Magnesium sulfate protection of fetal rat brain from severe maternal hypoxia

Objective: To determine whether severe maternal hypoxia affects fetal rat p hysical characteristics and causes neuronal damage, and whether magnesium s ulfate can decrease these effects. Methods: At 17 days gestation, rats were randomly assigned to one of four g roups that received saline injections and room air (n = 6), magnesium sulfa te and room air (n = 5), saline and hypoxia (n = 5) or magnesium sulfate an d hypoxia (n = 5). Maternal magnesium sulfate or saline injections were giv en for 4 hours. In groups 3 and 4 this was followed by a hypoxia chamber pr otocol that included a gas mixture of 9% oxygen and 3% carbon dioxide for 2 hours. After 72 hours of recovery, fetuses were delivered abdominally, per fused transcardially, and brains removed intact. Fetal body and brain weigh t and size were measured. Brains were embedded in paraffin, sectioned, and stained. A neuropathologist masked to the protocol performed histologic gra ding of brain regions. Results: Exposure to the hypoxia chamber resulted in decreased maternal oxy gen tension and pH (from 82.8 +/- 20.0 to 49.2 +/- 14.4 mmHg, and from 7.37 +/- 0.05 to 7.20 +/- 0.04, respectively; P < .005). Magnesium sulfate admi nistration resulted in higher magnesium levels in blood (from 1.52 +/- 0.2 to 3.77 +/- 0.7 mg/dL; P < .001). The hypoxia protocol resulted in a signif icant decrease in fetal body and brain size, but not weight. Hypoxia also c aused an increase in the proportion of fetal rats that had brain injury, in cluding shrinkage of cells and karyorrhexis in the hippocampus and thalamus (from 0% to 38.1% and 38.9%, respectively; P < .05). Magnesium sulfate red uced these effects on fetal brain histopathology and size. Conclusion: Severe maternal rat hypoxia resulted in significant fetal neuro nal damage and decreased fetal body and brain size. Maternal magnesium sulf ate administration reduced the effect of hypoxia on fetal brain histopathol ogy and size without affecting body size. (Obstet Gynecol 2000;96:124-8. (C ) 2000 by The American College of Obstetricians and Gynecologists.).