Effects of a circulating-water garment and forced-air warming on body heat content and core temperature

Abstract

Background: Forced-air warming is sometimes unable to maintain perioperative normothermia. Therefore, the authors compared heat transfer, regional heat distribution, and core rewarming of forced-air warming with a novel circulating-water garment.

Methods: Nine volunteers were each evaluated on two randomly ordered study days. They were anesthetized and cooled to a core temperature near 34 degrees C. The volunteers were subsequently warmed for 2.5 h with either a circulating-water garment or a forced-air cover. Overall, heat balance was determined from the difference between cutaneous heat loss (thermal flux transducers) and metabolic heat production (oxygen consumption). Average arm and leg (peripheral) tissue temperatures were determined from 18 intramuscular needle thermocouples, 15 skin thermal flux transducers, and "deep" hand and foot thermometers.

Results: Heat production (approximately 60 kcal/h) and loss (approximately 45 kcal/h) were similar with each treatment before warming. The increases in heat transfer across anterior portions of the skin surface were similar with each warming system (approximately 65 kcal/h). Forced-air warming had no effect on posterior heat transfer, whereas circulating-water transferred 21+/-9 kcal/h through the posterior skin surface after a half hour of warming. Over 2.5 h, circulating water thus increased body heat content 56% more than forced air. Core temperatures thus increased faster than with circulating water than forced air, especially during the first hour, with the result that core temperature was 1.1 degrees +/- 0.7 degrees C greater after 2.5 h (P < 0.001). Peripheral tissue heat content increased twice as much as core heat content with each device, but the core-to-peripheral tissue temperature gradient remained positive throughout the study.

Conclusions: The circulating-water system transferred more heat than forced air, with the difference resulting largely from posterior heating. Circulating water rewarmed patients 0.4 degrees C/h faster than forced air. A substantial peripheral-to-core tissue temperature gradient with each device indicated that peripheral tissues insulated the core, thus slowing heat transfer.

Figures

Fig. 1

Heat production and cutaneous heat loss before and during warming. Circulating-water or forced-air warming began at elapsed time zero. Results are presented as means with 95% confidence intervals. Heat production was similar throughout the study. Asterisks (*) indicate times when heat loss differed significantly between the two treatment days, P

Fig. 2

Systemic heat balance, as determined by the difference between heat production and heat loss before and during warming. Circulating-water or forced-air warming began at elapsed time zero. Results are presented as means with 95% confidence intervals. Asterisks (*) indicate the times when the change in overall heat content was significantly greater with the circulating-water garment, P

Fig. 3

Core temperatures before and after warming. Circulating-water or forced-air warming began at elapsed time zero. Results are presented as means with 95% confidence intervals. Asterisks (*) indicate times when core temperatures differed significantly on the treatment days, P

Fig. 4.

Cumulative changes in measured peripheral (extremities) and core (trunk) heat content before and during warming. Circulating-water or forced-air warming began at elapsed time zero. Results are presented as means with 95% confidence intervals. Asterisks (*) denote the times when changes in peripheral and core heat contents differed significantly between the treatment days, P

Fig. 5

Core-to-peripheral tissue temperature gradient before and during warming. Average temperature of extremity tissues was considered peripheral temperature. Circulating-water or forced-air warming began at elapsed time zero. Results are presented as means with 95% confidence intervals. Although the gradient was slightly greater with circulating-water even before active warming, the shape of the curves was similar.