected based on overall heat balance, assuming that the change was evenly distribu ted throughout the body. Results: Vasoconstriction and passive tissue cooling decreased heat loss but not to the level of heat production, C onsequently, heat loss exceeded metabolic heat production throughout t he study. Core temperature decreased approximate to 1.3 degrees C duri ng the 2-h prevasoconstriction period; however, core temperature remai ned virtually constant during the subsequent 3 h. In the 3 h after vas oconstriction, arm and leg heat content decreased 57 +/- 3 kcal, and v asoconstriction constrained 22 +/- 8 kcal to the trunk and head. Concl usions: These results confirm the efficacy of thermoregulatory vasocon striction in preventing additional core hypothermia. Decreased cutaneo us heat loss and constraint of metabolic heat to the core thermal comp artment contributed to the plateau.