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.