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 ultraso nographic examination in 15 patients (Mg group) during magnesium sulfate to colysis for the treatment of preterm labor. The maternal serum magnesium co ncentration was 5.7 +/- 0.5 mg/dl at the time of the examination. Sixteen f etuses in normal pregnancies at similar gestational ages were used as the c ontrol group. The fetal heart rate and the middle cerebral artery pulsatili ty index in the Mg group were lower than in the control group (p < 0.01). F ractional 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 th e 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 b lood 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 rig ht ventricular function is depressed, the fetus maintains its cardiac outpu t during prolonged hypermagnesemia by increasing its left ventricular funct ion. These results indicate the different fetal intracardiac and peripheral circulation, especially in the brain, from normal fetuses.