Correspondence  |   May 2003
Comparison of External Heat Exchange Systems: In Reply:—
Author Notes
  • MetroHealth Medical Center, Cleveland, Ohio.
Article Information
Correspondence   |   May 2003
Comparison of External Heat Exchange Systems: In Reply:—
Anesthesiology 5 2003, Vol.98, 1298-1299. doi:
Anesthesiology 5 2003, Vol.98, 1298-1299. doi:
In Reply:—
The purpose of our study was to evaluate an experimental forced air warming technique in surgical patients during general anesthesia. 1 Distal esophageal temperature was chosen as the primary outcome measure to quantify the degree of intraoperative core hypothermia, because most thermoregulatory research incorporates core temperature measures and because nearly all the major adverse effects of mild hypothermia (e.g.  , decreased resistance to wound infections, morbid cardiac outcomes) have been described in terms of core temperature. 2,3 Compared with the standard commercial forced air warming system, the experimental technique resulted in similar mean core temperatures (> 36°C) between groups at the end of surgery. We did not measure thermal conductance of the heat exchange systems used, and as Dr. English points out, accurate discrimination of thermal conductance between external heating methods requires measurement of area-weighted average temperature of multiple skin sites. Therefore, our study was able to quantify an important and relevant outcome measure, core temperature, but was not able to quantify the amount of heat exchange at the skin.
Although the laboratory investigation of Dr. Kempen 4 (which inspired us to perform this study) showed that forced air warming with hospital bed sheets was able to heat standardized thermal bodies twice as effectively as commercial blankets using identically warmed 38°C forced air, the efficacy of forced air warming systems in maintaining intraoperative normothermia in anesthetized patients depends not only on heat exchange area, but also on the total energy flux exiting the warming hose, insulating effect of sheets or blankets covering the patient, degree of thermoregulatory vasoconstriction, airflow resistance beneath the blankets, thickness of the convective thermal boundary layer, and velocity distribution within and outside the convective thermal boundary layer. Moreover, metabolic heat production in combination with effective heat conservation may contribute substantially to the maintenance of perioperative normothermia regardless of heat transfer at the skin from external warming devices. 5 Indeed, the relatively high incidence of core temperature < 36°C in both groups at the end of surgery (27–31%) in our study 1 was likely the result of limited surface area available for forced air warming, combined with infusion of ∼ 2 l of room temperature IV fluids. Infusing unwarmed (or inadequately warmed) IV fluids has been shown to increase the incidence and severity of perioperative hypothermia. 6,7 We agree with Dr. English that any further evaluation of our experimental forced air warming technique should address not only safety and efficacy, but also thermal balance and body heat content.
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Smith CE, Gerdes EG, Sweda S, Myles C, Punjabi A, Pinchak AC, Hagen JF: Warming intravenous fluids reduces perioperative hypothermia in women undergoing ambulatory gynecological surgery. Anesth Analg 1998; 87: 37–41Smith, CE Gerdes, EG Sweda, S Myles, C Punjabi, A Pinchak, AC Hagen, JF