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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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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 stimulation ...