The circulatory effects of hematocrit variations in normovolemic newborn lambs

This study describes the hemodynamic effects of polycythemia without hypervolemia acutely created in seven newborn lambs by means of an exchange transfusion with maternal packed cells. Pressures are measured continuously in the main pulmonary artery, aorta, right atrium, and a pulmonary vein. Pulmonary and systemic flows, and left-to-right and right-to-left shunts through the ductus arteriosus and/or the foramen ovale were calculated according to the Fick principle. The changes in the vascular resistances were noted. With increasing hematocrit a constant drop was observed in pulmonary and systemic flows and in the amount of blood shunting left to right through the ductus. This was associated with an increase in vascular resistances that was more pronounced in the pulmonary circulation. At a hematocrit of about 70 per cent, a turning point was observed: Pulmonary and systemic vascular resistances became equal and a right-to-left shunt appeared through the ductus. This created a peripheral hypoxia associated with a fall in the systemic vascular resistance and pressures. The same hemodynamic changes noted in this study can be expected in newborn infants with symptomatic polycythemia. For a rational therapeutic approach, it is important to know whether or not the polycythemic neonate is hypervolemic. In the absence of hypervolemia, simple bleeding could be harmful by decreasing an already very low cardiac output.