All mechanically ventilated patients in Trauma and surgical ICU at 185-Hospital (Kasr Alainy Hospitals) will start on pressure controlled ventilation mode (PCV) with inspiratory pressure achieving tidal volume 6-8 ml/kg for 2 hours then they will be randomized into : A-group (APRV group) & B-group (BiPAP group). which are described later in the 2 arms of the study.