RESTRAINING EFFECT OF INTACT PERICARDIUM DURING ACUTE VOLUME LOADING

To determine the effect of the intact pericardium on ventricular end-diastolic pressures (EDP) during acute volume loading, we measured left ventricular (LV) and right ventricular (RV) micromanometer pressure and LV volume using a conductance catheter in eight open-chest, anesthetized dogs. A range of LV pressure and volume was obtained by intravascular volume expansion with the pericardium intact and then over a similar range after removal of the pericardium. Pericardial pressure (P(per)) was calculated using static equilibrium analysis as the difference between LVEDP with the pericardium present and absent at a constant LV volume. At the beginning of the fluid infusion (LVEDP 7.3 +/- 1.7 mmHg and RVEDP 4.4 +/- 2.6 mmHg, mean +/- SD), P(per) was not different from zero (- 1.0 +/- 2.3 mmHg, P not significant). The onset of pericardial restraint (P(per) greater-than-or-equal-to 0 mmHg) occurred when LVEDP was 9.1 +/- 2.9 mmHg and RVEDP was 4.1 +/- 2.9 mmHg. At low cardiac volumes before fluid infusion, RV transmural pressure was positive and significantly greater than the near zero P(per). After the onset of pericardial restraint, however, RVEDP and P(per) increased similarly and were related according to P(per) = 1.1 (+/- 0.34) RVEDP - 4.2 (+/- 2.6) mmHg, standard deviation 0.6 +/- 0.8 mmHg, r = 0.98 +/- 0.10. These data indicate that the intact pericardium behaves in two functionally distinct ways. At low cardiac volumes, P(per) is zero and the pericardium does not affect LV filling. RV transmural pressure is positive and greater than P(per). Under these conditions, RVEDP does not reflect the external pressure of the left ventricle. At higher cardiac volumes, the pericardium substantially restrains LV filling. Under these latter circumstances, the change in RVEDP closely approximates the change in the external pressure of the left ventricle.