What is the place of nerve stimulation (NS) for ultrasound-guided blocks? Few studies have compared the ultrasound-only vs. ultrasound with concomitant nerve stimulation techiques. 1-4 Studies involving SS blocks have showed that concomitant neurostimulation prolongs procedural time but has little beneficial effect on block success, and may even reduce block success. 4 This most likely relates to US-only blocks facilitating real time observation of LA spread (or lack of it), and therefore, needle repositioning to promote appropriate perineural spread. However, previous studies have randomised patients to US-only or concomitant neurostimulation regardless of the difficulty encountered with both US and NS in an individual patient. That is, previous studies have not incorporated methodology to incorporate concomitant neurostimulation when it is most useful (when US nerve imaging proves difficult), or to omit concomitant neurostimulation when US imaging is straightforward.
For perineural catheter placement, concomitant neurostimulation has produced mixed results in terms of postoperative indices of perineural catheter precision. Concomitant neurostimulation is particularly useful for inexperienced practitioners, as it provides useful feedback regarding what the operator believes they are seeing on the US screen.
Our current approach to concomitant neurostimulation is as follows:
We typically do not use concomitant neurostimulation unless difficulty is experienced identifying the target neural structures. The exception is SS infraclavicular block (or nerves where NS is unhelpful e.g. pure sensory nerves, thoracolumbar nerves) where the appropriate needle endpoint is posterior to the 3rd part of the axillary artery. This endpoint is typically easily imaged in all patients. Once this endpoint is reached, full LA deposition occurs regardless of brachial plexus cord visualisation. For other blocks, if there is an impression of the target nerve on the US screen, but the operator is not entirely certain the structure is actually the nerve, a brief appropriate motor response is sought (0.8-1 mA, pulse width 0.1 ms, 2 Hz - except sciatic: 1.5 mA, pulse width 0.3 mA). A good example of this situation is interscalene block, with the anatomical variant of the C5 root/superior trunk passing through the anterior scalene muscle (rather than between the anterior and middle scalene muscles). Concomitant neurostimulation can readily confirm this anatomical variation. The operator can then be confident the structure appearing as the target nerve is actually the nerve. If US imaging proves futile, a traditional sustained motor response of between 0.2-0.5 mA is sought before LA deposition.
The approach is as for SS blocks, but we have a lower threshold for incorporating concomitant neurostimulation. SS block success is dependent on the operator visualising appropriate perineural LA spread, often with the in-plane technique, which facilitates needle tip imaging. Perineural catheter placement, on the other hand, often involves the out-of-plane technique, with success being dependent on accurate needle tip placement prior to catheter advancement a short distance beyond needle tip. Because needle tip accuracy is critical to accurate catheter placement, and the out-of-plane technique may render needle placement less precise, concomitant neurostimulation can be a valuable adjunct to US for accurate catheter placement. Like SS blocks, our usual practice is to aim for a brief motor response (0.8-1.0 mA, pulse width 0.1 ms, 2 Hz) to confirm the target neural structure is what we suspect it to be (e.g. anterior deltoid, biceps, triceps confirms the C56 roots during interscalene catheter placement). If US imaging proves futile, a traditional sustained motor response of between 0.2-0.5 mA is sought before catheter advancement.
Flushing the needle with dextrose 5% facilitates the elicitation of motor responses. If a motor response is still difficult to elicit, injecting a ml or two of dextrose can help.
1. Dingemans E, Williams SR, Arcand G, et al. Neurostimulation in ultrasound-guided infraclavicular block: a prospective randomized trial. Anesth Analg 2007;104:1275-80, tables of contents.
2. Fredrickson MJ, Ball CM, Dalgleish AJ, et al. A prospective randomized comparison of ultrasound and neurostimulation as needle end points for interscalene catheter placement. Anesth Analg 2009;108:1695-700.
3. Sites BD, Beach ML, Chinn CD, et al. A comparison of sensory and motor loss after a femoral nerve block conducted with ultrasound versus ultrasound and nerve stimulation. Reg Anesth Pain Med 2009;34:508-13.
4. Chan VW, Perlas A, McCartney CJ, et al. Ultrasound guidance improves success rate of axillary brachial plexus block. Can J Anaesth 2007;54:176-82.