The Relative Potencies for Motor Block After Intrathecal Ropivacaine, Levobupivacaine, and Bupivacaine

In Brief 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. IMPLICATIONS: We demonstrated a clinical profile for motor block potency for the pipecolylxylidines when administered spinally: low, intermediate, and high for ropivacaine, levobupivacaine, and bupivacaine, respectively. Intrathecal ropivacaine and levobupivacaine are significantly less potent than bupivacaine, which may explain the lesser motor blocking effects of intrathecal ropivacaine and levobupivacaine.