Severe right ventricular pressure loading in fetal sheep augments global myocardial blood flow to submaximal levels

Background. It has previously been shown that the fetal right ventricle (RV) is sensitive to changes in arterial pressure and that its stroke volume is significantly reduced with acute increases in pulmonary arterial pressure. However, the myocardial blood flow (MBF) response to increases in pulmonary arterial pressure have not been investigated in the fetus. Methods and Results. To assess whether the RV afterload sensitivity to arterial pressure is associated with limitation in MBF, seven fetal lambs were instrumented at 130 days of gestation with a pulmonary arterial occluder and intravascular catheters. RV stroke volume was measured by an electromagnetic flow probe and MBF by 15-μm labeled microspheres. MBF was determined at baseline and during incremental increases in pulmonary arterial pressure. Maximal MBF was determined in seven additional fetuses during adenosine infusion. The highest tolerated pressure was associated with a 50% reduction in RV stroke volume. The highest pulmonary arterial occlusion pressure resulted in a doubling of MBF to all regions of the heart (266±99 to 504±158, 193±69 to 387±100, and 171±66 to 338±134 ml/min/100 g for the RV, septum, and left ventricle, respectively). The best correlation for increases in both RV and global MBF was the RV heart rate-systolic pulmonary pressure product. Adenosine infusion was associated with a threefold increase in global MBF that was significantly greater than the MBF achieved during pulmonary arterial occlusion. Conclusions. The fetal RV sensitivity to acute pressure loading is not associated with limitation of MBF. The fetal myocardium has a remarkable flow reserve that allows for preservation of function during acute increases in arterial pressure.