Background: In this study, we sought to determine the median effective dose (ED50) for motor block of intrathecal ropivacaine, levobupivacaine, and bupivacaine and to define their motor-blocking potency ratios.
Methods: We enrolled 104 parturients undergoing elective cesarean delivery with combined spinal-epidural anesthesia and randomized them to one of three groups to receive intrathecal 0.5% (wt/vol) ropivacaine, levobupivacaine, or bupivacaine. The initial dose was 4 mg, and the testing interval was set at 1 mg. Efficacy was determined by the occurrence of any motor block in either lower limb (modified Bromage and hip motor function scale) within 5 min after the spinal injection.
Results: As assessed using up-down analysis, intrathecal ED50 for motor block was 5.79 mg for ropivacaine (95% CI 4.62-6.96), 4.83 mg for levobupivacaine (95% CI 4.35-5.32) and 3.44 mg for bupivacaine (95% CI 2.55-4.34) (P < 0.0007). The relative motor blocking potency ratios were ropivacaine/bupivacaine 0.59 (95% CI, 0.42-0.82), ropivacaine/levobupivacaine 0.83 (95% CI 0.64-1.09), and levobupivacaine/bupivacaine 0.71 (95% CI 0.51-0.98).
Conclusions: There is a clinical profile of potency for motor block for the pipecolylxylidines when administered spinally: low, intermediate, and high for ropivacaine, levobupivacaine, and bupivacaine, respectively.