Figure 1. Plot illustrating the effect of fetal perfusate pH on the maternal-to-fetal clearance of bupivacaine, lidocaine, 2-chloroprocaine, and antipyrine. Steady state drug transfer was established at a pH of 7.4 in both the maternal and fetal circuits, after which the fetal perfusate pH was changed to 7.2, 7.0, 6.8, and back to 7.4 in 30-min intervals. A linear increase in clearance was observed for each of the local anesthetic agents throughout the range of pHs studied, but not for antipyrine. Data expressed as mean plus/minus SEM.