Effect of Maternal Magnesium Sulfate Treatment on Fetal Heart Rate Variability

The effect of maternal magnesium sulfate treatment on fetal heart rate variability (FHRV) was evaluated by comparing the maternal serum magnesium level drawn during labor to the degree of FHRV seen concurrently on fetal rate heart monitor strips. The records of 57 patients treated with intravenous magnesium sulfate for hypertensive disorders of pregnancy were studied. Fetal heart rate variability was measured by both subjective assessment and objective measurement. The difference in beats/min between the average high and low fetal heart rates, at the time the magnesium level obtained, was used as the objective measurement. Labor records were reviewed and neonatal outcome recorded. Data from individual patients not treated with analgesics were analyzed separately in addition. Statistical methods used included analysis of variance, Student's t-test, and Chi square analysis. The range of magnesium levels was 2.3-7.4 mEq/L. Thirty-four women had single determinations of magnesium levels and 23 had multiple determinations. Statistical analysis showed no significant change in FHRV with increasing maternal magnesium levels. In addition, no significant effect of maternal magnesium level on FHRV was noted in the group that did not receive meperidine, morphine, or epidural anesthesia. Neonatal outcome, assessed by Apgar scores, did not vary with maternal magnesium level. We conclude that maternal magnesium sulfate treatment, in the therapeutic ranges normally obtained, does not significantly affect fetal heart rate variability.