Feto-maternal distribution of ropivacaine and bupivacaine after epidural administration for cesarean section

Ropivacaine is a new amino amide local anesthetic less lipophilic and with a lower affinity for plasma proteins than bupivacaine. The purpose of this study was to examine the feto-maternal distribution of ropivacaine and bupivacaine after epidural administration for cesarean section. Healthy parturients were randomly allocated in a double-blinded manner to receive either 0.5% ropivacaine or 0.5% bupivacaine through a lumbar epidural catheter. The total and free concentrations of ropivacaine and bupivacaine were determined in the maternal and umbilical plasma. The total dose (mg/kg) required for adequate surgical anesthesia and the resulting maximum total concentrations did not differ significantly between ropivacaine and bupivacaine. The free plasma clearance of bupivacaine was higher than that of ropivacaine (7382 vs 3344 ml/min, P=0.0001). The apparent terminal elimination half-life of ropivacaine was shorter than that of bupivacaine (6.0 vs 8.8 h, P=0.007). At delivery the maternal free plasma concentration of ropivacaine was more than twice that of the free concentration of bupivacaine (0.072 vs 0.032 mu g/ml, P=0.002). The free concentration of ropivacaine was about twice that of bupivacaine in the umbilical venous (0.06 vs 0.03 mu g/ml, P=0.001) and umbilical arterial (0.05 vs 0.02 mu g/ml, P=0.007) plasma. The more rapid plasma clearance of bupivacaine compared to ropivacaine, leading to lower maternal plasma concentrations and hence to lower umbilical concentrations at delivery, could be explained by the higher lipid solubility, hence greater distribution volume, of bupivacaine.