Quantitative effect of Positive End Expiratory Pressure on Central Venous Pressure in patient with closed and open thorax

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

Central venous pressure is an important indicator ofvolume status of patients. Application of positive end expiratory pressure isknown to influence the measurement of Central Venous Pressure. Therefore, theexact volume status of the patient based on central venous pressure may bedifficult to determine in patient with positive end expiratory pressure. Undermechanical ventilation, positive end expiratory pressure is known to increasethe mean airway pressure in lung. Similarly, Central venous pressure being an intramuralpressure of caval vein, is equal to transmural pressure when extramuralpressure is zero as in case of 0 cm H­­2O positive end expiratorypressure during expiration. The increase in mean airway pressure in lung mayhave proportionate or variable effect on intramural pressure of great veins.The exact quantity of extramural pressure which may influence the intramuralpressure 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 expiratorypressure, the extramural pressure is expected to remain zero. Previous studies reported variableeffect of positive end expiratory pressure on central venous pressure in closed thorax.

Thereforein this study, the effect of positive end expiratory pressure on Central venous pressure will be determined in same patientwith closed and open thorax. In such a scenario, it may be possible to exactlyquantify the effect of positive end expiratory pressure on Central venous pressure in closed thorax compared to openthorax and this will enable Central venous pressure monitoring more reliable for volumestatus in cardiac surgery patients. The study will further help in understandingthe physiology of positive end expiratory pressure effect on Central venouspressure.