Increased urinary flow without development of polyhydramnios in response to prolonged hypoxia in the ovine fetus.

Objective: In the ovine fetus subjected to 24 hours of hypoxia, urinary flow is normal within a few hours from the onset of hypoxia and there is a maintained inhibition of swallowing. We hypothesized that 4 days of fetal hypoxia would lead to polyhydramnios.
Study Design: Five late-gestation fetal sheep were subjected to hypoxia for 4 days and 7 other late-gestation fetal sheep served as time control animals. Fetal hypoxia was produced on postsurgical days 5 through 9 by continuous intratracheal nitrogen insufflation to the ewe. On days 3, 5, 7, and 9 after surgery, amniotic fluid volume, fetal urinary flow rate, and the compositions of maternal and fetal blood, amniotic fluid, and fetal urine were measured. A 3-factor analysis of variance was used for statistical analysis.
Results: During the period of experimental hypoxia the mean (+/-SE) fetal PaO(2) was 16.0 +/- 0.6 mm Hg, versus 21.2 +/- 0.7 mm Hg in control sheep (P <.001). Fetal hypoxia was associated with increased urinary flow on days 7 and 9, averaging 1410 +/- 310 and 2101 +/- 345 mL/d, respectively, versus 585 +/- 92 and 699 +/- 78 mL/d, respectively, in control animals (P <.001). Amniotic fluid volume was unchanged with time and averaged 960 +/- 159 mL in hypoxic fetuses on postsurgical days 7 through 9 and 851 +/- 130 mL in control animals (P =.60). Fetal blood lactate increased in the hypoxic animals, averaging 3.4 +/- 2.1 mmol/L versus 1.6 +/- 0.3 mmol/L in control animals (P =.02). Fetal urinary excretions of sodium, potassium, chloride, and lactate increased significantly during hypoxia, by 170% to 400%.
Conclusion: Four days of nitrogen-induced hypoxia in the ovine fetus resulted in excess fetal urinary flow approximating 1000 mL/d greater than normal without the development of polyhydramnios. Because amniotic fluid volume did not change and hypoxia is a known inhibitor of fetal swallowing, we speculate that intramembranous absorption of amniotic water, electrolytes, and lactate increased.