Placental transfer of lidocaine: effects of fetal acidosis.

To investigate whether fetal acidosis increases the placental transfer of lidocaine, resulting in higher fetal blood levels of the drug, lidocaine was infused intravenously into ten pregnant ewes to maintain plasma levels of 2-4 microgram/ml. After maternal-fetal equilibrium was reached, the fetus was made acidotic by infusing lactic acid intravenously. Fetal blood pH decreased from 7.35 to 7.10. With fetal acidemia, fetal blood lidocaine levels increased significantly from 1.60 +/- 0.11 microgram/ml to 2.72 +/- 0.26 microgram/ml. The fetal-maternal lidocaine ratio increased from 0.76 to 1.21. Correction of the acidosis by bicarbonate infusion returned the fetal-maternal ratios to control values. It is concluded that acidosis in the fetus may result in trapping of ionized lidocaine in the fetal circulation and increase the transfer of lidocaine across the placenta.