Volume-dependent effects of positive airway pressure on intracavitary left ventricular end-diastolic pressure.

To test the hypothesis that the effects of positive end-expiratory airway pressure (PEEP) on intracavitary left ventricular end-diastolic pressure (LVEDP) depend on the ventricular filling conditions under which PEEP is applied, the effects of PEEP on pressure in and around the left ventricle were determined before and after stepwise expansion of intravascular blood volume in 10 closed-chest dogs. Over a range of 0 to 20 cm of water, PEEP progressively increased both intrapericardial and intracavitary right ventricular end-diastolic pressures. These increases in pressure around the left ventricle were approximately linear and were relatively unaffected by volume loading. At the same time, PEEP always decreased transmural LVEDP by decreasing ventricular filling. However, transmural LVEDP fell more when ventricular volume was initially large, due to the nonlinear relationship between left ventricular transmural pressure and volume. As a result, intracavitary LVEDP (which reflected the sum of decreased transmural LVEDP and increased external pressure) increased when baseline ventricular volume was small and decreased when baseline ventricular volume was large. At intermediate volumes the fall in transmural pressure equaled the rise in external pressure, and intracavitary LVEDP did not change. These findings demonstrate that changes due to PEEP in intracavitary LVEDP are a complex function of increased intrathoracic pressure, decreased ventricular filling, and the operative level of left ventricular compliance.