References 2–6 of our colleagues' letter all involved epidural infusion,1,–,5with references 2–4 suggesting analgesia is improved using repeated bolus doses compared with a simple basal infusion. But all subjects had multi-orifice epidural catheters.1,–,3References 5 and 6 suggest that with the same infusion method, multi-orifice epidural catheters improve analgesia compared with single-orifice designs. But all had the same local anesthetic administration technique.4,5In other words, whereas for epidural infusions repeated boluses appear to improve analgesia compared with a simple basal infusion, all of these studies involved only multi-port catheters, and thus it remains unknown if using a single-orifice catheter would have resulted in different findings (the authors of those reports did not purport such a theory).1,–,3Therefore, we do not believe the data supports our colleagues' statement that a “better differential sensory-motor block” is “thought to be enhanced by multi-orifice flow.”
However, even if our colleagues' speculation was correct for epidural infusion, local anesthetic pharmacodynamics varies considerably among various introduction modalities. For example, local anesthetic dose, as opposed to volume or concentration, appears to be the primary determinant of block effects during continuous femoral and posterior lumbar plexus nerve blocks.6,7In contrast, the effects are mixed for epidural local anesthetic infusions: total dose is the primary determinant of analgesia quality and dermatomal spread, whereas concentration is the primary determinant of motor block and sympathectomy/hypotension.8It is for this very reason that we undertook our investigation: data from the literature involving central nervous system infusions (unfortunately) cannot be directly applied to those of the peripheral nervous system, such as the continuous femoral nerve blocks (cFNB) of our study.9We therefore do not believe that the references cited in our colleagues' letter, or any published data, supports their supposition that for continuous peripheral nerve blocks “a multi-orifice catheter is required” to “maximize” benefits.
Regarding continuous peripheral nerve blocks, there are two investigations involving popliteal-sciatic local anesthetic administration suggesting that repeated boluses improve analgesia compared with a basal infusion (our colleagues cited one of these two).10,11However, both of these studies involved single-orifice perineural catheters. Therefore, there are actually more investigations suggesting the superiority of repeated boluses that used single-orifice perineural catheters than there are studies (just one involving interscalene catheters) using a multiple-orifice design.12It appears that the references cited by our colleagues contradict their theory regarding the importance of multi-orifice catheters on bolus-versus -basal administration effects.10,–,12Regardless, dramatic pharmacodynamic differences exist even among various continuous peripheral nerve block catheter anatomic locations, and thus the results for popliteal-sciatic and interscalene catheters are not automatically applicable to cFNB.13
Our colleagues' speculation that the results of our study might be different with a multi-orifice catheter may prove accurate in the future. However, given (1) our investigation's randomized, triple-masked, split-body study design using extremely precise sensory and motor outcome measures with established reliability and validity; (2) the lack of data suggesting that the number of catheter orifice(s) impact the success of bolus-versus -basal local anesthetic administration for the peripheral (or central) nervous system; and, (3) the dramatic, often opposite, differences in pharmacodynamics among various continuous peripheral nerve block catheter anatomic locations, we believe that our concluding statement is both accurate and warranted: “this study did not find evidence to support the hypothesis that varying the method of local anesthetic administration–basal infusion versus repeated bolus doses–influences cFNB effects to a clinically significant degree. Thus, it is doubtful that, when using a cFNB, varying the method of local anesthetic administration will provide an increased sensory-to-motor block ratio and minimize motor block and the risk of falling while optimizing cutaneous analgesia.”9