Objective To determine the relative motor block potency of levobupivacaine and racemic bupivacaine using the minimal local anesthetic effective concentration (MLAC) model. Methods Fifty ASA I^II adult women undergoing gynecological major abdominal surgery were randomly allocated into either group levobupivacaine or group bupivacaine. Two epidural catheters were advanced 3cm into the spaces in cephalic direction at the T_ 11-12 and L_ 3-4 level. Each patient received a total 20 ml fractioned bolus of either levobupivacaine or bupivacaine within 8 min through the L_ 3-4 catheter. The first women in each group received 0.375% levobupivacaine or bupivacaine, and the next local anesthetic concentration was determined by the response of the previous patient in the same group to higher or lower concentration according to up-down sequential allocation, and the ratio of adjacent concentration rank is 0.92. Thirty minutes after infusion the level of motor block on lower extremities were scored with modified Bromage Motor scale. Results Using the formula of Dixon and Mood, the MMLAC for levobupivacaine was 0.324 (0.305, 0.344)% and for bupivacaine was 0.284 (0.269, 0.301)%. The ratio of MMLACs was 1.14:1. Conclusion This study demonstrates that the S-enantiomer of bupivacaine is a little less potent in motor block than racemate according to their MMLAC.
