Abstract
Non-invasive respiratory support including high flow nasal therapy (HFNT), continuous positive airway pressure (CPAP) and Bilevel positive airway pressure (BiPAP), exerts distinct physiological effects and requires specific settings and technicalities. HFNT, delivered through dedicated nasal cannulas, provides low levels of positive airway pressure, anatomical dead space washout, allows good patient tolerance and can be used during CPAP or BiPAP breaks. CPAP and BiPAP, administered through various interfaces (e.g., facemasks, oro-nasal masks, or helmets), can deliver higher positive pressure, thereby increasing end-expiratory lung volume, reducing intrapulmonary shunt and oxygenation, with potential benefits on respiratory mechanics as well. BiPAP also delivers pressure support, aiding CO₂ clearance and respiratory muscle unloading, which is especially useful in hypercapnic respiratory failure. Increased intrathoracic pressure also reduces right ventricle preload and left ventricle afterload, which is beneficial in patients with impaired left ventricular function. Non-invasive respiratory support indications depend on the cause of acute respiratory failure. In hypercapnic respiratory failure with respiratory acidosis, BiPAP via facemask is strongly recommended to prevent intubation and reduce mortality. In cardiogenic pulmonary edema, either CPAP or BiPAP is recommended, while HFNT can be useful for patients requiring prolonged support or when CPAP/BiPAP is not tolerated even after ventilator and interface settings optimization. In de-novo acute hypoxemic respiratory failure, HFNT should be considered as the first-line treatment, regardless of the aetiology: however, in COVID-19-related AHRF CPAP can be considered to avoid intubation. The choice of non-invasive respiratory support interface in both cardiogenic and non-cardiogenic AHRF should aim at minimizing leaks, optimizing CO(2) clearance, and maximizing patient tolerance. Monitoring is essential during non-invasive respiratory support to assess patient's response to treatment and to avoid delaying invasive respiratory support when needed, particularly in hypoxemic patients to avoid intubation delays and prevent patient-self-inflicted lung injury: physiological parameters, clinical scores, and lung ultrasound may help assess the risk of NIV failure. Monitoring tidal volume is valuable but challenging because of leaks. Though not widely used, esophageal pressure monitoring can assess patient effort and transpulmonary pressure. Additionally, electrical impedance tomography is an emerging tool for detecting asynchronous breathing and pendelluft phenomena.