Background and Objectives: Adequate comparison of blocking capabilities of local anesthetics should be done with some knowledge of their relative potencies. The objective of this clinical trial was to simultaneously determine the motor blocking minimum local anesthetic concentrations (MMLAC) and the relative potency ratios for racemic bupivacaine, levobupivacaine, and ropivacaine during labor.

Methods: We studied parturients with singleton term pregnancies in vertex presentation. Each patient received a 20 mL epidural bolus of bupivacaine, levobupivacaine, or ropivacaine determined by the MMLAC model. Baseline and 30 min after injection, measurements of pain and muscle strength were performed, with assessment of motor strength in the legs at 30 min being the primary outcome measure.

Results: There were no differences in demographic, hemodynamic, or obstetric characteristics between the patients receiving the three local anesthetics. The estimated MMLAC with the 95% confidence intervals (CI) were: Bupivacaine: 0.26% wt/vol (0.22-0.30); Levobupivacaine: 0.30% wt/vol (0.25-0.36); Ropivacaine: 0.34% wt/vol (0.29-0.38). ANOVA of MMLAC estimates was significant (F = 3.32, P = .046), and when ranked by analgesic potencies, a significant linear trend (P = .014) to increasing motor blocking potencies from ropivacaine to levobupivacaine to bupivacaine was also found.

Conclusions: This study confirms a motor blocking hierarchy for the three pipecoloxylidines.