Control of body temperature: use of the respiratory tract as a heat exchanger

The theoretical potential of the respiratory tract as a heat exchanger is enormous because the large alveolor surface area is in intimate contact with pulmonary blood flow. However, this potential is severely limited by some powerful physiologic mechanisms that ensure thermal isolation of alveolar gas, by the detrimental effects of dry gas and extremes of temperature on respiratory epithelium, and by the unfavorable thermal properties of respiratory gases in general. Optimal respiratory cooling using hyperventilation with cold helium-oxygen-CO₂ through a double lumen tube increased the rate of body heat loss by only 1.1°C/hr. Although respiratory cooling alone cannot effect heat transfer of sufficient magnitude to produce rapid cooling far induced hypothermia, it may find use as an adjunct in treating hyperthermic conditions and in induced hypothermia. Respiratory warming does not suffer the limitations of respiratory cooling and should find use in inhalation warming of hypothermic patients and in maintaining the body temperature of patients, especially small babies under anesthesia, who are unable to defend their own central temperature.