Comparison of External Heat Exchange Systems

Montreal General Hospital, Montreal, Quebec, Canada. englishm@sympatico.caTo the Editor:—The finding by Kabbara et al. 1that “standard hospital blankets heated to 38°C forced air were equally as effective (in maintaining core temperature) as commercial blankets heated with forced air to 43°C” does not mean that the heat exchange capabilities of these two systems are the same. Core temperature (Tc) cannot discriminate between external heating systems such as these, because it is insulated from any heat exchange at the skin by the prevailing thermal conductance, the heat exchange rate between core and skin. Thermal conductance is the reason that in any heat exchange cascade, Tc is the last temperature to change.Accurate discrimination requires measurement of the mean skin temperature (Tmsk), the area -weighted average temperature of multiple skin sites representative of the body surface area. Tmsk is the relevant temperature because the skin is the heat exchange site of the systems being compared and because Tmsk, together with thermal conductance, determines the rate of change in Tc. Being “equally as effective” at maintaining Tc does not mean that the two systems are “equally as effective” as heat exchangers at the skin. Without measuring Tmsk, one cannot determine whether these systems maintained Tc by design or by default, and 29% of all patients still became hypothermic.In whole body rewarming of hypothermic, postoperative patients, Ereth et al. 2also found no difference in Tc between one group warmed with a Bair Hugger (Augustine Medical Inc., Eden Prairie, MN) at 43°C and another with blankets warmed to 40°C and changed every 15 min. Tmsk was significantly higher in the Bair Hugger group but, despite this active warming, Tc did not increase more rapidly, Ereth argued, because of “thermoregulatory vasoconstriction” (i.e. , low thermal conductance). 2In fact, because Tmsk < Tc, direct heating of Tc would have been impossible. In Ereth's study, the Bair Hugger was rendered ineffective because it was insulated from the patient by an intervening cotton blanket. While Kabbara does not say whether his Bair Hugger was insulated in this way, this practice has become common and could explain the lack of a significant difference in this study.Commercial blankets are designed to provide a low resistance airflow path to distribute heat consistently over the skin area under the cover. Such a consistent heated area cannot be guaranteed by blowing air between blankets, because of the variable airflow resistance of such an ad hoc arrangement. It is a basic law of physics, which this paper will not change, that all heat exchange is a function of area. The authors rely heavily on a model by Kempen, 3which was biased in favor of the noncommercial system because it measured only in the low resistance airflow path and ignored the effect of airflow distribution. A model whose centerpiece was a “standard thermal body” from which “heat transfer” was estimated. But this estimate was irrelevant, because the standard thermal body has no clinical counterpart; inappropriate, because the standard thermal body was unperfused and, therefore, depended on temperature, not the heat transfer rate; and inaccurate, because the standard thermal body was unstirred .Tc is one important temperature in thermoregulation. But in any “further evaluation” of their suggested method, I trust that the authors will address not only the physiology of thermoregulation but also the physics of heat exchange. It would then be possible to attribute a plausible cause to a perceived effect.

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