No correlation between minimal electrical charge at the tip of the stimulating catheter and the efficacy of the peripheral nerve block catheter for brachial plexus block: a prospective blinded cohort study

Background: Stimulating catheters offer the possibility of delivering an electrical charge via the tip of the catheter. This may be advantageous as it allows verifying if the catheter tip is in close proximity to the target nerve, thereby increasing catheter performance. This prospective blinded cohort study was designed to investigate whether there is a correlation between the minimal electrical charge at the tip of the stimulating catheter, and the efficacy of the peripheral nerve block (PNB) catheter as determined by 24 h postoperative morphine consumption. Methods: Forty adult patients with ASA physical health classification I-III scheduled for upper extremity surgery under combined continuous interscalene block and general anesthesia were studied. Six patients were excluded from analysis. After inserting a stimulating catheter as if it were a non-stimulating catheter for 2-5 cm through the needle, the minimal electrical charge necessary to obtain an appropriate motor response was determined. A loading dose of 20 mL ropivacaine 0.75% ropivacaine was then administered, and postoperative analgesia was provided by a continuous infusion of ropivacaine 0.2% 8 mLh(-1) via the brachial plexus catheter, and an intravenous morphine patient-controlled analgesia (PCA) device. Main outcome measures include the minimal electrical charge (MEC) at the tip of the stimulating catheter necessary to elicit an appropriate motor response, and the efficacy of the PNB catheter as determined by 24 h postoperative PCA morphine consumption. Results: Mean (SD) [range] MEC at the tip of the stimulating catheter was 589 (1414) [30 - 5000] nC. Mean (SD) [range] 24 h morphine consumption was 8.9 (9.9) [0-29] mg. The correlation between the MEC and 24 h postoperative morphine consumption was Spearman's Rho r(s) = -0.26, 95% CI -0.56 to 0.09. Conclusion: We conclude that there is no proportional relation between MEC at the tip of the blindly inserted stimulating catheter and 24 h postoperative morphine consumption.