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The Median Effective Dose of Bupivacaine, Levobupivacaine, and Ropivacaine After Intrathecal Injection in Lower Limb Surgery

BACKGROUND: Intrathecal anesthesia is commonly used for lower limb surgery. Bupivacaine, levobupivacaine, and ropivacaine have all been used as intrathecal drugs, but their relative potency in this context has not been fully determined. In this study, we determined the median effective dose (ED50) of these three local anesthetics for intrathecal anesthesia in lower limb surgery and hence their relative potencies. METHODS: Seventy-five patients scheduled for lower limb surgery under combined spinal-epidural anesthesia were randomly allocated to one of three groups receiving intrathecal bupivacaine, levobupivacaine, or ropivacaine. The dose of local anesthetic was varied using up-down sequential allocation technique. The dose for the first patient in each group was 8 mg, and the dosing increment was set at 1 mg. Subsequent doses in each group were determined by the outcome in the previous patient using success or failure of the spinal anesthesia as the primary end point. A success was recorded if a bilateral T12 sensory block to cold was attained within 20 min after intrathecal injection, and the surgery proceeded successfully until at least 50 min after the intrathecal injection without supplementary epidural injection. The ED50 was calculated using the method of Dixon and Massey. RESULTS: The ED50s were 5.50 mg for bupivacaine (95% confidence interval [CI]: 4.90–6.10 mg), 5.68 mg for levobupivacaine (95% CI: 4.92–6.44 mg), and 8.41 mg for ropivacaine (95% CI: 7.15–9.67 mg) in intrathecal anesthesia. The relative anesthetic potency ratios are 0.97 (95% CI: 0.81–1.17) for levobupivacaine/bupivacaine, 0.65 (95% CI: 0.54–0.80) for ropivacaine/bupivacaine, and 0.68 (95% CI: 0.55–0.84) for ropivacaine/levobupivacaine. CONCLUSION: This study suggests that in intrathecal anesthesia for lower limb surgery, ropivacaine is less potent than levobupivacaine and bupivacaine, whereas the potency is similar between levobupivacaine and bupivacaine.