Positive end expiratory pressure (PEEP) is the positive pressure applied to lung alveoli causing it to remain distended at the end of expiration in mechanically ventilated patients. Positive end expiratory pressure decreases the work done in collapsing and distending alveoli at each cycle, ensuring maximum volume for ventilation and perfusion. The lungs and heart are enclosed in pleural cavity and pericardial cavity respectively within the large thoracic cavity, making each component interdependent on each other. The extrinsic positive airway pressure applied create pressure within the alveoli and pleural cavity which is subsequently transmitted to the adjacent structures causing increase in intra cardiac and large vessels intramural pressures. This pressure is being measured by comparing it to atmospheric pressure, called as Central Venous Pressure (CVP). 

Central venous pressure is an important indicator of volume status of patients. Application of positive end expiratory pressure is known to influence the measurement of Central Venous Pressure. Therefore, the exact volume status of the patient based on central venous pressure may be difficult to determine in patient with positive end expiratory pressure. Under mechanical ventilation, positive end expiratory pressure is known to increase the mean airway pressure in lung. Similarly, Central venous pressure being an intramural pressure of caval vein, is equal to transmural pressure when extramural pressure is zero as in case of 0 cm H­­₂O positive end expiratory pressure during expiration. The increase in mean airway pressure in lung may have proportionate or variable effect on intramural pressure of great veins. The exact quantity of extramural pressure which may influence the intramural pressure in closed thorax has been found to be variable across the literature. Furthermore, in case with open thorax even with the application of positive end expiratory pressure, the extramural pressure is expected to remain zero. Previous studies reported variable effect of positive end expiratory pressure on central venous pressure in closed thorax. 

Therefore in this study, the effect of positive end expiratory pressure on Central venous pressure will be determined in same patient with closed and open thorax. In such a scenario, it may be possible to exactly quantify the effect of positive end expiratory pressure on Central venous pressure in closed thorax compared to open thorax and this will enable Central venous pressure monitoring more reliable for volume status in cardiac surgery patients. The study will further help in understanding the physiology of positive end expiratory pressure effect on Central venous pressure.