MILD PRESSURE LOADING ALTERS RIGHT VENTRICULAR-FUNCTION IN FETAL SHEEP

Right ventricular function before and during 10 days of mild pressure loading (10 mm Hg increase in mean pulmonary arterial pressure) was compared with right ventricular function in unloaded near-term fetal sheep. Pressure loading did not alter fetal arterial blood gases or vascular pressures. The right ventricular function curve (stroke volume versus mean right atrial pressure) was not significantly altered by loading. However, the relation between right ventricular stroke volume and increased arterial pressure was dramatically shifted upward, indicating improved ventricular function after the 10-day loading period. Normalized free wall of the loaded right ventricles became thicker (1.2 +/- 0.2 versus 0.9 +/- 0.2 mm/kg, p < 0.01) and heavier (2.7 +/- 0.4 g/kg versus 2.2 +/- 0.4 g/kg, p < 0.05) than control, and the ratio of the equatorial radius of curvature to wall thickness decreased (3.2 +/- 0.5 versus 4.5 +/- 0.9, p < 0.005). Left ventricular free wall and septal weights and thicknesses were not significantly changed. The in vitro diastolic pressure-volume curves of both ventricular chambers of loaded hearts shifted to the left, indicating smaller ventricles than controls at physiological filling pressures. These data suggest the transduction of right ventricular loading effects to the left ventricle. Improved right ventricular function after loading is predicted by the law of Laplace based on the decreased radius of curvature-to-wall thickness ratio.