Amniotic fluid volume responses to intra-amniotic infusion of lactate in fetal sheep.

OBJECTIVE

In human and ovine fetuses, severe anemia is associated with elevated fetal blood and amniotic lactate levels and polyhydramnios. In ovine fetuses, intravascular infusion of sodium lactate elevates fetal plasma and amniotic lactate levels and produces polyhydramnios. The present study tested the hypothesis that an elevated amniotic lactate concentration in the absence of an increased fetal plasma lactate would be associated with an increase in amniotic fluid volume (AFV).

METHODS

Eight chronically catheterized, late-gestation fetal sheep were studied over 5 days. Twice each day, we measured blood gases and pH, electrolytes, glucose, lactate, and blood urea nitrogen (BUN) concentrations, as well as osmolality of fetal blood, maternal blood, amniotic fluid, and fetal urine. Amniotic fluid volume was measured once daily. During experimental days 2 to 4, lactic acid was infused into the amniotic compartment to achieve an amniotic lactate concentration of approximately 20 mmol/L. Statistical analysis was by analysis of variance and regression.

RESULTS

Amniotic fluid lactate levels averaged 2.2 +/- 0.4 mmol/L (mean +/- standard error) before infusion and 18.9 +/- 3.3 mmol/L during the 72-hour infusion, falling to 5.8 +/- 1.2 mmol/L postinfusion (P < .001). Fetal plasma lactate averaged 1.8 +/- 0.1 mmol/L on day 1 and increased by 1.4 +/- 0.6 mmol/L on day 4 (P < .001). Fetal urine flow was unchanged and averaged 0.54 +/- 0.08 mL/min over the 5 days. Amniotic fetal volume was 821 +/- 186 mL on day 1, increased nonsignificantly by 99 +/- 95 mL on day 4, and remained unchanged on day 5.

CONCLUSIONS

The present study suggests that if amniotic lactate acts osmotically to increase AFV, the effect is small. Thus, the primary site of action of elevated fetal lactate levels appears to be at the placenta rather than the intramembranous pathway.