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Electric Nerve Stimulation Does Not Correctly Predict Needle-Nerve Distance and Potential Local Anesthetic Spread for Interscalene Brachial Plexus Blockade

Fielmuth, Stefan, MD*; Szalata, Marek, MD*; Sievert, Heidi, MD*; Beier, David, MD; Rehberg, Sebastian, MD; Hahnenkamp, Klaus, MD; Mauermann, Knut, MD*; Meissner, Konrad, MD

doi: 10.1213/ANE.0000000000001982
Regional Anesthesia and Acute Pain Medicine: Brief Report

This study evaluated electric nerve stimulation as a nerve location tool. After eliciting motor response in 43 patients undergoing shoulder surgery, the needle tip’s position, distance from the closest nerve, and spread of saline were evaluated using ultrasound imaging. The needle’s tip resided 1 to 4 mm from the closest nerve in 21, in direct contact with it in 7, and 6 to 18 mm away in 15 patients. In 21 patients, subsequent saline dissection did not reach the brachial plexus. Thus, the success rate of electric nerve stimulation for correct needle-nerve distance identification was 48.8%, with correct fluid spread reached in only 51.2% of patients.

Published ahead of print April 3, 2017.

From the *Klinik für Anästhesiologie und Intensivmedizin; Klinik für Orthopädie, Dietrich Bonhoeffer Klinikum Neubrandenburg, Neubrandenburg, Germany; and Klinik für Anästhesiologie, Universitätsmedizin Greifswald, Greifswald, Germany.

Published ahead of print April 3, 2017.

Accepted for publication January 19, 2017.

Funding: Institutional.

The authors declare no conflicts of interest.

This study was presented, in part, at the 2013 ESRA Annual Meeting in Glasgow, United Kingdom.

Address correspondence to Konrad Meissner, MD, Klinik für Anästhesiologie, Universitätsmedizin Greifswald, Ferdinand-Sauerbruch-Str., D-17475 Greifswald, Germany. Address e-mail to

Traditional nerve stimulation using electric nerve stimulation remains important for peripheral nerve blocks, because it supposedly allows for the detection of a close but still safe proximity of the needle tip to the neural target, a distance of between 1 and 4 mm.1–3 For many reasons, the technique for localizing peripheral nerve structures before the administration of local anesthetics for peripheral blockade underwent a paradigm shift toward ultrasound-guided procedures in recent years.4 A fading motor response below 0.2 mA was deemed reliable to detect intraneural injection, whereas other studies have questioned the precision of electric nerve stimulation for indicating a safe needle-nerve distance.5 , 6 However, even though a recent study identified motor response at 0.2 mA stimulation in over 50% of patients with the needle tip 1 mm away from the nerve,7 data regarding the specificity of muscle twitches as indicating a close enough needle-nerve distance are lacking. We therefore hypothesized that success rates and safety of the needle-nerve distance identification, topographic accuracy, and subsequent fluid spread after electric nerve stimulation for interscalene blockade are significantly lower than estimated from former trials using stimulation and dissecting tools with no objective reference.

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After approval by the local institutional review board and informed written consent, the interscalene approach to the brachial plexus was performed using a 21-G facet needle (Sonolong, Pajunk, Geisingen, Germany) and a nerve stimulator (Multiswitch, Pajunk, Geisingen, Germany; 1.0 mA, 0.1 ms, 2 Hz) at the lateral border of the sternocleidomastoid muscle at cricoid level and aimed at the interscalene space in a tangential manner toward the supraclavicular fossa. While eliciting muscle twitches at the deltoid and/or biceps muscles, the current intensity was gradually reduced. When a current of 0.5 mA was reached with the motor response remaining, the intensity was further reduced to 0.2 mA with an immobilized needle. If the motor response ceded, stimulation was shut off without repositioning. A motor response remaining below 0.2 mA required repositioning of the needle to avoid direct nerve contact and another such cycle. The interscalene groove was then scanned with no apparent pressure in a short-axis view using a 12 MHz linear array connected to an ultrasound machine (LogiqE, GE Systems, Jiangtsu, China). Subsequently, 10 mL of normal saline was injected to visualize the spread of fluid toward the targeted neural structures from this position (“hydrodissection”). The primary outcome measure was needle-nerve distance when electric stimulation supposedly succeeded, and the secondary outcome measure was distribution of fluid from there. The intended measures were a distance between 1 and 4 mm to the nerve and a spread toward the C 5/6 roots. Any hydrodissection not resulting in a spread around the C 5/6 root was noted as inaccurate, regardless of the actual topographic position. Any distance of > 4 mm or <1 mm (or direct needle-nerve contact) was defined as not successful, enabling both analysis for avoiding direct nerve contact and for correct proximity. For both parameters combined, an exact one-sided binomial test with an α-level of 5% and 95% confidence limits was computed to reject the null hypothesis of an at least 75% successful needle-nerve distance identification rate. For ethical reasons, the study ended there, because exposure of patients to potentially insufficient local anesthetic doses was not attempted after the potential fluid spread had been deemed insufficient, in which case the needle was corrected under ultrasound guidance before blockade. Sample size calculation was performed using the 2-sided exact 1-sample binomial test for success rates. Assuming an acceptable 25% failure rate for correct needle-nerve distance identification, but a true success rate of only 50%, an amount of 32 patients was needed to reject the null hypothesis with a power of 80% and an α-level of 5%.

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A total of 44 consecutive patients presenting for elective shoulder surgery were enrolled in the study. In 43 patients, appropriate muscle responses in an electric current intensity ranging from 0.2 to 0.5 mA (median 0.4) could be obtained at the deltoid (27 patients), biceps (12 patients), or both muscles (4 patients).

Ultrasound imaging revealed the intended needle-nerve distance of between 1 and 4 mm in 21 of the 43 patients (Table). The needle tip resided >4 (6–18) mm away from the target in 15 of the 43 patients, in particular, anteriorly to the anterior scalene muscle in 2 patients, within the anterior scalene muscle in 5 patients, within the median scalene muscle in 7 patients, and within the sternocleidomastoid muscle in 1 patient. Distances of <1 mm (or direct contact with the nerve) had been inadvertently established in 7 patients.



The subsequent hydrodissection demonstrated a sufficient fluid spread, irrespective of their actual distance from the nerve, in 22 patients (marked with “X” in the Figure). Interestingly, these include only 8 of the “correct distance” patients described above and 7 patients with a distance longer than 4 mm. Of note, all 7 patients with <1 mm distances (or direct needle-nerve contact) demonstrated a sufficient hydrodissection, despite (or because of) their false-negative result for ruling out direct nerve contact. No intraneural injection (ie, halo due to nerve swelling) was observed. Among the 21 patients with incorrect locations, needle tip positions were superficially to the deep cervical fascia in 8 patients, and intramuscularly in the anterior or middle scalene muscle in 13 patients, regardless of their actual needle-nerve distance. In 21 of the 43 patients (marked with “+” in the Figure), hydrodissection did not reach the neural target, either because of a distance >4 mm between the needle tip and the nerve (in 15 patients), or while being blocked off by the deep cervical fascia (in 7 patients) despite a correct distance to the plexus.



The combined success rate for accurate needle-nerve distance (1–4 mm) identification by electric nerve stimulation was 48.8% (95% confidence interval, 33.3–64.5), with a rate for <1 mm or direct needle-nerve contact of 16.3% (95% confidence interval, 6.8–30.7), and for >4 mm of 34.9% (95% confidence interval, 21.0–50.9). The null hypothesis of a success rate >75% was therefore rejected (P = .0002). Extending the distance concept toward a subsequent successful fluid spread unveiled that a correct distance per se does not preclude insufficient hydrodissection. Despite similar absolute numbers for hydrodissection (22/43 subjects), the latter were, in part, different individuals and did include 7 patients with an observed contact of the needle tip and the nerve (see Table). Thus, topographic position and lack of obstruction for the injected fluid bolus were highly relevant for successful hydrodissection, rather than sheer needle-nerve distance or appropriate nerve stimulation.

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The present study assessed the accuracy of stimulation-based interscalene blockade with respect to visualized needle-nerve distance and hydrodissection. Results did not meet the intended criteria in almost 50% of patients. Thus, the basic principles of electric nerve localization—current intensity being proportional to the needle-nerve distance, and stimulating currents between 0.1 and 0.4 mA linked to distances of <2 mm—were proven wrong.2,3,8 Electric current conduction, propagation, and initiation of a motor response is probably influenced by anatomic structures, such as the arrangement of muscles, connective tissues, and resistive barriers in the vicinity of the needle’s path, more so than assumed.5 Unlike study setups chosen to observe ongoing stimulator-based needle placement using ultrasound, the present study is unbiased toward “live observation,” but cannot state whether or not the patients belonging to the insufficient hydrodissection and/or wrong distance/anatomic location groups would have experienced inadequate blockade. Despite reported safety numbers and success rates, electric nerve stimulation turned out to be a rather indirect localizing method, and not a precise guidance for needle placement.

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The authors are indebted to Dr Mitchell Fingerman, Department of Anesthesiology, Division of Regional Anesthesia, Washington University in St. Louis, St. Louis, MO, for valuable advice in designing the study and writing the manuscript, and to Dr Marcus Vollmer, Department of Bioinformatics, Universitätsmedizin Greifswald, for statistical advice.

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Name: Stefan Fielmuth, MD.

Contribution: This author helped initiate the study, write the protocol, conduct the study, analyze the data, and write the manuscript.

Name: Marek Szalata, MD.

Contribution: This author helped write the protocol, conduct the study, and process images.

Name: Heidi Sievert, MD.

Contribution: This author helped write the protocol, conduct the study, and analyze the data.

Name: David Beier, MD.

Contribution: This author helped write the protocol, conduct the study, and analyze the data.

Name: Sebastian Rehberg, MD.

Contribution: This author helped analyze the data and write the manuscript.

Name: Klaus Hahnenkamp, MD.

Contribution: This author helped analyze the data and write the manuscript.

Name: Knut Mauermann, MD.

Contribution: This author helped conduct the study and write the manuscript.

Name: Konrad Meissner, MD.

Contribution: This author helped write the protocol, conduct the study, and write the manuscript.

This manuscript was handled by: Richard Brull, MD, FRCPC.

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