Ritodrine concentrations in maternal and fetal serum and amniotic fluid.

Ritodrine hydrochloride concentrations were measured by radioimmunoassay in maternal and fetal sera and amniotic fluid from eight, pregnant, volunteers. Each patient agreed to have 420 micrograms . kg-1 of ritodrine infused intra-venously over two hours prior to elective cesarian section. During infusion a blood sample was taken every 30 minutes from a maternal peripheral vein. Once anesthesia was established the infusion was discontinued. During surgery, samples were drawn from maternal peripheral vein, uterine vein, umbilical artery and vein, and amniotic fluid. All the children delivered were healthy and had a normal course during the neonatal period. Ritodrine concentrations in maternal and fetal compartments at the time of cesarian section were 54.2 +/- 11.8 ng/ml (mean +/- S.D.) in maternal peripheral vein, 37.5 +/- 10.3 ng/ml in uterine vein, 15.8 +/- 4.8 ng/ml in umbilical artery, 16.6 +/- 8.9 ng/ml in umbilical vein, and 15.2 +/- 8.7 ng/ml in amniotic fluid. There was a statistically significant difference (p less than 0.02, paired-t test) between the mean ritodrine concentrations in sera from the maternal peripheral vein and uterine vein. This difference probably reflects transplacental passage. The mean ratio of fetal vein and peripheral maternal vein serum ritodrine concentrations was 0.30 +/- 0.13 (mean +/- SD). The similarity between mean ritodrine concentrations in amniotic fluid, and sera from umbilical arterial and venous blood was striking. These results suggest that the fetus does not conjugate the drug.