HOW CAN WE AVOID INTRANEURAL INJECTION DURING NERVE BLOCKS? That was the focus of a study we recently performed at our facility. Our aim was to find the optimal nerve stimulation levels to avoid this outcome. Interestingly, the results not only shed light on our original question, but on the roles of stimulation and ultrasound in peripheral nerve blocks.

Recent animal studies suggest that a motor response with currents < 0.2 mA only occurs when the needle is intraneural. We hypothesized that the level of stimulating threshold outside the nerve differs significantly from the level inside the nerve and can be used to predict whether the needle tip is extraneural or intraneural.

To test our hypothesis, we used a Stimuplex® A needle attached to a B. Braun HNS11 Nerve Stimulator to determine the minimal stimulation threshold needed to elicit a motor response just outside and inside the most superficial part of the brachial plexus during highresolution, ultrasound-guided, supraclavicular block. We enrolled 39 patients in the study.

We found that a motor response with a stimulation threshold of 0.2mA or less with pulse duration of 0.1mA appears to be a reliable predictor of intraneural needle position in patients with normal sensory-motor examination in the brachial plexus. Of the 39 patients, no motor responses occurred when the needle was outside the nerve with a current of < 0.2mA. In addition, we found that stimulation currents greater than 0.2 and as high as 1.0 or greater could not rule out intraneural position. Just under two-thirds of patients had stimulation thresholds that differed significantly (>0.3mA) with needle position.

Interestingly, in 23% of our cases, ultrasound indicated that the needle tip was intraneural, yet a current higher than 0.5mA was required to produce a motor response. This suggests that nerve stimulation does not necessarily gaurantee that the needle is outside the nerve and supports the use of ultrasound to help confirm the needle tip position.

So, what does this mean clinically? Practically speaking:

• If a motor response occurs with current less than 0.2mA, injection should not take place;
• Clinicians should not assume that intraneural injection is avoided just because a motor response is elicited with a threshold current between 0.2 and 0.5mA.

In sum, both ultrasound and nerve stimulation used synergistically can help provide more "situational awareness" during nerve blockade. Since neither ultrasound nor nerve stimulation alone can totally prevent intraneural injection, the combination of both techniques along with a blunt tip needle, low injection pressure, and slow injection can all help

Reference
Bigeleisen PE, Moayeri N, Groen GJ. Extraneural versus Intraneural Stimulation Thresholds during Ultrasound-guided Supraclavicular Block. Anesthesiology. 2009; 110:1235-43.

Dr. Bigeleisen is a Professor of Anesthesiology and Biomedical Engineering at the University of Pittsburgh and at the University of Rochester. His research focuses on ultrasound imaging and device invention.