Minimum Effective Concentration of Bupivacaine in Ultrasound-Guided Femoral Nerve Block after Arthroscopic Knee Meniscectomy: A Randomized, Double-Blind, Controlled Trial.

BACKGROUND Adequate analgesia is important for early hospital discharge after meniscectomy. A femoral nerve block may reduce the need for systemic analgesics, with fewer side effects; however, motor block can occur. Ultrasound-guided femoral nerve block may reduce the required local anesthetic concentration, preventing motor block. OBJECTIVE The primary objective of this study was to determine the lowest effective analgesic concentration of bupivacaine in 50% (EC50) and in 90% (EC90) of patients for a successful ultrasound-guided femoral nerve block in arthroscopic knee meniscectomy. STUDY DESIGN This was a prospective, randomized, double-blind, controlled trial. SETTINGS This study was conducted at Hospital São Domingos. METHODS A total of 52 patients undergoing arthroscopic knee meniscectomy were submitted to ultrasound-guided femoral nerve block using 22 mL bupivacaine. The bupivacaine concentration given to a study patient was determined by the response of the previous patient (a biased-coin design up-down sequential method). If the previous patient had a negative response, the bupivacaine concentration was increased by 0.05% for the next case. If the previous patient had a positive response, the next patient was randomized to receive the same bupivacaine concentration (with a probability of 0.89) or to have a decrease by 0.05% (with a probability of 0.11). A successful block was defined by a numerical pain intensity scale score < 4 (0 = no pain; 10 = worst imaginable pain) in 3 different evaluations. If the pain intensity score was = 4 (moderate or severe pain) at any time, the block was considered failed. General anesthesia was induced with 30 µg/kg alfentanil and 2 mg/kg propofol, followed by propofol maintanance, plus remifentanil if needed. Postoperative analgesia supplementation was performed with dipyrone; ketoprofen and tramadol were given if needed. DATA MEASUREMENTS The following parameters were evaluated: numerical pain intensity score, duration of analgesia, supplementary analgesic dose in 24 hours, and need for intraoperative remifentanil. RESULTS The EC50 was 0.160 (95% CI: 0.150 - 0.189), and EC90 was 0.271 (95% CI: 0.196 - 0.300). There was no difference in numerical pain intensity score for the different concentrations of bupivacaine. A successful block was achieved in 45 patients, with no difference according to bupivacaine concentration. Time to first analgesic supplementation dose was longer for bupivacaine concentrations = 0.3% (543.8 ± 283.8 min.), compared to 0.25% (391.3 ± 177.8 min.) and < 0.25% (302.3 ± 210.1 min.). There were no differences in supplementary analgesic dose in 24 hours nor in the use of intraoperative remifentanil according to bupivacaine concentration. LIMITATIONS The analgesic effect was measured only during the first 2 hours. CONCLUSIONS Bupivacaine EC50 for ultrasound-guided femoral nerve block was 0.160 (95% CI: 0.150 - 0.189), and EC90 was 0.271 (95% CI: 0.196 - 0.300).