Physiological Dead Space Measured by Volumetric Capnography in BiPAP and APRV .

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. Measurement tools : The following data will be recorded : * Demographic data ( Age , Sex , Weight and Height ). * P/F ratio before inclusion. * All patients then will be connected to Volumetric capnography which is included in the metabolic module on General Electric ventilator (Engstrom Carestation, GE Health care, USA). and physiological dead space will be recorded after 30 minutes and at the end of the 3 hours on each APRV \& BiPAP modes . * Peak air way pressure. * Mean airway Pressure. They will be also recorded after * Dynamic Compliance. 30 Minutes and at the end of the * P/F ratio , PH and pCO2. 3 hours On APRV \& BiPAP modes. * Minute ventilation. * Blood pressure , Heart rate and central venous pressure will be recorded before inclusion . then , they will be also recorded every 2 hours .