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

Thomas R. Moore, MD

Robert A. Bace, PhD

Division of Perinatal Medicine, Department of Reproductive Medicine, University of California San Diego, La Jolla, California

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.

Key Words: Fetus • ovine • amnioinfusion • sodium • potassium • chloride • pH

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