ABSTRACT
Forty-two preeclamptic patients between 36 and 41 weeks gestation were investigated
for baseline fetal heart rate (FHR) variability 1 hour after the initiation of magnesium
therapy, at midlabor, and at the time of delivery. With a standard regimen of intravenous
magnesium sulfate therapy, the mean maternal serum magnesium levels at the three different
sampling times were statistically different (f = 6.94, p < 0.01 by one-way analysis
of variance), documenting the cumulative effect of continuous intravenous magnesium
administration. The majority of the fetuses (86%) that exhibited a decrease in long-term
FHR variability were associated with a maternal serum magnesium level above the lower
limit of the therapeutic range (4.8 mg/dl). Of all fetuses whose maternal serum magnesium
levels were 4.8 mg/dl or greater at delivery, only 40% exhibited a decrease in FHR
variability. A higher mean maternal magnesium level, a higher mean cord blood magnesium
level, and a higher total dose of magnesium sulfate were observed in the group of
fetuses showing a decrease in FHR variability than in the group showing no change
in FHR variability. The good fetal outcome seen in both study populations suggests
that the effect of magnesium sulfate on FHR variability is a transient, reversible
phenomenon that should not be considered as a sign of fetal distress.
