Changes in Lung Aeration and Inspiratory Effort With and Without Awake 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.