Fetal blood volume, urine flow, swallowing, and amniotic fluid volume responses to long-term intravascular infusions of saline.

The purpose of this study was to determine the effects of a long-term infusion into the fetal circulation on fetal and amniotic fluid dynamics. In 10 chronically catheterized fetal sheep averaging 130 +/- 1 (SE) days' gestation, a balanced, isotonic electrolyte solution (Isolyte S) was infused continuously for 5 days into a fetal vein at a rate of 0, 1, 2, 4, and 0 L/day, respectively. During the infusion, fetal blood volume increased by a maximum of 6.4 +/- 2.0% (p less than 0.001), and the daily swallowing of amniotic fluid doubled (p less than 0.001). Fetal urine flow increased (p less than 0.0001) above preinfusion rates by a volume equal to the infusion rate plus the increase in swallowing, whereas renal excretion of sodium and chloride increased by the amount infused. The increase in the plasma concentration of atrial natriuretic factor (p less than 0.0001) and the decrease in arginine vasopressin (p less than 0.05) were not linearly related to urine flow changes. Amniotic fluid volume increased (p less than 0.0001) by 20% of the infused volume. All values returned to normal on day 5 except amniotic fluid volume, which remained elevated. Estimated allantoic fluid volume at the end of day 5 was 800 ml above normal. Thus it appears that on a long-term basis, the ovine fetus eliminates infused water and electrolytes through its kidneys rather than across the placenta. Although all of the infused volume left the fetus through its kidneys, only 30% of the infused volume remained in the amniotic and allantoic fluid compartments, suggesting transfer to the mother by unknown mechanisms.