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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

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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).

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Fig. 22-1
Graphic Jump Location

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

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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

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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 ...

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