In Response

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We would like to thank our colleagues for their thoughtful and informative letter1 addressing our recent article.2 The letter notes that “there are a few pertinent points in study design which need to be looked into before the results are extrapolated to postoperative surgical patients [emphasis added].”1 However, of the 4 points delineated, none directly address our study’s design. Rather, 3 of the 4 points (the first, second, and fourth) question the ability to extrapolate results involving healthy volunteers to postsurgical patients due to multiple reasons that the authors describe. We concur with our colleagues on these 3 points. As we emphasized in our article: “Whether cutaneous sensation correlates well with postoperative pain following various abdominal procedures remains unknown, making extrapolation to clinical practice more difficult.” Our concluding sentence specifically cautions that “further research is warranted … in a post-surgical patient population.”2
We would like to address further our colleagues’ third point, which, although not clearly specified, appears to question the validity of our negative results given the “positive” results of 3 previously published clinical studies. It appears inconsistent that our colleagues question the validity of extrapolating from the transversus abdominis plane (TAP) in volunteers to the TAP in surgical patients (for which we agree), yet voice no issue extrapolating data from studies involving perineural popliteal,3 femoral,4 adductor canal,5 and even the epidural space of cadavers6 to the perineural TAP.2 On this point, we differ with our colleagues, considering that multiple previous investigations demonstrate that the optimal method of local anesthetic administration often varies with anatomic location.7 For example, when using femoral catheters, a basal infusion (exclusively or added to bolus doses) does not necessarily add infusion benefits.8 However, for popliteal sciatic catheters, only a basal infusion is required to maximize infusion benefits,9 while infraclavicular catheters require both a basal infusion and bolus doses to optimize analgesia.10 Additional studies provide contrasting results as well, emphasizing the complexity of these issues.7 Clearly, data from one anatomic catheter location may not automatically be applied to another.
However, even if the various catheter locations were analogous, the preponderance of published data fails to demonstrate much benefit, if any, of administering repeated bolus doses compared with a basal infusion via perineural catheters.7 Our colleagues referenced 3 positive clinical studies. One involved adductor canal catheters, which found that intermittent 15 mL of 0.5% ropivacaine boluses every 6 hours reduced morphine consumption in the 24 hours following knee surgery compared with a continuous 2.5 mL/h basal infusion with an equivalent ropivacaine dose.5 Critical to evaluating these results is the fact that continuous adductor canal blocks appear to require a higher basal infusion rate of local anesthetic than their femoral counterparts. Recent studies demonstrate that even with a relatively high rate of 8 mL/h, analgesia for total knee arthroplasty remains suboptimal compared to a higher rate,11 and contrast injected at pressures comparable to 8 mL/h resulted in somewhat limited spread in the adductor canal.12 Therefore, it is highly questionable whether the study cited by our colleagues was positive due to the inherent superiority of repeated bolus doses as they opine, or rather inadequacy of the extraordinarily low basal infusion rate of 2.5 mL/h, when even 8 mL/h has been found to be suboptimal.5 Confidence in these findings is further decreased with the publication of contrary evidence from a much larger clinical trial published subsequently that utilized a 9 mL/h basal infusion in postoperative patients with adductor canal catheters. A 27-mL ropivacaine bolus every 3 hours resulted in no statistically—or clinically—significant difference compared with the basal infusion.
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