The long-term goal of this project is to understand the mechanisms underlying the pathophysiology of chronic anemia in the fetus. The overall hypothesis we wish to test for this period is that coronary vascular growth in fetal anemia is sufficient to maintain oxygen delivery, myocardial metabolism and function. (1) To determine if left ventricular function is altered during chronic anemia we will study left ventricular stroke volume-left atrial pressure relationships and left ventricular sensitivity to afterload. To determine if oxygen consumption per gm of heart tissue remains similarly related to cardiac stroke work consumption per gm of heart tissue we will measure metabolism across the left ventricle. (2) To quantify the increase in coronary vascularity during chronic anemia we will use two methods: the physiologic measure of maximal conductance and standard morphologic measures of capillary density. To test if coronary flow reserve is maintained in chronically anemic fetuses by vascular growth we will measure myocardial blood flow and coronary pressure-flow relationships at rest and in response to maximal vasodilation with adenosine during progressive fetal anemia. To differentiate changes due to growth from changes due to decreased viscosity we will study adenosine coronary pressure-flow relationships in chronically anemic fetuses in which the hematocrit is restored to normal values by transfusion of red blood cells. (3) We hypothesize that the oxygen sensing erythropoietin enhancer HIF-1 (hypoxia inducible factor-1) increases VEGF (vascular endothelial growth factor) stimulating angiogenesis, thus leading to new blood vessels. To test if HIF-1 and VEGF are increased in anemic hearts we will measure HIF-1 and VEGF by Western analysis in control and anemic tissues. (4) To determine if blocking VEGF reduces coronary blood flow responses to chronic anemia we will measure maximal conductance in anemic fetuses in which anti-VEGF antibody is infused by arterial injection into the coronary artery. These studies will extend the understanding of the mechanisms by which the fetal heart changes its circulation and metabolism during chronic anemia in order to do more work with a lower coronary arterial oxygen content. The project will also add information concerning the trophic factors responsible for blood vessel growth in the heart.