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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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NERVE STIMULATORS

In recent years, the use of nerve stimulators has moved from the status of occasionally used device to one of common use and often critical importance. The growing emphasis on techniques that employ multiple injections near individual nerves or placement of stimulating catheters has provided impetus for this change. The primary impediment to successful use of a nerve stimulator in a clinical practice is that it is at least a three-handed, or two-individual, technique ( Fig. 1-10 ). This situation has not changed, although there are devices available that allow foot control of the stimulator current, eliminating the need for a third hand or a second individual. In situations requiring a second set of hands, correct operation of contemporary peripheral nerve stimulators is straightforward and easily taught during the course of the block. There are a variety of circumstances in which a nerve stimulator is helpful (e.g., in children and adults who are already anesthetized when a decision is made that regional block is an appropriate technique, in individuals who are unable to report paresthesias accurately, when performing local anesthetic administration on specific nerves, and when placing stimulating catheters for anesthesia or postoperative analgesia). Another group who may benefit from the use of a nerve stimulator are patients with chronic pain, in whom accurate needle placement and reproduction of pain with electrical stimulation or elimination of pain with accurate administration of small volumes of local anesthetic may improve diagnosis and treatment.

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Figure 1-10  Nerve stimulator technique.


When nerve stimulation is used during regional block, insulated needles are most appropriate as the current from such a needle results in a current sphere around the needle tip, whereas uninsulated needles emit current at the tip as well as along the shaft, potentially resulting in less precise needle location. A peripheral nerve stimulator should allow between 0.1 and 10 mA of current in pulses lasting approximately 200 msec at a frequency of one or two pulses per second. The peripheral nerve stimulator should have a readily apparent readout of when a complete circuit is present, a consistent and accurate current output over its entire range, and a digital display of the current delivered with each pulse. This facilitates finding the general location of the nerve while stimulating at 2 mA and allows refinement of needle positioning as the current pulse is reduced to 0.5 to 0.1 mA. The nerve stimulator should have the polarity of the terminals clearly identified, as peripheral nerves are most effectively stimulated using the needle as the cathode (negative terminal). Alternatively, if the circuit is established with the needle as anode (positive terminal), approximately four times as much current is needed to produce equivalent stimulation. The positive lead of the stimulator should be placed at a site remote from the site of stimulation by connecting the lead to a common electrocardiographic electrode (see Fig. 1-10 ).

The use of a nerve stimulator is not a substitute for complete knowledge of the anatomy and careful site selection for needle insertion. In fact, as much attention should be paid to the anatomy and technique when using a nerve stimulator as when not using it. Large myelinated motor fibers are stimulated by less current than small unmyelinated fibers, and muscle contraction is most often produced before the patient experiences discomfort. The needle should be carefully repositioned at a point where muscle contraction can be elicited with 0.5 to 0.1 mA. If a pure sensory nerve is to be blocked, a similar procedure is followed; however, correct needle localization requires the patient to report a sense of pulsed “tingling or burning” over the cutaneous distribution of the sensory nerve. Once the needle is in the final position and stimulation is achieved with 0.5 to 0.1 mA, 1 mL of local anesthetic is injected through the needle. If the needle is accurately positioned, this amount of solution should rapidly abolish the muscle contraction and/or the sensation with pulsed current.

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