Figure 2. Plot illustrating the relation of the calculated fraction of the unionized bupivacaine, lidocaine, and 2-chloroprocaine in the fetal perfusate on maternal-to-fetal clearance. Clearance was shown to be inversely and linearly related to the unionized fraction of drug in the fetal circuit. The correlation coefficient, r, and slope for a regression line for each agent is noted below each curve. Data expressed as mean plus/minus SEM.