Composition and volume of fluid swallowed by fetal sheep

Oesophageal cannulations were performed in seven fetal sheep between 109 and 128 d of gestation. Fluid swallowed by the fetuses was collected externally and periodically returned to the fetal stomach by a peristaltic pump. The volume of fluid swallowed daily was calculated from the movement of water in a parallel pump circuit. Samples of oesophageal fluid were removed from the collection bag at the same time as tracheal and amniotic fluid samples. Oesophageal fluid was very viscous and was usually intermediate in colour between tracheal and amniotic fluids. After reduction of viscosity by dithiothreitol, the compositions of the three fluids were analysed. Mean concentrations of Na+, Cl- and protein, and mean osmolarity of oesophageal fluid, were intermediate between values obtained in tracheal and amniotic fluids. [K+] was often greater in oesophageal fluid than would be expected by the relative contribution of tracheal and amniotic fluids as indicated by [Na+] and [Cl-] in the three fluids. It is suggested that the high viscosity of oesophageal fluid and its relatively high [K+] may result from fetal salivary secretions. Ionic concentrations in individual sets of samples show that the proportions of tracheal and amniotic fluids swallowed by the fetus are variable from day to day. The mean daily volumes of fluid swallowed by the seven fetuses ranged from 98 to 577 ml, and are in agreement with previously published data.