Minimum effective anesthetic concentration (MEAC) for sciatic nerve block: Subgluteus and popliteal approaches

Abstract

Background: We tested the hypothesis that using a subgluteus approach to the sciatic nerve requires a lower concentration of mepivacaine to obtain complete anesthesia as compared with the popliteal approach. Methods: With midazolam premedication (0.05 mg·kg-1 iv), 48 patients undergoing hallux valgus repair were randomly allocated to receive a sciatic nerve block using either a posterior popliteal (group Popliteal, n = 24) or subgluteus (group Subgluteus, n = 24) approach with 30 mL of local anesthetic injected after elicitation of plantar flexion of the foot with a current ≤ 0.5 mA. A 20G catheter was inserted for 2-4 cm to supplement the block if required. The concentration of the injected solution was varied for consecutive patients using the up-and-down staircase method according to the response of the previous patient (initial concentration: 1%; up-and-down steps: 0.1%). Successful nerve block was defined as complete loss of pinprick sensation in both tibial and common peroneal nerve distributions with concomitant inability to perform plantar or dorsal flexion of the foot 30 min after injection. Results: The minimum effective anesthetic concentration of mepivacaine resulting in complete block of the sciatic nerve in 50% of cases (ED50) was 0.95% ± 0.014% (95% confidence intervals [CI95]: 0.77%-1.12%) in group Subgluteus and 1.53% ± 0.453% (CI95: 0.96%-2.00%) in group Popliteal (P = 0.026). The ED95 for adequate nerve block calculated with probit transformation and logistic regression analysis was 1.12% (CI95: 0.71%-1.99%) in group Subgluteus and 1.98% (CI95: 1.39%-2.31%) in group Popliteal. Conclusion: A subgluteus approach to the sciatic nerve facilitates a reduction of the minimum effective concentration of local anesthetic required to produce an effective surgical block within 30 min after the injection as compared with the posterior popliteal approach.