Fetal Electrolyte and Acid-Base Responses to Amnioinfusion: Lactated Ringer's Versus Normal Saline in the Ovine Fetus

Abstract

Objective: We hypothesized that amnioinfusion with normal saline would increase fetal plasma sodium and chloride concentrations, resulting in a hyperchloremic acidosis, and that these alterations would not occur after amnioinfusion with lactated Ringer's solution.

Methods: Chronically catheterized fetal sheep (137 ± 1 days' gestation; mean ± SE) were divided into three groups: control (n = 8), infused with normal saline (n = 10), and infused with lactated Ringer's solution (n = 10). the protocol consisted of a 30-minute pre-infusion period, a 1-hour amnioinfusion, and a 1-hour recovery period. During amnioinfusion, warmed solution was infused at a rate of 100 mL/minute for 1 hour. Fetal plasma and amniotic fluid electrolyte concentrations and osmolalities were measured every 20 minutes. Statistical analysis was by analysis of variance and linear regression.

Results: Amniotic fluid electrolyte concentrations changed significantly (P < .001) in both amnioinfusion groups, resulting in amniotic fluid compositions that were essentially the same as the infused fluid 20 minutes after starting the amnioinfusion. Significant increases in fetal plasma Na⁺ and Cl^- concentrations (2-3 mEq/L) occurred in the normal-saline infusion group relative to both the control and lactated Ringer's groups (P < .001). the lactated Ringer's group demonstrated only a modest increase in plasma Na⁺ (P = .04) and no change in plasma Cl^- concentration. Fetal arterial pH decreased (- 0.015 U) in the normal-saline group, and the change in fetal pH was linearly related to the change in plasma Cl^- concentration (r = - 0.532, P = .004).

Conclusions: Normal-saline amnioinfusion can significantly alter fetal plasma electrolyte concentrations and blood pH, whereas amnioinfusion with lactated Ringer's solution results in minimal changes in fetal electrolytes and acid-base balance. The fetal plasma changes that occur during saline infusion are in the physiologic but not the pathologic range.