Filling and arterial pressures as determinants of RV stroke volume in the sheep fetus

Right ventricular (RV) function was investigated in nine fetal lambs (125-130 days gestation) that were instrumented with pulmonary artery electromagnetic flow sensors and vascular catheters. Control arterial CO₂ and O₂ tension, pH, and hematocrit values were 46.1 ± 1.6 (SD) Torr, 20.6 ± 1.8 Torr, 7.39 ± 0.02, and 31 ± 5.3%, respectively. Control values for right ventricular output (247 ± 75 ml.min^-1.kg^-1), stroke volume (SV, 1.5 ± 0.4 ml.kg^-1), right atrial pressure (3.7 ± 1.2 mmHg), heart rate (166 ± 18 beats.min^-1), and arterial pressure (AP, 43 ± 4 mmHg) were unchanged by administration of atropine and propranolol. Withdrawal and infusion of fetal blood with or without concomitant infusion of nitroprusside or phenylephrine produced RV function curves at low, normal, and high arterial pressures. All function curves had a steep ascending limb and a plateau. The breakpoint joining the limbs of the control curve was right atrial pressure 3.4 ± 1.2 mmHg and SV 1.5 ± 0.4 ml.kg^-1. Increased AP shifted the breakpoint downward. Linear regression of SV on AP from 15 to 95 mmHg at right atrial pressure greater than breakpoint was SV = -0.016 ml.kg^-1 mmHg^-1.AP + 2.25 ml.kg^-1.