Maternal and postnatal hypermagnesemia and the meconium plug syndrome.

Thirty dated pregnant Srague-Dawley rats received IM MgSO4 (750 mg/kg/ X 5 days) until birth. Fifteen untreated pregnant Sprague-Dawley rats of similar gestation served as controls. A total of 453 rats were born. Ninety neonates (two from each of 45 liters) were randomly removed before suckling, sacrificed, and the water content of the meconium and serum magnesium determined. The remaining 363 bewborn rats were observed for delayed passage of meconium or abdominal distention. Hypermagnesemic (6.3 +/- 0.5 mg/100 ml) neonates had neither abdominal distention nor delayed passage of meconium. The water content of meconium in hypermagnesemic rats (84.7 +/- 4.2% and in untreated controls (85.5 +/- 3.8%) were statistically similar. Dated pregnant mongrel dogs were given magnesium sulfate by continuous intravenous infusion and by bolus intravenous injection. The water content of meconium, transit time, and basic electrical rhythm (BER) of treated neonates were similar to untreated neonatal controls. The water content of meconium, transit time, and BER of neonatal dogs made hypermagnesemic (12.0 +/- 1.2 mg/100 ml) in the postnatal period were similar tp untreated neonatal controls. In adult and newborn pupies, receiving a continuous intravenous magnesium sulfate infusion, deep tendon reflexes ceased at serum magnesium levels of 9.0-10.0 mg/100 ml, spontaneous respiration at 14.0-15.0 mg/100 ml, and BER was suppressed (transiently) at 20.0-25.0 mg/10 ml. These data suggest that high (sublethal) doses of MgSO4 in pregnant and newborn animals have little effect on BER, meconium transit time or water content of the neonates. No correlation between MgSO4 treatment and the meconium plug syndrome was observed in the experiments.