Swallowing, urine flow, and amniotic fluid volume responses to prolonged hypoxia in the ovine fetus

OBJECTIVE: Four days of hypoxia produce an extensive fetal polyuria with little change in amniotic fluid volume in the ovine fetus. We hypothesized that fetal swallowing and intramembranous absorption would increase with prolonged hypoxia to offset the polyuria. STUDY DESIGN: After a 24-hour normoxic period, nine ovine fetuses were subjected to 4 days of hypoxia induced by lowering maternal inspired oxygen content. Seven fetuses were monitored for 5 days as normoxic time controls. Measurements included fetal swallowed volume by a computerized system with Transonic flow probes, urine production by gravity drainage, and amniotic fluid volume by an indicator dilution technique. Data were averaged over 12-hour intervals, and a three-factor repeated-measures analysis of variance was used for statistical testing. RESULTS: During days 2 to 5, arterial oxygen tension was 20.7±1.1 (SE) mm Hg in the normoxic and 13.9±0.8 mm Hg in the hypoxic fetuses (P<.0001). Urine flow was unchanged over time in the normoxic fetuses and increased gradually from 693±88 to 2189±679 mL per day during hypoxia (P<.0001). The prehypoxia swallowed volume was similar in the two groups, averaging 447±95 mL per day. Although transiently decreased in eight of nine hypoxic fetuses, the 12-hour average swallowed volumes were not significantly different at any time in the hypoxic versus normoxic fetuses (P = .62). Amniotic fluid volume increased in the hypoxic fetuses relative to that in the normoxic fetuses (520±338 mL vs −226±136 mL, P<.01), although the increase was small (P<.01) relative to the excess volume of urine (4269±1306 mL). Estimated intramembranous absorption increased from 209±95 mL per day during normoxia to average 1032±396 mL per day during hypoxia. CONCLUSIONS: The current study supports the concept that prolonged hypoxia produces a progressive fetal polyuria with relatively small changes in amniotic fluid volume. Concomitantly, hypoxia does not induce prolonged changes in fetal swallowing; rather, intramembranous absorption greatly increases, thereby preventing severe polyhydramnios.