In Response

Abstract

We welcome the debate offered by Machi et al.1 regarding the validity of our study findings.2 Unfortunately, the authors have misunderstood key parts of the article. The article's brief report format did not enable us to describe the anesthetic methodology not pertinent to the study's primary outcome: 20-minute aggregate block score. Specifically, because lidocaine was used for the infraclavicular block, elective ropivacaine distal block supplementation was primarily for postoperative analgesia3 and not block failure. Thus, the surgical anesthesia success rate (defined as no intraoperative pain requiring infiltration or sedation) was not 49% vs 55%, but was 96% vs 86%. Regardless, we urge extreme caution comparing study success rates, as apparent differences can be due to the block testing method (pinprick2,3 vs cold stimulus4), the timing of testing (as early as 15 minutes3 or as late as 45 minutes), use of elective supplementation, and the definition of surgical anesthesia success (level of analgesia/sedation permitted). Our study used stringent criteria during early block progression: complete loss of pinprick sensation and all movement at 20 minutes, and a similarly stringent definition for surgical anesthesia success. Machi et al. challenge the frequency with which we were able to visualize all 3 cords, because it was less than the authors' experience of 99%. Consequently, they question the triple-injection arm, which allowed arbitrary local anesthetic placement at 2, 6, and 8 o'clock when cord visualization was not possible. In fact, this triple-injection method was based on Sandhu and Capan's5 influential observational study which reported that the cords were “easily imaged in all patients” (n = 126) at approximately 2, 6, and 8 o'clock, and the work of Perlas et al.,6 another experienced ultrasonographer who, using a cart-based machine, reported cord visualization in only 27% of patients. Furthermore, this triple-injection technique was almost identical to that used in another similar randomized trial.4 We agree that the single-injection technique is not a true “single” injection, because some needle manipulation was permitted posterior to the artery in a cephalocaudad direction “not extending beyond the cepahalocaudad borders of the artery.” However, nowhere is it stated that the “single-injection technique was manipulated to involve up to 50% circumference of the artery”; rather, “some needle manipulation was permitted to promote a shallow saucer-shaped spread dorsal to the artery.” Regardless, we fail to appreciate how this minor needle manipulation would invalidate our findings. Finally, evidence-based medicine gives the highest weight to evidence supported by more than 1 large blinded randomized controlled trial. Although small (n < 30) observational studies have suggested significant cord position variability, and the presence of septa—which might provide an anatomical rationale for multiple injections—2 large (n = 100) randomized trials instead, now support the use of a single injection posterior to the axillary artery.2,4 Michael J. Fredrickson, FANZCA Philip Wolstencroft, FANZCA Departments of Anaesthesiology University of Auckland Auckland, New Zealand Department of Anaesthesia Auckland City Hospital Auckland, New Zealand [email protected]