Conventional methods for peripheral nerve or plexus blockade have
involved the identification of surface anatomic landmarks. Such landmarks
serve as an approximate starting point for a search for the targeted nerve
or nerves by needle exploration. The objective of needle exploration is to
reach a finite endpoint that indicates the tip of the needle is sufficiently
close to the targeted nerve or nerve plexuses. Two distinct types of
1. An anatomic endpoint based on encountering
anatomic relations to the targeted nerve or nerves. Examples of blocks that
make use of anatomic endpoints include field block, transarterial techniques,
or ultrasonographic guidance.
2. A functional endpoint based on a nerve
response to mechanical or electrical stimulation. The main types of
functional endpoints used clinically are either sensory response to
mechanical stimulation of the nerve (mechanical paresthesia) or a motor
response to electrical stimulation.
Designated anatomic landmarks are limited because they vary from patient
to patient and do not always correlate with the location of the underlying
nerve or plexus. In addition, traditional landmark measurements are
sometimes complicated, requiring linear measurements with a ruler, bisecting
lines, and they are not always normalized to patient size or body habitus.
For many blocks, accepted descriptions of the technique include insertion of
the block needle at a certain distance from a designated palpable
landmark, without regard to patient size. Consequently, with many
techniques, dexterity and delicate proprioception are often required for
successful block performance.
Techniques such as ultrasonography or other imaging techniques or
percutaneous localization utilizing transcutaneous electrical stimulation
help to decrease needle exploration. Transcutaneous electrical stimulation,
in contrast to an imaging technique such as ultrasonography, utilizes a
functional neural response, either motor or sensory. Prelocalization of the
nerve prior to needle insertion serves to decrease the amount of invasive
search with the needle, increasing patient comfort while decreasing the
potential for complications. The purpose of this chapter is to discuss how
transcutaneous electrical stimulation helps to localize the underlying nerve
or plexus through the skin, in a noninvasive manner before the needle is
introduced transcutaneously. The chapter will necessarily discuss basic
elements of nerve stimulation; however, for more in-depth coverage of
principles of nerve stimulation and nerve stimulators, the reader is
referred to Chapters 5 (Electrophysiology of Nerve Stimulation) and 17
(Equipment for Peripheral Nerve Blocks).
Elicitation of a paresthesia is an all-or-nothing phenomenon, ie, the
needle either contacts the nerve or it does not. By contrast, use of
electrical nerve stimulation yields graded information, which may be useful
at a distance from the targeted nerve. Furthermore, visual cues of motor
responses from untargeted nerves allow for redirection of the needle. This
concept has been extended to the use of transcutaneous electrical
stimulation to yield visual cues and motor responses, noninvasively, through
the intact overlying skin.
Transcutaneous electrical stimulation to elicit a motor response has
been used to assist in determining the optimal entry point ...