The Physiological Effects of Different Non-invasive Respiratory Support in Patients With Acute Hypoxemic Respiratory

Patients with acute hypoxemic respiratory failure (AHRF) typically present with pathophysiological alterations characterized by the coexistence of respiratory dysfunction and hypoxemia. Respiratory dysfunction leads to dyspnea, increased work of breathing, use of accessory respiratory muscles, and hypercapnia, while gas exchange impairment results in hypoxemia. Studies have shown that hypercapnia, acidosis, and hypoxemia can all enhance inspiratory effort, which further increases negative intrathoracic pressure. In these patients, regional differences in airway resistance and lung compliance are often present, causing redistribution of air within the lungs. This redistribution manifests as gas movement from non-dependent to dependent regions, known as "pendelluft," which amplifies regional alveolar strain and ventilation heterogeneity. This phenomenon becomes more pronounced during noninvasive respiratory support when spontaneous breathing is preserved.

Noninvasive respiratory support strategies mainly include high-flow nasal oxygen (HFNO), noninvasive positive pressure ventilation (NIV), and continuous positive airway pressure (CPAP). HFNO delivers high-flow gas through nasal cannulas, generating a certain level of positive end-expiratory pressure (PEEP) and flushing out anatomical dead space to improve gas exchange, thereby reducing inspiratory effort, lowering the work of breathing, and enhancing oxygenation. NIV, typically using pressure support ventilation (NIV-PSV), is a patient-triggered, pressure-targeted mode that provides inspiratory positive pressure above PEEP. By augmenting tidal volume and reducing inspiratory effort, NIV improves gas exchange; however, leaks may limit the effective delivery of PEEP, and full inspiratory synchronization can increase transpulmonary driving pressure and tidal volume. CPAP, by contrast, delivers a constant positive pressure during both inspiration and expiration. Compared with HFNO, CPAP generates higher PEEP, which facilitates alveolar recruitment and more effectively improves oxygenation. Relative to NIV, CPAP may reduce transpulmonary driving pressure and tidal volume.

Different noninvasive respiratory support strategies exert varying effects on respiratory drive and regional lung strain, leading to differences in the occurrence and magnitude of pendelluft. Physiological studies have suggested that CPAP may offer greater benefits in improving oxygenation and reducing inspiratory effort; however, whether it can mitigate the occurrence and extent of pendelluft remains uncertain. Therefore, this study was conducted to visualize and quantitatively assess pendelluft in real time using electrical impedance tomography (EIT), aiming to verify whether CPAP has a superior effect in reducing pendelluft in patients with AHRF.