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

Background and Rationale : Mechanical ventilation is an essential component of the care of patients with respiratory failure.Biphasic positive Airway Pressure (BiPAP) and Airway Pressure release ventilation (APRV) are relatively new modes of mechanical ventilation which can be used in treatment of patients with impaired oxygenation.The effect of using BiPAP and APRV modes on reducing the physiological dead space had not been previously investigated. The investigators hypothesize that using APRV mode will decrease physiological dead space more than BiPAP mode in the mechanically ventilated critically ill patients. Objectives : To assess the physiological dead space with each mode. To assess lung mechanics during the use of the two modes. To assess the effectiveness of ventilation during the use of the two modes. Study population \& Sample size : Sixty adult patients more than 18 years old who are mechanically ventilated patients with P/F ratio less than 300. This sample size was calculated based on the assumption that APRV will decrease dead space by 20% with alpha error 0.05 and power 80%. The mean and Standard deviation of the volume of the dead space assessed in a previous study using BIPAP was 40 Study Design : A randomized controlled non-blinded study with cross-over design. In the Trauma and surgical ICU at 185-Hospital (Kasr Alainy Hospitals). Methods : 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 one of the two study groups the BIPAP group or the APRV group . Possible Risk (s) to study population : By adjusting the ventilator parameters properly and continuous monitoring of the patients in the study, there will be no risk facing the patients. Outcome parameter (s): Primary outcome: Physiological dead space will be measured in the