Body morphology and the speed of cutaneous rewarming

Background: Infants and children cool quickly because their surface area (and therefore heat loss) is large compared with their metabolic rate, which is mostly a function of body mass. Rewarming rate is a function of cutaneous heat transfer plus metabolic heat production divided by body mass. Therefore, the authors tested the hypothesis that the rate of forced-air rewarming is inversely related to body size. Methods: Isoflurane, nitrous oxide, and fentanyl anesthesia were administered to infants, children, and adults scheduled to undergo hypothermic neurosurgery. All fluids were warmed to 37°C and ambient temperature was maintained near 21°C. Patients were covered with a full-body, forced-air cover of the appropriate size. The heater was set to low or ambient temperature to reduce core temperature to 34°C in time for dural opening. Blower temperature was then adjusted to maintain core temperature at 34°C for 1 h. Subsequently, the forced-air heater temperature was set to high (43°C). Rewarming continued for the duration of surgery and postoperatively until core temperature exceeded 36.5°C. The rewarming rate in individual patients was determined by linear regression. Results: Rewarming rates were highly linear over time, with correlations coefficients (r²) averaging 0.98 ± 0.02. There was a linear relation between rewarming rate (°C/h) and body surface area (BSA; m2): Rate (°C/h) = -0.59 · BSA (m²) + 1.9, r² = 0.74. Halving BSA thus nearly doubled the rewarming rate. Conclusions: Infants and children rewarm two to three times faster than adults, thus rapidly recovering from accidental or therapeutic hypothermia.