Fetal lamb ventricles respond differently to filling and arterial pressures and to in utero ventilation.

Right and left ventricular function were investigated in 12 fetal lambs (127-140 days gestation) instrumented with electromagnetic flow sensors on the ascending aorta and the main pulmonary artery, and with vascular catheters. Nine fetuses were equipped with a postductal aortic occluder and the trachea was cannulated in eight. Control arterial blood values were pH 7.36 +/- 0.02 (SD), PCO2 49.3 +/- 2.3 torr, PO2 18.4 +/- 1.7 torr, and hematocrit 37.3 +/- 4.4%. Biventricular function curves relating stroke volume to mean right and left atrial pressure were generated by rapid withdrawal and reinfusion of fetal blood. Both function curves were composed of steep ascending and plateau limbs that intersected at a breakpoint. Stroke volumes at the breakpoints were 0.94 +/- 0.19 ml.kg-1 and 0.63 +/- 0.15 ml.kg-1 for right and left ventricle, respectively (p less than 0.001). During postductal aortic occlusion, arterial pressure increased by 19.3 +/- 7.9 torr while right ventricular stroke volume decreased by approximately 48% and left ventricular stroke volume decreased by approximately 9%. In utero ventilation increased arterial pressure, heart rate, PO2, and oxygen content. Right atrial pressure increased from 3.9 +/- 1.3 to 5.8 +/- 2.9 torr (p less than 0.05); left atrial pressure from 3.5 +/- 1.5 to 10.0 +/- 4.4 torr (p less than 0.05). Aortic flow nearly doubled (112 +/- 29 to 211 +/- 35 ml.min-1.kg-1) (p less than 0.05), and the left ventricular function curve shifted upward. The right ventricular function curve was shifted downward during ventilation. We conclude that the fetal ventricles differ significantly in their outputs, response to changes in arterial pressure, and to the onset of in utero ventilation.