Fetal cardiovascular and fluid responses to maternal volume loading with lactated Ringer's or hypotonic solution

To determine whether elevations in maternal vascular pressures or reductions in maternal osmolality would promote fluid transfer to the fetus, we intravenously infused either lactated Ringer's solution or diluted (hypotonic) lactated Ringer's solution continuously over 4 hours into late-gestation pregnant sheep. During the Ringer's solution infusion, the increases in maternal arterial (20.7 +/- 1.7 mm Hg, mean +/- SE) and venous (6.6 +/- 0.9 mm Hg) pressures were significantly greater (p less than 0.00001) than those during the hypotonic infusion (6.6 +/- 1.5 and 1.7 +/- 0.6 mm Hg, respectively). The maternal osmolality changes during the Ringer's infusion (-5.7 +/- 1.2 mOsm/kg at 1 hour and +6.8 +/- 1.1 mOsm/kg at 1 hour and -15.9 +/- 2.5 mOsm/kg at 5 hours). Fetal vascular pressures and blood volume were unchanged during either infusion. Fetal heart rate decreased by 15 to 20 beats/min by 1.5 hours of infusion in both groups but remained decreased only in the hypotonic group. Fetal urine flow decreased at the end of the Ringer's infusion and increased during the hypotonic infusion. These urine flow changes correlated with opposite changes in fetal plasma osmolality. The four-quadrant amniotic fluid index tended to increase in both groups, with an overall nonsignificant increase of 32% +/- 16% 1 hour after the infusions. In summary, our findings suggest that (1) acute increases in maternal vascular pressures do not appear to promote fluid transfer to the ovine fetus and (2) acute decreases in maternal osmolality result in a small shift of fluid into the fetus as evidenced by an increase in fetal urine flow.