Letters to the Editor: Letters & Announcements
Although we concur with Chin et al.1 that it “would have been useful to know the distribution of sensory loss and its correlation with the site of surgical pain,” we disagree that we failed to “specify if the motor response used as the endpoint for catheter placement was random, standardized or matched to the site of surgery.” As noted in the Methods section,2 the catheter-placement protocol was identical to that previously published (Ref. 1 of article under discussion)3; and the protocol specified that the catheter insertion was standardized with the preferred stimulation-induced muscular motion correlating with the medial cord.3 Our reasoning for targeting the medial cord is that when using the coracoid technique and the Stimucath (Arrow International, Reading, PA) stimulating catheter, we have found it difficult to keep the catheter tip along the posterior and lateral cords when inserting the catheter past the needle tip (probably for anatomic reasons). Thus, although the specific muscle response and correlation with the surgical site was not investigated/recorded, the insertion protocol was standardized and the overwhelming majority of catheters placed along the median cord.
Chin et al. also noted that “it is also possible that patients in this group [0.4% concentration] self-administered more boluses of local anesthetic in an attempt to improve analgesia, and hence received a larger mass and volume of local anesthetic.” We agree with their assessment and specifically noted among the study limitations that, “although each patient-controlled bolus dose delivered the same ropivacaine dose for both treatment groups (8 mg available every 30 min), the actual delivered doses for each group are unavailable. Therefore, it is possible that patients assigned to 0.4% ropivacaine self-administered a greater number of bolus doses resulting in a higher total dose of delivered ropivacaine. This methodological weakness decreases confidence in the rejection of our null hypothesis.”1 We are thus retesting the original hypothesis in a new study involving hospitalized patients, which gives us access to the important bolus-dose use information.
But whatever the mechanism may be, our study conclusions remain clinically useful. “In other words, regardless of the total amount of local anesthetic delivered to each treatment group, providing a 0.2% ropivacaine infusion at a basal rate of 8 mL/h resulted in a lower incidence of patient-identified insensate limbs relative to a 0.4% ropivacaine infusion at a basal rate of 4 mL/h.”1
Brian M. Ilfeld, MD, MS
Edward R. Mariano, MD, MAS
Vanessa J. Loland, MD
Department of Anesthesiology
University of California San Diego
San Diego, California
Daniel I. Sessler, MD
Department of Outcomes Research
For the PAINfRE™ Investigators
1.Chin KJ, Perlas A, Chan V, Brull R. Continuous infraclavicular plexus blockade. Anesth Analg 2009;109:1347–8
2.Ilfeld BM, Le LT, Ramjohn J, Loland VJ, Wadhwa AN, Gerancher JC, Renehan EM, Sessler DI, Shuster JJ, Theriaque DW, Maldonado RC, Mariano ER. The effects of local anesthetic concentration and dose on continuous infraclavicular nerve blocks: a multicenter, randomized, observer-masked, controlled study. Anesth Analg 2009;108:345–50
3.Ilfeld BM, Morey TE, Enneking FK. Infraclavicular perineural local anesthetic infusion: a comparison of three dosing regimens for postoperative analgesia. Anesthesiology 2004;100:395–402