[Humidification of the respiratory tract in anesthesia]

The upper airways humidify, warm and purify the air breathed in. When these are bypassed by an endotracheal intubation catheter, cold dry air goes straight to the bronchi; this leads to local and generalized changes, which are all the more pronounced the longer the intubation lasts. Drying of the tracheobronchial mucosa leads to an early decrease in mucus production, with an increase in its viscosity, a progressive arrest of ciliary function with, finally, a destruction of the vibratory cilia. These lesions create a build-up of thick mucus which hinders humidification of the gas flow and increases bronchial resistances. The disturbed VA/Q relationship leads to a decreased secretion of surfactant. The usual loss of water by way of the lungs is about 30 mg.l-1 of air. Evaporation of this water uses up calories. Most authors suggest that, so as to compensate for the heat and water loss, inhaled gases should be warmed to between 25 and 30 degrees C and be saturated with water to 100%, i.e. 23-30 mg water per litre of air. The water and heat loss can be reduced by using either partial or total rebreathing of expired gases, or a device for heat and moisture exchange, or a humidifier. The most efficient circuits are those which include a soda lime canister. They can be made even more efficient by using different devices. Although this humidification is sufficient for large tidal volumes and a surrounding temperature greater than 22 degrees C, it is not enough for Bain's circuit.(ABSTRACT TRUNCATED AT 250 WORDS)