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

Abstract

The dose-response relationship for spinal ropivacaine in patients undergoing surgery of the lower extremity has not been fully determined. 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. 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. 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.