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 ED50 was 7.6 mg (95% CI: 6.2-8.7 mg) and for ED95 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 ED50 and ED95 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.