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Minimal Current Intensity to Elicit an Evoked Motor Response Cannot Discern Between Needle-Nerve Contact and Intraneural Needle Insertion

Wiesmann, Thomas MD*; Bornträger, Andreas MD*; Vassiliou, Timon MD*; Hadzic, Admir MD, PhD; Wulf, Hinnerk MD*; Müller, Hans-Helge MD; Steinfeldt, Thorsten MD*

doi: 10.1213/ANE.0b013e3182a94454
Regional Anesthesia: Research Report

BACKGROUND: The ability of an evoked motor response (EMR) with nerve stimulation to detect intraneural needle placement reliably at low current intensity has recently been challenged. In this study, we hypothesized that current intensity is higher in needle-nerve contact than in intraneural needle placement.

METHODS: Brachial plexus nerves were exposed surgically in 6 anesthetized pigs. An insulated needle connected to a nerve stimulator was placed either with 1 mm distance to the nerve (control position), adjacent to nerve epineurium (needle-nerve contact position), or inside the nerve (intraneural position). Three pulse duration settings were applied in random fashion (0.1, 0.3, or 1.0 milliseconds) at each needle position. Starting at 0.0 mA, electrical current was increased until a minimal threshold current resulting in a specific EMR was observed. Fifty threshold current measurements were scheduled for each needle position-pulse duration setting.

RESULTS: Four hundred-fifty threshold currents in 50 peripheral nerves were measured. Threshold current intensities (mA) to elicit EMR showed small differences between the needle-nerve contact position [median (25th–75th percentiles); 0.1 milliseconds: 0.12 (0.08–0.18) mA; 0.3 milliseconds: 0.10 (0.06–0.12) mA; 1.0 milliseconds: 0.06 (0.04–0.10) mA] and the intraneural position (0.1 milliseconds: 0.12 [0.10–0.16] mA; 0.3 milliseconds: 0.08 [0.06–0.10] mA; 1.0 milliseconds: 0.06 [0.06–0.08] mA) that are neither statistically significant nor clinically relevant. Regardless of the pulse duration that was applied, the 98.33% confidence interval revealed a difference of at most 0.02 mA. However, threshold current intensities to elicit EMR were lower for the needle-nerve contact position than for the control position (0.1 milliseconds: 0.28 [0.26–0.32] mA; 0.3 milliseconds: 0.20 [0.16–0.22] mA; 1.0 milliseconds: 0.12 [0.10–0.14] mA).

CONCLUSIONS: The confidence interval for differences suggests minimal current intensity to elicit a motor response that cannot reliably discern between a needle-nerve contact from intraneural needle placement. In addition, an EMR at threshold currents <0.2 mA (irrespective of the applied pulse duration) indicates intraneural needle placement or needle-nerve contact.

Published ahead of print November 27, 2013

From the *Faculty of Medicine, Department of Anesthesiology and Critical Care Therapy, Philipps University Marburg, Marburg, Germany; St. Luke’s and Roosevelt Hospital Center, University Hospital of Columbia University, College of Physicians and Surgeons, New York City, New York; and Institute of Medical Informatics, Biostatistics and Epidemiology (IBE), Ludwig-Maximilians-University (LMU), Munich, Germany.

Accepted for publication August 7, 2013.

Published ahead of print November 27, 2013

Funding: Institutional grants.

Conflict of Interest: See Disclosures at the end of the article.

This report was previously presented, in part, at the ESRA 2011, Dresden, Germany.

Reprints will not be available from the authors.

Address correspondence to Thomas Wiesmann, MD, Department of Anesthesiology and Intensive Care Therapy, Philipps University Marburg, University Hospital Giessen-Marburg, Campus Marburg, Baldingerstrasse, 35032 Marburg, Germany. Address e-mail to

© 2014 International Anesthesia Research Society