The importance of delivering warm humidified gas to patients ventilated through an endotracheal or tracheostomy tube is widely accepted. Mechanical ventilation with endotracheal intubation bypasses the upper airway and the normal heat- and moisture-exchanging process of inspired gases. A continuous loss of moisture and heat occurs, which predisposes patients to serious airway damage (Shelly etai 1988)

The development of mechanical ventilation in the intensive care unit (ICU) has resulted in a number of complications which are the source of a major increase in morbidity, mortality, and the cost of hospital care. Some of these complications are due to the poor conditioning of the temperature and humidity of inspired gases (Table 1).

Table 1 Complications associated with poor conditioning of inspiratory gases

A fixed volume of gas at a given temperature can only contain a limited and precise amount of water vapor. The temperature of this gas determines its saturation point, i.e. the maximum concentration of water vapor that it can contain. The relative humidity (RH) is the humidity expressed as a percentage of the maximum water vapor that a gas can contain at a given temperature. Absolute humidity (AH) is the weight of water vapor that a unit volume of gas contains at a given temperature.

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