Heated Humidifier

Abstract

During nasal respiration, the inspired air is warmed and humidified by evaporation of water from the surfaces of the mucous membranes. The air in the pulmonary periphery thus becomes saturated with water vapor. The point at which gases reach 37 °C and 100 % relative humidity (corresponding to an absolute humidity of 44 mg/L) is called the “isothermic saturation boundary” (ISB). The ISB is located well below the carina during quiet breathing. The evaporation leads to loss of energy, which results in cooling of the mucous membranes. This fall in temperature allows recovery of water and heat through condensation during the subsequent expiration. Delivery of cool, dry gases to the patient with a bypassed upper airway can have dire consequences, including alterations in tracheobronchial structure and function. Common findings include inspissation of secretions, airways plugged with mucus, ciliary dyskinesis, epithelial desquamation, and tracheal tube occlusion [1]. For intubated patients in whom the upper airway is bypassed—which otherwise would have supplied 75 % of the heat and moisture to the lower respiratory tract—a heated humidifier can supply the same.