LEG HEAT-CONTENT CONTINUES TO DECREASE DURING THE CORE TEMPERATURE PLATEAU IN HUMANS ANESTHETIZED WITH ISOFLURANE

Background. Sufficient hypothermia during anesthesia provokes thermore gulatory responses, but the clinical significance of these responses r emains unknown. Nonshivering thermogenesis does not increase metabolic heat production in anesthetized adults. Vasoconstriction reduces cuta neous heat loss, but the initial decrease appears insufficient to caus e a thermal steady state (heat production equaling heat loss). Accordi ngly, the authors tested the hypotheses that: 1) thermoregulatory vaso constriction prevents further core hypothermia; and 2) the resulting s table core temperature is not a thermal steady state, but, instead, is accompanied for several hours by a continued reduction in body heat c ontent. Methods. Six healthy volunteers were anesthetized with isoflur ane (0.8%) and paralyzed with vecuronium. Core hypothermia was induced by fan cooling, and continued for 3 h after vasoconstriction in the l egs was detected. Leg heat content was calculated from six needle ther mocouples and skin temperature, by integrating the resulting parabolic regression over volume. Results. Core temperature decreased 1.0 +/- 0 .2-degrees-C in the 1 h before vasoconstriction, but only 0.4 +/- 0.3- degrees-C in the subsequent 3 h. This temperature decrease, evenly dis tributed throughout the body, would reduce leg heat content 10 kcal. H owever, measured leg heat content decreased 49 +/- 18 kcal in the 3 h after vasoconstriction. Conclusions. These data thus indicate that the rmoregulatory vasoconstriction produces a clinically important reducti on in the rate of core cooling. This core temperature plateau resulted , at least in part, from sequestration of metabolic heat to the core w hich allowed core temperature to remain nearly constant, despite a con tinually decreasing body heat content.