Factors affecting lidocaine transfer across the normal term human placenta
were studied using the dual perfused isolated single cotyledon. Experiments
were performed using perfusates which provided equal protein binding in bo
th the maternal and fetal circuits as well as perfusates that approached th
e actual in vivo maternal/fetal protein binding gradient. Additional experi
ments were performed to investigate the effects of increasing maternal lido
caine concentration (5, 10, 40, 80 mu g/mL) on maternal to fetal (M-->F) li
docaine transfer across the human placenta. Lidocaine crossed the placenta
rapidly in both the M-->F and fetal to maternal (F-->M) directions. When pr
otein binding was similar in the two circuits, M-->F transfer ratios (lidoc
aine transfer/antipyrine transfer) were significantly lower than the transf
er ratios seen in the F-->M direction (0.59+/-0.04 versus 0.84+/-0.06, P<0.
05). Transfer ratios (M-->F: 0.83+/-0.06, F-->M: 0.96+/-0.06) were not redu
ced when the physiological maternal/fetal protein binding gradient was pres
ent. Lidocaine transfer was not diminished by increasing maternal concentra
tions and, in contrast to bupivacaine, was not significantly affected by it
s binding.