Humidification During Invasive Mechanical Ventilation: Less Lung Inflammation With Optimal Gas Conditioning.

During normal breathing, the upper airway warms and moistens the inspired air such that, at the level of the main carina, humidity reaches 44 mg H2O/L (100% relative humidity at 37°C, the average core temperature of humans). Conversely, during invasive mechanical ventilation, orotracheal or tracheostomy tubes bypass the upper airways, determining a decrease of approximately 75% of respiratory gas conditioning.1 As a result, at least some degree of artificial humidification is necessary inasmuch as it avoids mucociliary failure, epithelial damage, airway obstruction, and reduction in respiratory compliance. The optimal level of humidity and temperature, however, still remains to be determined. Some authors2,3 have suggested that normal values during spontaneous breathing should also apply to invasive mechanical ventilation. Likewise, based on experimental and clinical studies, the 2012 American Association for Respiratory Care guideline1 recommends a range of gas moisture between 33 and 44 mg H2O/L and temperature between 34 and 41°C with a relative humidity of 100%. This range, however, has never been put to the test in terms of relevant outcomes, such as airway and lung inflammation. In the current issue of RESPIRATORY CARE, Jiang et al4 evaluated 5 different levels of gas conditioning in healthy rabbits under invasive mechanical ventilation. They assessed the effects of a wide range of absolute humidity on airway epithelium damage and on lung inflammation using cytokines in the bronchoalveolar lavage fluid, histologic lung injury score, and wet to dry ratio of the lungs. They found less lung inflammation and less epithelial damage in the group with more physiological gas conditioning, considering the core temperature of rabbits. Mechanical ventilation is a supportive treatment and can be life-saving in many conditions. However, depending on the ventilator settings and on the underlying lung disorder, mechanical ventilation can in itself promote or perpetuate lung injury, a consequence known as ventilatorinduced lung injury. This injury is attributed to excessive stress (tension) and strain (deformation) of the lung cells,