Role of the pericardium and intact chest wall in the hemodynamic response to positive end-expiratory pressure ventilation.

The mechanism for the reduction of both systemic arterial blood pressure and cardiac output (CO) and for the elevation of intravascular pressure which accompanies positive end-expiratory pressure ventilation (PEEP) remains uncertain. The role of the intact pericardium and the chest wall has not been defined. This study examines cardiac transmural pressure with PEEP (0, 4, 8, and 16 cm H2O) administration in nine closed chest, anesthetized, normovolemic dogs utilizing direct measurements of intrapericardial pressure. The experiment was repeated following pericadiectomy and left thoracotomy. The closed chest dogs exhibited significant reductions in blood pressure and CO without detectable changes in transmural pressure. With combined pericardiectomy and left thoracotomy, the responses of both CO and blood pressure to PEEP were similar to those observed with the chest closed. Absolute right atrial pressures were similar at all levels of PEEP and were independent of the presence of the pericardium and the intact chest wall. The rise in absolute left atrial pressure observed with these structures intact was completely abolished by pericardiectomy and left thoracotomy. Thus the major hemodynamic effects of PEEP, namely changes in blood pressure and CO, cannot be accounted for by changes in transmural pressure or influences from the intact chest wall and pericardium.