SWALLOWING OF LUNG LIQUID AND AMNIOTIC-FLUID BY THE OVINE FETUS UNDERNORMOXIC AND HYPOXIC CONDITIONS

OBJECTIVE: The lungs of the mammalian fetus secrete large volumes of f luid daily. The purpose of this study was to estimate the fraction of the lung liquid that is swallowed as it exits the fetal trachea versus that which enters the amniotic fluid under normoxic and hypoxic condi tions. STUDY DESIGN: In chronically catheterized fetal sheep at 119 to 133 days' gestation the volume of fluid swallowed by the fetus was mo nitored five times per day for three consecutive 24-hour periods: cont rol, hypoxia, and recovery. The Na+, K+, and Cl- concentrations of the swallowed fluid, lung liquid, and amniotic fluid were measured simult aneously. The fraction of the swallowed fluid that originated from the lungs or amniotic fluid was calculated from 24-hour average compositi ons and the assumption that the fetus swallowed only amniotic fluid an d lung liquid. RESULTS: During the control, hypoxia, and recovery peri ods the fetuses swallowed 264 +/- 43 (SE), 92 +/- 23, and 271 +/- 24 m l/kg of fetal weight per day, respectively. As determined from Cl- con centrations, this swallowed fluid was composed of 17.7% +/- 2.7%, 24.8 % +/- 5.8%, and 11.9% +/- 3.4% lung liquid, respectively, with the rem ainder being amniotic fluid. Throughout the three 24-hour observation periods there was an inverse relationship between the net 24-hour swal lowed volume and the fraction of the swallowed fluid that originated f rom the lungs. Calculations based on Na+ concentrations yielded essent ially the same results with slightly more scatter, whereas calculation s based on K+ concentrations were unreliable. CONCLUSIONS: (1) Chlorid e concentrations provide the best of the three index values for a comp ositional analysis of fluids swallowed by the fetus. (2) Under normoxi c conditions around 18% of swallowed fluid is derived from the fetal l ungs. (3) On the basis of published fluid secretion rates for the feta l lung, an average of 50% of the liquid that exits the fetal trachea i s swallowed and the rest mixes with the amniotic fluid.