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 (ED(50)) 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 ED(50) was calculated using the method of Dixon and Massey. RESULTS: The ED(50)s 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.

Spinal ropivacaine for lower limb surgery: a dose response study.

BACKGROUND: The dose-response relationship for spinal ropivacaine in patients undergoing surgery of the lower extremity has not been fully determined. METHODS: We performed a prospective, randomized, double-blind study of 60 patients scheduled for lower limb surgery under combined spinal-epidural anesthesia. Patients were assigned to receive 1 of 5 doses of intrathecal ropivacaine: 2, 4, 7, 10, or 14 mg diluted to 2.8 mL with normal saline. A dose was considered successful if a sensory block to cold was achieved bilaterally at the T12 dermatome within 20 min and surgery proceeded without supplementation for at least 50 min. RESULTS: Anesthesia was successful in 0, 0, 42, 83, and 100% of the 2, 4, 7, 10, and 14 mg groups, respectively. The derived value for ED(50) was 7.6 mg (95% CI: 6.2-8.7 mg) and for ED(95) was 11.4 mg (95% CI: 9.7-18.3 mg). The cephalic level of sensory block and the degree of motor block increased with larger doses of ropivacaine. CONCLUSION: The ED(50) and ED(95) for spinal ropivacaine in lower limb surgery of 50 min duration or less were 7.6 and 11.4 mg, respectively. This provides a useful guide for clinicians to choose the optimal dose of spinal ropivacaine under different clinical situations.