Figure 6. Absence of inhibitory effects of mepivacaine on placental transfer of bupivacaine. Steady-state bupivacaine transfer was established with a maternal bupivacaine concentration of 4 micro gram/ml, after which mepivacaine, a structurally analogous amide LA, was added in 45-min intervals at the concentrations of 4, 40, and 80 micro gram/ml. No significant decrease in bupivacaine clearance was seen. These data are consistent with passive diffusion. Data expressed as mean plus/minus SEM of 16 clearance observations at each concentration during four open placental perfusions.