The effect of umbilical vein occlusion on fetal oxygenation, cardiovascular parameters, and fetal electroencephalogram.

In 11 fetal sheep experiments, the blood pressure in the fetal aorta (FA) and in the umbilical vein (UV) was measured following umbilical vein occlusion (UVO), as was the fetal heart rate (FHR), pH, Pco2, and oxygen saturation (So2) in both fetal vessels, and umbilical blood flow (Qumb) of the common UV. The fetal electroencephalogram was recorded continuously throughout the experiment. The results (No. = 17) were grouped according to the response of FA So2 into moderate (So2greater than40 per cent, mean 49.8, S.D. 6.3, and severe So2less than40 per cent, mean 20.4, S.D. 9.2). After 8 to 10 seconds, the fetal blood pressure in FA increased. Umbilical vein blood pressure increased to 25 mm. Hg (S.D. 8) and 35 mm. Hg (S.D. 9) in the moderate and severe groups, respectively. As a result of the decreased perfusion pressure (FA-UV blood pressure) across the fetal side of the placenta, the Qumb fell from 147 ml. per kilogram per minute (S.D. 57) and 30 ml. per kilogram per minute (S.D. 55) to 84 ml. per kilogram per minute (S.D. 48) and 120 ml. per kilogram per minute (S.D.22), respectively. The fall in FA So2 was related to the decrease in Qumb: FA So2=37.6 x log Qumb - 22.8 (2alphaless than 0.001). There was a mild decrease in So2 from 70 to 55 per cent when Qumb fell from 250 to 120 ml. per kilogram per minute. Below 80 to 120ml. per kilogram per minute, the fall in FA So2 was almost linear. The So2 in the UV remained constant so that arteriovenous difference for oxygen (AV Do2) increased. Oxygen consumption decreased almost linearly when Qumb fell below 80 to 120 ml. per kilogram per minute. The fetal electroencephalogram showed no significant change in voltage and the faster activities. From these observations, it is concluded that a decrease in Qumb following UVO jeopardizes the fetus only if a critical Qumb of 80 to 120 ml. per kilogram per minute and fetal artery So2 of 50 to 60 per cent is achieved.