Effect of umbilical blood flow on transplacental diffusion of ethanol and oxygen.

We studied the effect of varying umbilical blood flow (F) on transplacental diffusion of ethanol and O2 before and during partial occlusion of the fetal abdominal aorta. At steady state, samples of umbilical and uterine arterial and venous blood were drawn simultaneously from chronically indwelling catheters and analyzed for the concentration of ethanol. O2 capacity, O2 saturation, PO2, PCO2, and pH. A decrease in F from 507 to 289 ml/min (P less than 0.001) was associated with a significant (P less than 0.02) increase of the umbilical-uterine venous ethanol concentration difference. This increase was indicative of a 25% diffusional shunt for ethanol on the fetal side of the placenta. The decrease in F caused a decrease in the umbilical O2 delivery-to-control O2 uptake ratio from 3.1 to values between 2.2 and 0.8 and was associated with a decrease in fetal O2 uptake to values between 94 and 56% of control. Uterine venous PO2 increased in response to the decrease in fetal O2 uptake. Umbilical venous PO2 correlated with uterine venous PO2 (r = 0.8, P less than 0.02). The response of fetal O2 uptake to decrease in umbilical O2 delivery was nonlinear and confirmed previous studies, showing that the normal umbilical O2 delivery rate exceeds approximately twofold the minimum necessary to sustain a normal rate of fetal oxidative metabolism.