Blunted Fetal Response to Vibroacoustic Stimulation Associated with Maternal Intravenous Magnesium Sulfate Therapy

ABSTRACT

Maternal intravenous magnesium sulfate (MgSO₄) therapy has been associated with decreased fetal heart rate (FHR) variability and reactivity, resulting in an increased incidence of nonreactive nonstress tests, frequently requiring further assessment of fetal well-being. This study was designed to assess fetal response to vibroacoustic stimulation in association with maternal intravenous MgSO₄ therapy. Five gravidas with singleton gestations with normal fetal anatomy, intact membranes, and normal amniotic fluid volume at 31 weeks or more presenting with preterm labor were included. Vibroacoustic stimulation was performed prior to and during subsequent intravenous MgSO₄ therapy so that each fetus served as its own control. Ultrasound was applied during vibroacoustic stimulation to assess fetal movement response. Response to vibroacoustic stimulation was considered normal if a FHR acceleration of at least 15 beats/min for at least 15 seconds occurred within 15 seconds after the stimulation with prolonged fetal movements. Maternal serum magnesium levels were obtained prior to each stimulation. All fetal responses prior to treatment were normal. All subsequent fetal responses in association with intravenous MgSO₄ were abnormal, consisting of blunted FHR accelerations of 10 to 15 beats/min and ranging between 10 and 15 seconds in duration associated with brief limited fetal movements. Mean maternal serum magnesium levels (± standard error) during vibroacoustic stimulation, prior to and following intravenous MgSO₄, were 1.4 ± 0.1 and 5.2 ± 0.4 mEq/L, respectively (P = 0.008). All 5-minute Apgar scores were 8 or higher. This occurrence may reflect an effect of magnesium on the fetus as either a central nervous system depressant or alternatively as a peripheral neuromuscular relaxant. These results indicate that caution should be exercised in interpreting fetal response to vibroacoustic stimulation in the presence of therapeutic maternal serum magnesium levels.

Keywords

Magnesium sulfate - vibroacoustic stimulation - fetal heart rate