The investigators aimed to investigate the ventilation homogeneity and transpulmonary pressure during treatments of High-flow nasal cannula(HFNC) and (CPAP) on supine and prone position for COVID-19 or non-COVID-19 patients with acute hypoxemic respiratory failure (AHRF).

Awake prone positioning (APP) for non-intubated patients has been reported to improve oxygenation for patients with acute hypoxemic respiratory failure (AHRF),particularly for patients with COVID-19 induced AHRF, APP has been shown to decrease the need of intubation.Similarly, APP with helmet continuous positive airway pressure (CPAP) enables a reduction in the work of breathing and an improvement in oxygenation and sensation of dyspnea in COVID-19-associated acute respiratory distress syndrome (ARDS). Moreover, among intubated patients with ARDS assessed by electro-impedance tomography (EIT), prone positioning was found to reduce alveolar overdistention and collapse, resulting in improvement of ventilation homogeneity.However, the effects of APP on the ventilation homogeneity and work of breathing for non-COVID-19 patients with AHRF remain unknown.

High-flow nasal cannula (HFNC) has been proven to improve oxygenation and reduce intubation rate for patients with AHRF in multiple meta-analyses,and has been recommended to treat AHRF in several recently published guidelines.CPAP provides consistent level of positive end-expiratory pressure (PEEP), in comparison to variable level of PEEP generated by HFNC, thus CPAP was reported to have greater improvement of oxygenation than HFNC in 20 patients with AHRF, more importantly, they found that among patients who avoided intubation, the extent of oxygenation improvement was greater with HFNC+APP than with NIV+APP, in contrast, for patients who were intubated, the extent of oxygenation improvement was greater with NIV+APP than with HFNC+APP.This raised the concerns of the self-inflicted lung injury (SILI) generated by heterogenous aeration and large swings of transpulmonary pressure. Thus, the investigators aimed to investigate the ventilation homogeneity and transpulmonary pressure during treatments of HFNC and CPAP on supine and prone.

Inclusion Criteria:

• adult patients (18-90 years) who have acute hypoxemic respiratory failure, requiring FIO2 > 0.4 to maintain SpO2 at 90-95% during HFNC at 50 L/min

Exclusion Criteria:

• patients who need immediate intubation; refuse to participate in the study; unable to communicate; have contraindication to place the esophageal catheter; unable to use EIT, such as open-chest surgery with chest tube placement; have contraindication for prone positioning, including pregnant, post-abdomen surgery within a week; hypercapnic respiratory failure.