The effects of twenty-four hours of reduced uterine blood flow on fetal fluid balance in sheep

OBJECTIVE: Our aim was to determine the effects of a sustained reduction in uteroplacental perfusion, leading to fetal hypoxia, on determinants of amniotic fluid volume in sheep. STUDY DESIGN: Surgery was performed on five pregnant ewes 110 to 116 days after mating. At 127.3 ± 2.2 days uterine blood flow was reduced for 24 hours, which reduced fetal SaO₂ from 61.9% ± 1.2% to 24.9% ± 0.8%. RESULTS: Fetal urine production was increased from a control value of 193.0 ± 24.0 ml/kg per 24 hours to 279.3 ± 30.0 ml/kg per 24 hours during periods of reduced uterine blood flow and remained above control values for up to 48 hours after the reduced uterine blood flow period. A substantial loss of fetal water and electrolytes occurred through urine, which was associated with changes in the composition of fetal plasma and fetal tracheal, fetal swallowed, and amniotic fluids. Fetal swallowing was reduced throughout the reduced uterine blood flow period from a control value of 200.8 ± 56.0 ml/kg per 24 hours to 32.7 ± 8.4 ml/kg per 24 hours and returned to control levels after the cessation of the reduced uterine blood flow. CONCLUSION: We conclude that 24 hours of reduced uterine blood flow causes major changes in fetal renal function and fetal swallowing that, in spite of an expected reduction in lung liquid production, would increase the flow of fluid and electrolytes from the fetus into the amniotic sac. (AM J OBSTET GYNECOL 1994;170:1442-51.)