Effect of esophageal ligation on amniotic fluid volume and urinary flow rate in fetal sheep

OBJECTIVE: Although the fetus normally swallows large volumes of amniotic f luid each day, it is unclear whether amniotic fluid volume increases after fetal esophageal obstruction or whether fetal urine production changes. Our objective was to determine the effects of fetal esophageal ligation on amn iotic fluid volume and urinary flow rate over time. STUDY DESIGN: Seven late-gestation fetal sheep underwent esophageal ligatio n, and 7 served as time control animals. The urachus was ligated to elimina te urine flow to the allantoic cavity. On days 1, 3, 5, 7, and 9 after surg ery, we measured the composition of amniotic fluid, fetal urine, and fetal and maternal blood, as well as amniotic fluid volume and fetal urinary flow rate. A 3-factor analysis of variance was used for statistical analysis. RESULTS: Amniotic fluid volume did not change with time in the control grou p, averaging 876 +/- 142 mL (mean +/- SEM), and it decreased in the esophag eal ligation group (P = .020), averaging 309 +/- 75 mL on day 9. fetal urin ary flow rate was lower (P = .0063) in the esophageal ligation group (431 /- 27 mL/d) than in the control group (631 +/- 54 mL/d). There were no diff erences in fetal or maternal blood compositions between the two groups. Amn iotic fluid sodium and chloride increased in the ligated animals. CONCLUSION: Polyhydramnios did not occur after esophageal ligation, even th ough the fetuses excreted approximately 4000 mL of urine over the 9-day stu dy period. This suggests that intramembranous absorption is substantially i ncreased. With only small changes in amniotic solute concentrations, intram embranous solute absorption must occur simultaneously with water, suggestin g a near-zero reflection coefficient for solutes. We speculate that fetal u rine, lung secretions, or both contain a factor that increases intramembran ous permeability.