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

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

Methods: 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.

Results: 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.

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