Circulatory Changes during Growth in the Fetal Lamb

Abstract

The changes in circulation with advancing gestation were investigated in 44 fetal lambs in utero; gestational ages ranged from about 60 days to 150 days. Spinal analgesia was administered to the ewe, polyvinyl catheters were inserted into fetal vessels, and umbilical blood flow was measured by the steady-state diffusion Fick method during antipyrine infusion. Cardiac output, distribution of cardiac output, and actual organ blood flows were calculated from injections of nuclide-labeled microspheres (50µ diam) into a forelimb and into an umbilical or hindlimb vein. Umbilical Po 2 , Pco 2 and pH did not change significantly during gestation. Umbilical blood flow and total cardiac output increased in proportion to fetal weight. The proportion of the combined ventricular output distributed to the placenta decreased from about 50% in the youngest fetuses to about 40% just before term. The proportion of the cardiac output distributed to the lungs, as well as the actual flow in relation to lung weight, increased throughout gestation, with a more rapid rise after about 120 days. There was also a late increase in intestinal flow. Cerebral blood flow increased gradually throughout gestation, both as a proportion of cardiac output and in relation to brain weight. There were no significant changes in percent of cardiac output, or flow related to weight in the kidney, heart, or skin and muscular tissues. The studies suggest that, since lung blood flow is a relatively small proportion of total cardiac output, it is not important in regulating distribution of blood flow, but that the peripheral circulation in skin and muscle, which receives a large percent of fetal cardiac output, is the site where vasomotor responses may effect major redistribution of the fetal circulation.