Humidification and temperature of inspired gases in intensive care unit patients

Under normal circumstances, it can be assumed that the temperature in the upper trachea ranges from 30 to 33 °C and RH is approximately 95 per cent, providing a water content of 30 mg H2O/l. In mechanically ventilated patients, a gas temperature of 29 to 32 °C and 95 to 100 per cent RH should be adequate for inspired gases (AH, 25-30 mg H2O/l). However, the optimum humidity of inspired gas for patients in the ICU is still a matter of controversy and the minimum acceptable level has not yet been established. It has been suggested that 23 to 33 mg H 2O/l is a desirable range with a tracheal temperature of 32 °C. Some authors have proposed higher tracheal temperatures (33-37 °C), leading to AH up to 44 mg H2O/l. Literature values range from 17 to 44 mg H2O/l. In man, during nose breathing of room air, temperature in the subglottic space is 32 °C, RH is 98 per cent, and the water content is 32.8 mg H 2O/l. A temperature of 33 °C and a water content of 25 mg H2O/l were found in the larynx during nose breathing; at a point 9 cm below the vocal cords the temperature was 35 °C and the water content was 35 mg H 2O/l.

Currently, there is no routine method for continuous measurement of AH, RH, or tracheal temperature. If secretions become thick, the volume and frequency of instillations should be changed. A change in the fluidity of bronchial secretions, an increase in peak inspiratory pressure, and/or a flattening of pressure-volume curves are all signs of incorrect conditioning of inspired gases.

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