Pulmonary and pericardial limitations to diastolic filling of the left ventricle of the lamb.

To determine which tissues limit left ventricular (LV) filling during the early neonatal period we studied LV end-diastolic pressure-diameter relationships in two groups of halothane-anesthetized lambs (five 1-h-old lambs and six 2- to 17-day-old lambs). First, we assessed LV end-diastolic pressure-diameter relations over a range of 5-20 mmHg, when the chest and the pericardium were closed; second, after the chest wall and lungs had been retracted from the heart; and finally, after the pericardium was retracted. In the oldest lambs LV diameter increased significantly [3.5 +/- 0.3% (SE) at an end-diastolic pressure of 10 mmHg; P < 0.05] after retracting the chest wall and the lungs and when the pericardium was still intact. By contrast, retracting the chest wall and lungs did not significantly change LV diameter in the youngest lambs. In both age groups LV diameters were greatest (P < 0.05) after the pericardium was subsequently retracted (the increase was evident at all end-diastolic pressures and averaged 9.0% relative to the closed-chest, closed-pericardium series). These studies confirm that the thoracic tissues substantially limit LV filling in young lambs. Immediately after birth this limitation is almost exclusively related to the pericardium, whereas in older lambs the chest wall-lung combination and the pericardium each contribute.