Hygrometry behavior during high-flow nasal oxygen therapy and non-invasive mechanical ventilation: A narrative review of bench to clinical studies

Recently, there has been growing interest in knowing the best hygrometry level during high-flow nasal oxygen and non-invasive ventilation (NIV) and its potential influence on the outcome. Various studies have shown that breathing cold and dry air results in excessive water loss by nasal mucosa, reduced mucociliary clearance, increased airway resistance, reduced epithelial cell function, increased inflammation, sloughing of tracheal epithelium, and submucosal inflammation. With the Coronavirus Disease 2019 pandemic, using high-flow nasal oxygen with a heated humidifier has become an emerging form of non-invasive support among clinicians. However, we cannot always assume stable humidification. Similarly, there are no clear guidelines for using humidification during NIV, although humidification of inspired gas during invasive ventilation is an accepted standard of care. NIV disturbs the normal physiological system that warms and humidifies inspired gases. If NIV is supplied through an intensive care unit ventilator that utilizes anhydrous gases from compressed wall air and oxygen, the risk of dryness increases. In addition, patients with acute respiratory failure tend to breathe through the mouth during NIV, which is a less efficient route than nasal breathing for adding heat and moisture to the inspired gas. Obstructive sleep apnea syndrome is one of the most important indications for chronic use of NIV at home. Available data suggest that up to 60% of patients with obstructive sleep apnea syndrome who use continuous positive airway pressure therapy experience nasal congestion and dryness of the mouth and nose. Therefore, humidifying the inspired gas in NIV may be essential for patient comfort and compliance with treatment. We aimed to review the available bench and clinical studies that addressed the utility of hygrometry in NIV and nasal high-flow oxygen and discuss the technical limitations of different humidification systems for both systems.