Airway Heating and Humidification

Simple thermodynamic calculations indicate that less than 10% of metabolic heat production is lost through the respiratory tract. The loss results from both heating and humidifying inspiratory gases, but humidification requires two thirds of the heat.^[52] Because little heat is lost through respiration, even active airway heating and humidification minimally influence core temperature.^{[58] [164]} The apparent clinical efficacy of these devices probably results from artifactual warming of proximally positioned esophageal stethoscopes. ^[165]

Because respiratory heat loss remains virtually constant during anesthesia, the fraction of total heat lost through the respiratory tract decreases dramatically during large operations in which substantial heat is lost from evaporation within surgical incisions.^[53] Consequently, airway heating and humidification are even less effective than usual in patients most in need of effective warming. Cutaneous warming maintains normothermia so much better than respiratory gas conditioning does that intraoperative active airway heating and humidification are rarely if ever indicated. Airway heating and humidification are more effective in infants and children than adults,^[166] but cutaneous warming is also more effective in these patients and transfers more than 10 times as much heat. Hygroscopic condenser humidifiers and heat- and moisture-exchanging filters ("artificial noses") retain substantial amounts of moisture and heat within the respiratory system. In terms of preventing heat loss, these passive devices are about half as good as active systems^[166] ; however, they cost only a fraction as much. Heat retention is comparable in all clinically available heat and moisture exchangers.^[52]