Concentrations of ritodrine hydrochloride in maternal and fetal serum and amniotic fluid following intravenous administration in late pregnancy.

Seven healthy pregnant volunteers undergoing elective cesarean section at 39-40 weeks of gestation were studied for transplacental passage of ritodrine hydrochloride, which was administered by intravenous infusion at the rate of 72-149 micrograms/min for 161-335 min. The concentrations of ritodrine in the collected maternal and fetal blood and the amniotic fluid were radioimmunoassayed. The levels of ritodrine in maternal serum were between 22.5 and 51.0 ng/ml one hour after the initiation of infusion, 33.7-66.4 ng/ml after 2 h, 18.2-73.6 ng/ml after 4 h and 45.7-189.6 ng/ml at delivery, respectively. The umbilical blood and amniotic fluid concentrations of ritodrine at delivery were between 15.6 and 35.1 ng/ml in arterial blood, 12.5-29.6 ng/ml in venous blood and 10.0-49.1 ng/ml in amniotic fluid. The ratio of umbilical venous blood concentrations to maternal venous concentrations (CV/MV) ranged from 0.066 to 0.544 with the mean of 0.263 +/- 0.063 (M +/- S.E.). The results obtained substantiate the rapid and appreciable transfer of ritodrine to the fetus and amniotic fluid.