Placental passage of the oxytocin antagonist atosiban

OBJECTIVE: We wanted to determine the degree of placental transfer of atosiban (Antocin), an oxytocin antagonist, in pregnant women at term. We also assessed the effects of the infusion on umbilical cord blood gases at birth and the maternal hematocrit drop after cesarean section. STUDY DESIGN: Eight women undergoing elective cesarean section at term were studied. Each received an infusion of 300 μg/min of atosiban over 208 to 443 minutes; the infusion was continued up to the time of cord clamping. Uterine vein and umbilical blood samples were obtained simultaneously. They were assayed by specific radioimmunoassay. Cord blood gases were obtained and compared with those from a control group of women undergoing elective cesarean section. RESULTS: The mean (±SD) maternal uterine vein concentration was 331.9 ± 42.9 ng/ml, compared with 42 ± 13 ng/ml in the umbilical vein (p< 0.05). The mean maternal/fetal ratio was 12 ± 0.03, which was not affected by the length of infusion. There was no significant difference in the hematocrit drop between the cesarean delivery groups: 5.9 ± 0.4 for the control group versus 5.8 ± 1.1 for the atosiban group (p> 0.1). The mean cord pH was 7.27 for the atosiban group versus 7.27 for the control group (n = 141) (p> 0.01). One year follow-up of the infants (n = 7) was normal. CONCLUSIONS: Our results show minimal placental transfer of atosiban. Drug levels did not increase with longer infusions, and no effect was seen on umbilical cord gases. Administration of atosiban even at high doses up to the time of delivery did not increase maternal blood loss at cesarean section.