Plasma protein binding and placental transfer of bupivacaine

The maternal venous and umbilical venous plasma concentrations of bupivacaine were determined at delivery following epidural administration of the drug to 31 women in labor. In each case the umbilical venous plasma concentration of bupivacaine was lower than the maternal venous plasma concentration. There was no significant difference between the concentration of unbound bupivacaine in umbilical venous and maternal venous plasma at delivery. The difference in umbilical and maternal plasma concentrations of bupivacaine appears to be a consequence of greater bupivacaine binding to maternal than to fetal total plasma protein. The intersubject variation of the ratio of bupivacaine concentration in fetal plasma/bupivacaine concentration in maternal plasma was found to be related to individual variation in the extent of protein binding of bupivacaine in maternal and umbilical plasma and correlated positively with the variation of the ratio of total protein concentration in umbilical and maternal plasma.