FETAL ELECTROLYTE AND ACID-BASE RESPONSES TO AMNIOINFUSION - LACTATEDRINGERS VERSUS NORMAL SALINE IN THE OVINE FETUS

OBJECTIVE: We hypothesized that amnioinfusion with normal saline would increase fetal plasma sodium and chloride concentrations, resulting i n a hyperchloremic acidosis, and that these alterations would not occu r after amnioinfusion with lactated Ringer's solution. METHODS: Chroni cally 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, war med 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 o f variance and linear regression. RESULTS: Amniotic fluid electrolyte concentrations changed significantly (P <.001) in both amnioinfusion g roups, resulting in amniotic fluid compositions that were essentially the same as the infused fluid 20 minutes after starting the amnioinfus ion. Significant increases in fetal plasma Na+ and Cl- concentrations (2-3 mEq/L) occurred in the normal-saline infusion group relative to b oth the control and lactated Ringer's groups IP <.001). The lactated R inger's group demonstrated only a modest increase in plasma Na+ (P =.0 4) and no change in plasma Cl- concentration. Fetal arterial pH decrea sed (- 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 signifi cantly alter fetal plasma electrolyte concentrations and blood pH, whe reas amnioinfusion with lactated Ringer's solution results in minimal changes in fetal electrolytes and acid-bare balance. The fetal plasma changes that occur during saline infusion are in the physiologic but n ot the pathologic range.