Inclusion Criteria:  Intubated patients requiring MV for >24 h Age >18 years Acute hypoxemic respiratory failure, defined as a PaO2/FiO2 <300 with an FiO2 ≥0.3 and PEEP≥5 cmH2O. ARF caused by pulmonary insults.  Exclusion Criteria:  ARF from non-pulmonary origin. Contraindications from high PEEP (severe head trauma or severe chest trauma). Patients that cannot maintained supine position. Uncorrected hypovolemia Hemodynamic instability