Distal middle scalene catheter analgesia – separating infusion effect from catheter‐tip position: a reply

We thank Han and He [1] for their careful reading of our trial [2] and welcome the opportunity to address the three questions they raise. We accept our design does not isolate catheter-tip position. Patients allocated to the catheter group differed from those allocated to the single-injection group in tip location, duration of local anaesthetic exposure and cumulative dose; we acknowledged this in our report. Our question was narrower than the interpretation Han and He caution against. We asked whether clinically effective sustained analgesia is attainable when the tip lies approximately 25 mm from the interscalene groove, a position substantially more distal than that targeted in conventional continuous interscalene techniques, not whether proximity to the groove is unnecessary. Rhyner et al. [3] further illustrate that the contribution of continuous infusion is context dependent and cannot be inferred from study design alone. This uncertainty strengthens the case for the catheter-to-catheter comparison they propose, randomising groove-adjacent against distal tips with identical bolus, infusion and multimodal regimens, which we regard as the necessary next step. On the second point made by Han and He, our reporting was insufficient. Catheter-tip position was confirmed by ultrasound both at insertion and immediately before removal on the first postoperative day. At insertion, the tip was visualised within the middle scalene muscle approximately 25 mm from the interscalene groove. Before removal, the tip was re-imaged and remained within the middle scalene muscle in 29/30 patients; the single exception was a catheter dislodged during ambulation. We did not re-measure the distance from the groove at removal and can therefore confirm that the tip remained intramuscular but not that its distance from the groove was unchanged. Nor did we assess the dynamic distribution of the infusate and Han and He are correct on this point. Confirming where the tip lies is not the same as showing where the drug acts, and proximal tracking along fascial planes remains a plausible and untested explanation for our findings. We regard this as the central mechanistic question, and our data do not answer it. On respiratory safety, both groups received the same 20 ml bolus of levobupivacaine 0.25% at the same anatomical target, within 4 mm of the groove, as used in previous work [4]. The volume-dependent effects reported by Renard et al. [5] are pertinent to that initial bolus; bolus volume was therefore not a between-group variable in our trial. However, neither the diaphragmatic effect of that bolus nor that of the subsequent distal infusion was assessed sonographically. All patients were observed after tracheal extubation in the operating theatre with clinical respiratory assessment and pulse oximetry before transfer to the ward, and pulse oximetry was continued until the final assessment at 10:00 on the first postoperative day; no patient required clinical intervention for desaturation. This is a clinical observation rather than a measure of phrenic function and does not exclude subclinical hemidiaphragmatic paresis. Patients with chronic respiratory disease were not studied and diaphragmatic ultrasound with prespecified respiratory outcomes should be included in any subsequent trial. Our findings support the clinical feasibility of sustained analgesia with this distal-catheter strategy; they do not establish the independent effect of tip position or its mechanism. The anatomical and respiratory questions identified by Han and He require direct testing, and we agree they should be the focus of the next trial.