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 ultrasoundguided 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). Key words: Postoperative analgesia, femoral block, ultrasound-guided, bupivacaíne minimum concentration, arthroscopic meniscectomy