Minimum Effective Concentration of Ropivacaine For Ultrasound-Guided Adductor Canal + IPACK Block In Total Knee Arthroplasty

Abstract Background: This study aimed to investigate the minimum effective concentration (MEC90, defined as effective in 90% of patients) of ropivacaine during the combined procedure of adductor canal block (ACB) and infiltration between the popliteal artery and capsule of the posterior knee (IPACK) block for patients undergoing total knee arthroplasty. Methods: This double-blind, randomized dose-finding trial was based on a biased coin up-and-down sequential design, where the concentration of ropivacaine administered to a given patient depended on the previous patient’s response. Before surgery, the first patient received 20 ml of 0.2% ropivacaine for ACB and again for IPACK. If the block failed, the next subject received a 0.025% higher ropivacaine concentration; otherwise, the next subject received either a 0.025% smaller dose (probability of 0.11) or the same dose (probability of 0.89). Block success was defined as the patient did not suffer significant pain and did not receive rescue analgesia within 6 hours after surgery. MEC90 was estimated by isotonic regression, and the 95% confidence interval (CI) was calculated by bootstrapping. Results: Based on analysis of 52 patients, the MEC90 was 0.247% (95% CI 0.227–0.271%), MEC95 was 0.260% (95% CI 0.244–0.282%) and MEC99 was 0.272% (95% CI 0.260–0.291%). In contrast, four of nine trials in a recent systematic review reported ropivacaine concentrations below 0.247%.Conclusions: Our small trial suggests that 0.247% ropivacaine in 20 ml respectively can provide successful ACB + IPACK block in 90% of patients. However, given that many published trials have used lower concentrations, our findings should be verified in larger studies. Trial registration: This study was registered with the Chinese Clinical Trial Registry (http://www.chictr.org.cn/index.aspx). The clinical trial registration number was ChiCTR2100048757 (Date of registration: July 16, 2021).