MATERNAL AND UMBILICAL-CORD CONCENTRATIONS OF FENTANYL AFTER EPIDURAL ANALGESIA FOR CESAREAN-SECTION

The maternal and umbilical concentrations of fentanyl were measured after epidural analgesia for cesarean section, using a highly sensitive radioimmunoassay method. Sixteen parturients were anesthetized with a single epidural injection of a mixture of 85 mg bupivacaine 0.5%, 60 mg etidocaine 1%, and 100-mu-g fentanyl with epinephrine 1:200 000. Apparent maternal individual maximum peak concentration (C(max)) of fentanyl was 0.38 +/- 0.16 ng/ml (mean +/- SD) (range 0.12-0.59 ng/ml) and the time to reach C(max) (T(max)) was 24 +/- 14 min (range 5-60 min). Infants were born 19 to 42 min after epidural administration of fentanyl (mean 27 min). Fentanyl concentrations in neonates was 0.13 +/- 0.04 ng/ml for the umbilical vein and 0.06 +/- 0.03 ng/ml for the artery. The fetus extraction ratio was 53 +/- 19% (range 20-83%). The large difference between arterial and venous concentrations of fentanyl may be due to a metabolization by the fetus and/or an uptake of the drug in the fetal tissues. Thus, even if fentanyl levels reaching the fetus after cesarean section under epidural anesthesia, using local anesthetics with 100-mu-g of fentanyl, are within safe range values, the likehood of fentanyl uptake by fetal tissues calls for a cautious use of repeated fentanyl administration.