Skip to Main Content
Chapter 22

A nerve stimulator delivers a charge to a motor nerve fiber, which causes a flow of ions through the nerve membrane and initiates an action potential in the nerve fiber.1 The larger the delivered charge, the more motor fibers in the motor nerve fascicle will fire and the stronger the resulting motor response until a maximal motor response is reached. Several factors influence the total charge delivered to a nerve fiber. The duration and intensity of the current are the main variables. The impedance of the tissue between the stimulating surface (needle or catheter) and the nerve further influence the current intensity. This is reduced as the distance from the stimulating surface to the nerve is reduced. In addition, the type of tissue between the stimulating surface and the nerve, the type of electrode used, and the polarity of the electrode affect the total impedance of the system and thus affect the motor response to stimulation. Muscle mass is also important, such that a stimulus in a frail 80-year-old woman differs substantially from a motor response from a similar electrical response delivered to a young muscular patient.1

Beyond exposure to regional anesthesia during training, an important aspect in developing confidence and proficiency in the placement of peripheral nerve blocks is through observing a well-defined motor response to neurostimulation. At the level of the roots, the brachial plexus originates from five separate nerve bundles, mainly divided into posterior sensory fibers and anterior motor fibers2 (Figure 22–1). More distally, the five roots of the brachial plexus converge to form three trunks: the superior, middle, and inferior trunks. The brachial plexus then divides into the three cords, and finally the seven terminal branches are formed. The three cords are named according to their relation to the axillary artery: posterior, lateral, and medial (see Figure 22–1).

###### Fig. 22-1

Schematic representation of the nerves of the brachial plexus and upper extremity.

Success with brachial plexus blockade requires the identification and block of the appropriate roots, trunks, cords, and peripheral nerves for the proposed surgery.3 Accurately identifying the correct nerve bundles and avoiding injection of incorrect nerves may lead to higher success rates if the single stimulation technique of the infraclavicular block, for example, is followed,4,5 or it may lead to shorter latency periods if the multiple stimulation technique6 is chosen. In the case of blocks at the trunk level, it is also important to recognize false motor responses and avoid injecting local anesthetic when these are elicited.7

Whichever approach to blocks of the brachial plexus is chosen, elicitation of specific muscle twitches when each nerve bundle is stimulated is often confusing for trainees and experienced anesthesiologists alike. This chapter describes a simple and consistent way to teach and remember the motor responses elicited by electrical ...

### MyAccess Sign In

Username
Password

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

### About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

## Subscription Options

### AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more

$995 USD ### Pay Per View: Timed Access to all of AccessAnesthesiology 24 Hour Subscription$34.95

Buy Now

48 Hour Subscription \$54.95

Buy Now

### Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.