Levels of ritodrine hydrochloride in fetal blood and amniotic fluid following long-term continuous administration in late pregnancy.

Placental transfer of ritodrine hydrochloride (ritodrine) was investigated in late-pregnant women following continuous infusion. Five volunteer women undergoing cesarean section received a total dose of 650-5970 mg of ritodrine for 8-49 days. After the infusion was discontinued at cesarean section, maternal and fetal blood and amniotic fluid were collected for later determinations of ritodrine by radioimmunoassay. Maternal serum concentrations of ritodrine were between 99.0-332.4 ng/ml. Serum levels of ritodrine in cord artery and vein were 75.9-250.5 ng/ml and 74.7-213.0 ng/ml, respectively. Ritodrine levels in amniotic fluid ranged from 126.0 to 424.0 ng/ml. The concentration of ritodrine in cord vein to maternal vein (CV/MV) was 0.695 +/- 0.066 (m +/- SE), while the concentration ratio of the amniotic fluid to maternal vein (AF/MV) was 1.719 +/- 0.211. In the present study of continuous administration of ritodrine, remarkably higher concentrations in fetal circulation and amniotic fluid were observed compared to those in cases of short-term infusion.