Conditioning of Medical Gases during Spontaneous Breathing

Abstract

The humidification and heating (i.e., the conditioning) of medical gases is now well established clinical practice in intubated patients receiving invasive ventilatory support [1]. Under normal circumstances, room air is only partially humidified, with a relative humidity around 50%, absolute humidity of 19.4 mgH₂O/1 and room temperature (22 °C). Through the nose and upper airways, the inspired air is filtered for particles and microorganisms, warmed to body temperature (37°C), and fully saturated [2]. This can ensure optimal gas exchange and respiratory function, maintaining the gas mixture constant, at 37°C with absolute humidity of 44 mgH₂O/1 (i.e., relative humidity 100%), within the lower airways and alveoli. Nasal mucosa and turbinate bones in the nose, are mostly involved in these mechanisms. The nasal mucosa is always moist, due to its high vascularization and high concentration of mucous glands [3]. The surface area of turbinates, covered by the mucosa, can increase the turbulence of gas flow due to the convoluted surface. Both these factors increase the contact between the gas and mucosa [4]. As a result, inspiratory flow arriving in the oropharynx is already heated to 30–32°C and almost fully saturated (absolute humidity 28–34 mgH₂O/1, corresponding to 90–100% relative humidity) [5]. During the passage in the trachea, the gas is further heated to body temperature and charged of water vapor until the isothermic saturation boundary [6].