Pressures in Umbilical Arteries and Veins of the Fetal Lamb in Utero

¹ From the Department of Anatomy, University of Illinois College of Medicine, Chicago, Illinois

During the last third of gestation in the ewe (149 days), the first branch of an umbilical artery and of an umbilical vein was catheterized. Access was obtained by going through a small opening in the uterus to the vessels, leaving the membranes intact. Blood pressures in the main artery and vein were recorded, along with carotid blood pressure and respiratory activity in the ewe. Normal venous pressures were 20–35 mm Hg; arterial pressures, 50–70 mm Hg. The pressure gradient across the placenta, 10–20 mm Hg. Arterial and venous pulsations have been observed, most often synchronous, but at times out of phase. Pressure on the uterine contents by strong respiratory movements or use of Pitocin have caused equal rises of pressure in arteries and veins. When mild and severe hypoxia occur in the ewe (10–13% O₂ and 6% O₂ in the inspired air), changes in blood pressure occur, but more commonly in the artery than in the vein. Venous pressure falls with terminal hypoxia, indicating a failure of cardiac output. Venous pressure has been seen to remain near normal levels even though heart rate increases or decreases and even though arterial pressure go up or down. When umbilical vein pressure fails, however, fetal death is imminent.