Minimum effective concentration of ropivacaine for ultrasound-guided adductor canal + IPACK block in total knee arthroplasty

Objective This study aimed to investigate the minimum effective concentration (MEC 90, 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). The primary outcome was whether the block was successful. Block success was defined as the patient did not suffer significant pain and did not receive rescue analgesia within 6 h after surgery. MEC 90 was estimated by isotonic regression, and the 95% confidence interval (CI) was calculated by bootstrapping. Secondary outcomes were numerical rating scale (NRS) pain scores at postoperative 24 h and 48 h, postoperative morphine consumption, and time to hospital discharge. Secondary outcomes were compared between patients whose blocks succeeded with those which failed. Results Based on analysis of 52 patients, the MEC 90 was 0.247% (95% CI 0.227–0.271%), MEC 95 was 0.260% (95% CI 0.244–0.282%) and MEC 99 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%. Patients whose blocks succeeded ( n = 45) had significantly lower NRS pain scores, lower morphine consumption, and shorter hospitalization than patients whose blocks failed ( n = 7). 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.