The use of a selected ion monitoring technique to study the disposition of bupivacaine in mother, fetus, and neonate following epidural anesthesia for cesarean section.

It is well known that the concentration of bupivacaine in umbilical cord blood at birth is low compared with the concentration in maternal blood. It is not clear whether this low fetal/maternal ratio (F/M) is due to decreased placental transfer or increased uptake by fetal tissues. The purposes of this study were to develop an appropriate analytic method and to clarify this issue by studying the disposition of bupivacaine in mother, fetus and neonate following epidural anesthesia. The study population included 14 parturients who were delivered by Cesarean section, and their infants. Gas chromatography-mass spectrometry techniques were developed which could simultaneously determine bupivacaine and its metabolite 2,6-pipecolylxylidine (PPX) in maternal, fetal and neonatal body fluids to less than 4 ng/ml. The results indicate several points: First, that bupivacaine and PPX remain detectable in neonatal blood for at least three days. Second, that plasma levels of PPX decrease more slowly in mother and neonate than bupivacaine. Also, both mother an neonate excrete primarily PPX in urine, but a higher percentage of unchanged bupivacaine is excreted by the neonate. Finally, urinary excretion of PPX by the neonate remains relatively constant during the first 48 h of life. In contrast, the mother excretes the highest amount of PPX between 12-24 h postpartum. The persistence of bupivacaine and PPX in neonatal body fluids suggests that the low F/M ratio of bupavacaine at birth is due to considerable uptake of bupivacaine by fetal tissues and is not due to diminished placental transfer.