Effects of hemorrhage on umbilical venous return and oxygen delivery in fetal lambs.

The effect of acute hemorrhage (21.7 +/- 1.6%, mean +/- SE of the estimated blood volume) on the distribution of the umbilical venous return (UVR) and oxygen delivery to fetal organs (radionuclide microsphere technique) was studied in 10 chronically catheterized fetal lambs. Hemorrhage decreased UVR and total oxygen delivery to the fetus by 23 and 28%, respectively (P less than 0.001). A greater proportion of the UVR bypassed the liver through the ductus venosus (DV) (P less than 0.01). Consequently, DV blood flow was relatively maintained, and liver blood flow and oxygen delivery derived from the UVR decreased by 41 and 45%, respectively (P less than 0.002). The proportion of cardiac output constituted by DV blood increased by 30% (P less than 0.01). The changes in the distribution of the DV blood were similar to the changes in the distribution of cardiac output. As a result the proportion of organ blood flow and oxygen delivery derived from DV blood increased by 30% in both the upper and lower body organs. These studies show that fetal hemorrhage has a marked effect on the distribution of the UVR to the liver. The preferential distribution of the DV blood to the heart and brain is maintained but not enhanced. Our results indicate that changes in distribution of the DV-derived blood and oxygen following fetal hemorrhage is determined primarily by the responses of the arterial circulation, i.e., circulatory afterload.