The effect of magnesium sulfate on fetal heart rate parameters: A randomized, placebo-controlled trial

OBJECTIVE: We sought to determine the effect of magnesium sulfate on fetal heart rate baseline value, variability, and acceleratlon-deceleration patte rn. STUDY DESIGN: Normal, nonlaboring pregnant patients at >30 weeks' gestation were recruited. Baseline fetal heart rate monitoring for I hour was perfor med. After an 800-kcal meal, patients were randomized to re; ceive either a n intravenous loading dose of 6 g of magnesium sulfate in 100 mt of isotoni c sodium chloride solution or 100 mt of isotonic sodium chloride solution a lone. Subsequently, patients in the magnesium sulfate group received a 2-g/ h intravenous infusion for 3 hours at a rate of 125 mL/h. Patients randomiz ed to the sodium chloride solution group received a sodium chloride solutio n infusion at a similar rate (unlabeled intra; venous bags). Maternal blood was drawn at 0, I, and 3 hours for determination of total and ionized magn esium and calcium, electrolyte, and glucose levels. One hour of fetal heart rate monitoring was repeated at 1 and 3 hours of infusion. Tracings were i nterpreted without identifiers (of time or group) by using the National Ins titute of Child Health and Human Development fetal heart rate monitoring gu idelines. RESULTS: Magnesium sulfate administration resulted in decreased fetal heart rate baseline values and variability in the third hour. The fetal heart ra te baseline value was 134.4 +/- 6.3 versus 136.6 +/- 6.4 beats/min before i nfusion (P > .05), 134.4 +/- 7.1 versus 135.1 +/- 6.6 beats/min in the firs t hour (P > .05), and 134.6 +/- 7.1 versus 132.3 +/- 7.6 beats/min in the t hird hour (P < .05) in the sodium chloride solution group versus the magnes ium sulfate group, respectively. Petal heart rate Variability (grades 1-5) was 2.75 +/- 0.33 versus 2.82 +/- 0.29 before infusion (P > .05), 2.81 +/- 0.30 versus 2.84 +/- 0.28 in the first hour (P > .05), and 2.71 +/- 0.52 ve rsus 2.67 +/- 0.36 in the third hour in the sodium chloride solution group versus the magnesium sulfate group, respectively (P < .05). Magnesium sulfa te blocked the positive correlation between gestational age and number of a ccelerations found in control subjects, No significant decelerations were i dentified. CONCLUSIONS: Prolonged administration of magnesium sulfate was associated w ith decreased fetal heart rate baseline values and variability. Given the s mall magnitude of these changes, the clinical significance of these finding s is questionable. Magnesium sulfate inhibition of the increasing number of accelerations with gestational age needs to be considered when fetal well- being is assessed.