Fetal cardiovascular function during prolonged magnesium sulfate tocolysis

The purpose of this study was to evaluate the fetal cardiovascular function during prolonged magnesium sulfate tocolysis. We performed a fetal ultrasonographic examination in 15 patients (Mg group) during magnesium sulfate tocolysis for the treatment of preterm labor. The maternal serum magnesium concentration was 5.7 +/- 0.5 mg/dl at the time of the examination. Sixteen fetuses in normal pregnancies at similar gestational ages were used as the control group. The fetal heart rate and the middle cerebral artery pulsatility index in the Mg group were lower than in the control group (p < 0.01). Fractional shortening (FS) of the right ventricle in the Mg group was lower (p < 0.01), while FS of the left ventricle was higher (p < 0.01) than in the controls. The calculated blood flow through the tricuspid orifice in the Mg group was lower than in the control group (p < 0.01). In contrast, the blood flow through the mitral orifice in the Mg group was higher than in the control group (p < 0.01). In conclusion, in spite of the fact that the right ventricular function is depressed, the fetus maintains its cardiac output during prolonged hypermagnesemia by increasing its left ventricular function. These results indicate the different fetal intracardiac and peripheral circulation, especially in the brain, from normal fetuses.