HEAT-FLOW AND DISTRIBUTION DURING INDUCTION OF GENERAL-ANESTHESIA

Background: Core hypothermia after induction of general anesthesia res ults from an internal core-to-peripheral redistribution of body heat a nd a net loss of heat to the environment. However, the relative contri butions of each mechanism remain unknown, The authors evaluated region al body heat content and the extent to which core hypothermia after in duction of anesthesia resulted from altered heat balance and internal heat redistribution. Methods: Six minimally clothed male volunteers in an approximate to 22 degrees C environment were evaluated for 2.5 con trol hours before induction of general anesthesia and for 3 subsequent hours. Overall heat balance was determined from the difference betwee n cutaneous heat loss (thermal flux transducers) and metabolic heat pr oduction (oxygen consumption). Arm and leg tissue heat contents were d etermined from 19 intramuscular needle thermocouples, 10 skin temperat ures, and ''deep'' foot temperature, To separate the effects of redist ribution and net heat loss, we multiplied the change in overall heat b alance by body weight and the specific heat of humans. The resulting c hange in mean body temperature was subtracted from the change in dista l esophageal (core) temperature, leaving the core hypothermia specific ally resulting from redistribution. Results: Core temperature was near ly constant during the control period but decreased 1.6 +/- 0.3 degree s C in the first hour of anesthesia. Redistribution contributed 81% to this initial decrease and required transfer of 46 kcal from the trunk to the extremities. During the subsequent 2 h of anesthesia, core tem perature decreased an additional 1.1 +/- 0.3 degrees C, with redistrib ution contributing only 43%. Thus, only 17 kcal was redistributed duri ng the second and third hours of anesthesia, Redistribution therefore contributed 65% to the entire 2.8 +/- 0.5 degrees C decrease in core t emperature during the 3 h of anesthesia. Proximal extremity heat conte nt decreased slightly after induction of anesthesia, but distal heat c ontent increased markedly. The distal extremities thus contributed mos t to core cooling. Although the arms constituted only a fifth of extre mity mass, redistribution increased arm heat content nearly as much as leg heat content. Distal extremity heat content increased approximate to 40 kcal during the first hour of anesthesia and remained elevated for the duration of the study. Conclusions: The arms and legs are both important components of the peripheral thermal compartment, but dista l segments contribute most. Core hypothermia during the first hour aft er induction resulted largely from redistribution of body heat, and re distribution remained the major cause even after 3 h of anesthesia.