Phenytoin and magnesium sulfate effects on fetal heart rate tracings assessed by computer analysis

To compare the effects of maternally administered phenytoin and magnesium sulfate on the fetal heart rate (FHR) using computer analysis.

Thirty-six nonlaboring preeclamptic women between 27-41 weeks' gestation were treated with either phenytoin or magnesium sulfate in a nonrandomized fashion. All fetuses were deemed to be well by traditional electronic-biophysical criteria. One-hour FHR recordings were analyzed by computer analysis before treatment. No other medications were administered. Tracings recorded 1 hour after drug administration were analyzed. Therapeutic serum levels were achieved in both groups before FHR tracings were reevaluated. Statistical analysis used paired Student t test, with significance set at P < or = .05.

There were no differences in birth weight, gestational age, Apgar scores, or computer-analyzed FHR characteristics between the groups before treatment. Magnesium sulfate reduced significantly the frequency of accelerations of ten and 15 beats per minute; caused a 62% reduction in reactivity, defined as accelerations of 15 beats per minute in 20 minutes of FHR tracing (but no change in reactivity with accelerations of ten beats per minute); and reduced short- and long-term variability. Phenytoin reduced short-term variability only.

Phenytoin does not confound the computer analysis of FHR tracings and may offer some advantage over magnesium sulfate when used for prophylaxis against eclampsia.