Effects of Changes in Maternal–Fetal pH on the Transplacental Equilibrium of Bupivacaine

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Abstract
Increases in the maternal-fetal pH gradient that may occur during labor and delivery may increase the fetal concentration of local anesthetics. The effects of pH changes on the transplacental concentration equilibrium of bupivacaine were evaluated. They increased the maternal-fetal pH gradient in each of 6 pregnant ewes from a control value of 0.15 to 0.54 by hyperventilating the lungs of the ewe and infusing lactic acid into her fetus. After infusion of bupivacaine (0.15 mg/kg, i.v.) into the mother, the drug rapidly appeared in fetal blood with values significantly increased over control values at 1 and 5 min. The fetal/maternal (f/m) ratios were increased significantly at 5, 15 and 30 min. The f/m ratios had stabilized by 15 min in control and experimental states, suggesting that equilibrium was achieved. The consistently low f/m ratios are explained by the presumed similarity of the ovine maternal and fetal protein binding rates to those of man. The maternal and fetal pH values are major factors in the determination of the f/m ratios.

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