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Humidification issues are overlooked by many clinicians in the intensive care unit (ICU). Because the need to heat and humidify inspired gases during mechanical ventilation is unanimously accepted, this process is considered the basic, supportive standard of care, about which there is no real debate. Yet, considerable controversy has surrounded central issues concerning humidification such as the level of adequate humidification and how to provide it, the influence of humidification devices on the incidence of ventilator-associated pneumonia, and certain patients and clinical situations and their requirements, such as the need for humidification during noninvasive ventilation. This may account for important differences in the practice of humidification between countries.1,2 Fortunately, renewed interest has emerged over the past decade, as indicated by several clinical studies that have helped settle some controversies. This chapter will review the reasons for conditioning inspired gases by recalling the normal process of heating and humidifying air during spontaneous breathing, the physical principles of humidification, and the consequences of inappropriate conditioning. Devices to achieve this conditioning are covered and their advantages and potential drawbacks discussed. Finally, practical guidelines are provided.

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As mentioned in the next section, the upper respiratory tract is responsible for most of the conditioning of the inspired gases. Important features of this conditioning (reviewed by Irlbeck3) include heat, humidification, and filtration, in order to deliver to the lower respiratory tract a warm (32°C [89.6°F]), humid (95% relative humidity), and pathogen-free and particle-free gas. The last step is achieved in the lower respiratory tract. During invasive mechanical ventilation, the endotracheal tube bypasses the upper respiratory tract. This places the burden of supplying heat and humidity to the cold and dry medical gases on the lower respiratory tract, a task for which it is poorly suited.4

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The upper respiratory tract (nose, mouth, nasopharynx, oropharynx, laryngopharynx, and larynx, but mainly the nose) is responsible for most of the conditioning of inspired gas. Anatomic structure and physiologic function of the nose are intimately linked. The highly vascular mucosa of the nose is ciliated and rich in mucosal glands and goblet cells. Three curved bony plates on the lateral side of each nasal cavity (the superior, middle, and inferior concha or turbinate bones), covered with a mucous membrane, ensure the important function of satisfactorily conditioning inspired gases. Their large surface area and position in relationship with the air current enable sufficient contact with the inspired gas. This mucous membrane also lines the paranasal sinuses, trachea, and bronchi but not the pharynx, which does not take part in the air-conditioning process.

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Humidity can be defined as the moisture content of the atmosphere and by extension, water present as vapor in a gas mixture. Vaporization indicates the change of a liquid (or a solid) to a gas or vapor. There is no strict difference between the terms gas and vapor, although gas is generally used to describe a substance that appears in the gaseous ...

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