Positive end expiratory pressure decreases venous return and decreases cardiac output and hence blood pressure decreases and decreased perfusion so urine output can reduce. Pulmonary barotrauma is a frequentcomplicationofPEEPtherapy. Pneumothorax, pneumomediastinum, and interstitial emphysema may lead to rapid deterioration of a patient maintained on mechanical ventilation with an already compromised respiratory status. Positive end-expiratory pressure PEEP is a mode of therapy used in conjunction with mechanical ventilation. At the end of mechanical or spontaneous exhalation, PEEP maintains the patient's airway pressure above the atmospheric level by exeing pressure that opposes passive emptying of the lung. This pressure is typically achieved by maintaining a positive pressure flow at the end of exhalation. This pressure is measured in centimeters of water. PEEP therapy can be effective when used in patients with a diffuse lung disease that results in an acute decrease in functional residual capacity (FRC), which is the volume of gas that remains in the lung at the end of a normal expiration. FRC is determined by primarily the elastic characteristics of the lung and chest wall. In many pulmonary diseases, FRC is reduced because of the collapse of the unstable alveoli. This reduction in lung volume decreases the surface area available for gas exchange and results in intrapulmonary shunting (unoxygenated blood returning to the left side of the hea). If FRC is not restored, a high concentration of inspired oxygen may be required to maintain the aerial oxygen content of the blood in an acceptable range. Applying PEEP increases alveolar pressure and alveolar volume. The increased lung volume increases the surface area by reopening and stabilizing collapsed or unstable alveoli. This splinting, or propping open, of the alveoli with positive pressure improves the ventilation-perfusion match, reducing the shunt effect. After a true shunt is modified to a ventilation-perfusion mismatch with PEEP, lowered concentrations of oxygen can be used to maintain an adequate PaO2. PEEP therapy may also be effective in improving lung compliance. When FRC and lung compliance are decreased, additional energy and volume are required to inflate the lung. By applying PEEP, the lung volume at the end of exhalation is increased. The already paially inflated lung requires less volume and energy than before for full inflation. When used to treat patients with a diffuse lung disease, PEEP should improve compliance, decrease dead space, and decrease the intrapulmonary shunt effect. The most impoant benefit of the use of PEEP is that it enables the patient to maintain an adequate PaO2 at a low and safe concentration of oxygen (< 60%), reducing the risk of oxygen toxicity . Because PEEP is not a benign mode of therapy and because it can lead to serious hemodynamic consequences, the ventilator operator should have a definite indication to use it. The addition of external PEEP is typically justified when a PaO2 of 60 mm Hg cannot be achieved with an FIO2 of 60% or if the estimated initial shunt fraction is greater than 25%. No evidence suppos adding external PEEP during initial setup of the ventilator to satisfy misguided attempts to supply prophylactic PEEP or physiologic PEEP.