Placental transfer of local anaesthetics in the premature sheep fetus.

This study was designed to test whether there is any difference in the placental transfer of bupivacaine or lidocaine in the early compared to the late preterm maternal/fetal sheep preparation; and whether the premature lamb fetus reacts to a steady state local anaesthetic infusion differently from the same lamb near term. Eleven ewes were studied in two groups receiving bupivacaine (group A) or lidocaine (group B). Hysterotomy and insertion of fetal and maternal lines were performed at 110 days gestation, and studies were repeated weekly using the same local anaesthetic until delivery. We found no difference in maternal or fetal cardiovascular responses to bupivacaine or lidocaine in the early (mean 119 days) compared to the late (mean 132 days) preparations. The levels of bupivacaine and lidocaine in the fetal blood were similar in early and late fetuses, as were the fetal/maternal ratios of both drugs.