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ABSTRACT
Humidification for mechanical ventilation and high flow gas applications is more complicated than most clinicians are taught to believe and as a result is often not well understood. Several misconceptions survive despite growing evidence to the contrary. This article attempts to debunk some of these myths including why temperature is not a surrogate for humidity, why 100% humidity is not always optimal and the problems with humidity using heated wire circuits.
RÉSUMÉ
L'humidification de la ventilation mécanique et des dispositifs de gaz à grand débit est plus complexe que la plupart des cliniciens sont portés à le croire et par conséquent, elle est souvent mal comprise. De nombreuses idées fausses circulent en dépit de preuves démontrant le contraire. Cet article tend à discréditer certains mythes : pourquoi la température n'est pas un substitut à l'humidité, pourquoi une humidité de 100 % n'est pas toujours l'idéal, et les problèmes de l'humidité et des circuits filaires chauffés
MYTH 1: If gas temperature is 370C then humidity levels are sufficient
FACT: Humidity levels with gas/water interface humidifiers are not measured; they are assumed through temperature. However, gas temperatures do not reflect humidity levels, especially with heated wire circuits. Heated wires are designed to heat the temperature of the gas along the inspiratory limb to match the set temperature and can do this without regard to humidity levels. See further explanation in Myth #3 and Myth #9.
BOTTOM LI NE: The displayed temperature is not indicative of humidity.
MYTH 2: Heated wire circuits with heated passover humidifiers deliver near ideal humidity levels.
FACT: Absolute humidity levels with heated passover (gas/ water interface) humidifiers consistently measure low. A study by Francois Lellouche reported mean absolute humidity measures of 35-3 ±1.8 (mean ± sd) and as low as 32.0 ± 2.3 with high ambient temperatures from the most common heated passover system used today in North America.1 Other studies have found similar results for passover humidifiers.2,3,8,11 Since gas at BTPS (i.e. 100% relative humidity; 370C) has an absolute humidity of 44.0 mg/ L these levels are well below optimal levels and are close to the minimum humidity levels of 33 rng/L required during mechanical ventilation according to international standards.56
BOTTOM LINE: Heated passover humidifiers do not humidify as...