Morphometric Influences on Intraoperative Core Temperature Changes

Abstract

Intraoperative core hypothermia develops in three characteristic phases:1) core-to-peripheral redistribution of body heat that is most prominent during the first hour after induction of anesthesia; 2) subsequent slow linear decrease in core temperature resulting largely from heat loss exceeding metabolic heat production; and 3) core temperature plateau resulting when thermoregulatory vasoconstriction decreases cutaneous heat loss and constrains metabolic heat to the core thermal compartment. Accordingly, we tested the hypotheses that: 1) core cooling does not depend on body fat (BF) or the ratio of weight-to-surface area (Wt/SA) during the initial redistribution phase; 2) the core cooling rate is a function of the Wt/SA ratio during the second phase; and 3) the rate of core cooling during the plateau phase (after vasoconstriction) will be determined by the percentage of BF. In 40 patients undergoing elective colon surgery, the amount of redistribution hypothermia was inversely proportional to the percentage of BF (Delta T_C = 0.034 centered dot BF - 2.2, r² = 0.63) and the Wt/SA ratio (Delta T_C = 0.052 centered dot Wt/SA - 3.35, r² = 0.66). The core cooled linearly during the second phase, and the cooling rate was inversely proportional to the Wt/SA ratio (rate = 0.035 centered dot (Wt/SA) - 2.2, r² = 0.29). Thermoregulatory vasoconstriction was effective in virtually all patients independent of their morphology, and produced a fourfold reduction in the core cooling rate. These results indicate that patient morphometric characteristics substantially influence intraoperative core temperature changes, and that the effect depends on the hypothermia phase. (Anesth Analg 1995;80:562-7)