Amniotic fluid volume responses to amnio-infusion of amniotic fluid versus lactated Ringer's solution in fetal sheep

The present study tested the hypothesis that an intra-amniotic infusion of amniotic fluid (AF) would produce a more sustained increase in AF volume than an infusion of lactated Ringer's solution.

Five chronically catheterized, late-gestation fetal sheep were studied over two 5-day periods with AF volume measured daily. After baseline measurements on day 1, 1 L of either warmed, previously frozen AF or warmed lactated Ringer's solution was infused intra-amniotically over 60 minutes. Two days later, the other fluid was infused. During the second week, fluids were infused in the opposite order. Analysis of variance (ANOVA) was used for statistical testing.

Following intra-amniotic infusion (n = 20) of 1007 +/- 7 (SE) mL of either AF or Ringer's solution, intra-amniotic retention of the infused fluid was only moderate after 1 day (37.2% +/- 7.9%, P <.001) and was not significantly different from zero after 2 days (16.5% +/- 9.5%, P =.1). There were no significant differences in AF volume following infusion of AF versus lactated Ringer's solution or the order in which they were infused. AF compositional changes were similar except that pH and bicarbonate concentration were reduced as expected immediately after lactated Ringer's solution with a return to normal values after 1 day. AF lactate increased after lactated Ringer's solution infusion, declining to baseline values after 2 days. Fetal urine flow rate increased by 75% +/- 24% at 1 day postinfusion and there was no difference between infusates.

The expansion of AF volume over 2 days following amnio-infusion does not appear to depend on minor compositional differences or the presence of microconstituents such as hormones, cytokines, or growth factors that are normally present in AF.