Ovid: Peripheral Nerve Blocks: A Color Atlas

Nerve Stimulators and Insulated Needles

Admir Hadzic

Jerry Vloka

History

The first description of electrical stimulation to locate the brachial plexus was recorded by Perthes in 1912. However, the acceptance of this method to aid in performance of peripheral nerve blocks was not realized until the 1960s when electronic advances and the consequent introduction of more convenient solid-state units were made. Greenblatt and Denson demonstrated that motor nerves can be stimulated without eliciting pain and that the current required to stimulate the nerve depends on the distance between the needle and the target nerve. In the last two decades, peripheral nerve stimulation techniques have largely replaced paresthesia techniques for most major conduction blocks, particularly in lower extremity blockade. This approach is well accepted and is associated with favorable success rates. It is important to realize that nerve stimulators are not used as a replacement for the sound knowledge of anatomy, but to help to position the needle in closer proximity to the nerve without a required contact with the nerve (paresthesia) and with less discomfort to the patient.

Basic Electrophysiology

In order to propagate a nerve impulse, a certain threshold stimulus must be applied to the nerve. The ability to stimulate a nerve depends on the intensity of the current applied and the duration of the current. In mixed nerves it is possible to stimulate the motor component without eliciting pain by limiting the current intensity and duration. To stimulate motor fibers, a current of shorter duration (0.05 to 0.2 ms) is typically used. The use of a shorter pulse duration increases the likelihood of an increased proximity between the nerve fibers and the unshielded tip of the needle, but makes the localization of the nerve more challenging. Consequently, the nerve stimulator is usually set up with a current of 1 to 1.5 mA and a pulse duration of 0.1 to 0.3 ms. The intensity of the current is decreased along with the pulse duration to adjust the position of the needle. In contrast, the stimulation of sensory fibers requires longer pulse duration time (0.3 to 1.0 ms) than do motor fibers (0.05 to 0.1 ms). With such a setup it is possible to locate sensory nerves such as the radial nerve at the wrist, the lateral femoral cutaneous nerve, and the saphenous nerve by eliciting electrical paresthesia. The use of longer pulse duration is appropriate in patients with peripheral neuropathy, including diabetic patients. In these patients it is often necessary to

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stimulate with a pulse duration time of 0.3 to 1.0 ms to elicit a motor response. The access to multiple pulse duration times represents a major improvement of the nerve stimulator presently available. An important principle of peripheral nerve stimulation is the preferential “cathodal stimulation.” In other words, when the nerve is stimulated by an electrode, significantly less current is required to obtain a response to a nerve stimulation when the cathode (negative) rather than the anode (positive) is adjacent to the nerve. This principle has significant clinical applications, and it requires that clinicians pay particular attention to the polarity of the electrodes. Another fundamental principle is that the current intensity required to stimulate the nerve is in relationship to the distance of the needle from the nerve. As the stimulating tip moves away from the nerve, the relationship between the current and the distance from the nerve is governed by Coulombs law:

E = K(Q/r2),

where E is the current required to stimulate and r is the needle–nerve distance. This principle is used to estimate needle–nerve distance by employing a stimulus of known intensity and pulse duration. It should be noted that this relationship is not linear, which means that as the needle–nerve distance increases, a current of substantially greater intensity is required to stimulate the nerve.

Important Features of Nerve Stimulators for Regional Anesthesia

It is important to realize that the nerve stimulator-assisted nerve block techniques assume that nerve stimulators are accurate and user-friendly to maneuver during block performance. Skin resistance, electrode surface resistance, and gel conductivity can vary widely. For these reasons, nerve stimulators for use in regional anesthesia should be specifically engineered for that application, rather than be an “all-purpose” unit with neuromuscular block monitoring capabilities. A plethora of features present on some units does little to facilitate their use and adds to the complexity of their operation. Some of the desirable characteristics of nerve stimulators for regional blocks are outlined here (Fig. 2-1).

Figure 2-1. HNS-12 Nerve Stimulator. Variable pulse widths of 0.1, 0.3, and 1.0 ms. Displays current delivered by the nerve stimulator and received by the patient. Especially interesting for use on patients with peripheral neuropathy and for sensory nerve blocks.

Current intensity—It is important that the nerve stimulator deliver accurate current over a range of 0 to 5 mA.

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Variable pulse duration time—To allow for stimulation of motor as well as sensory nerves in patients with normal nerve conduction and peripheral neuropathy, the nerve pulse duration time should be variable rather than fixed.
Constant current output—This feature allows for an automatic compensation of the voltage output when tissue or connection impedance changes during nerve stimulation, ensuring accurate delivery of the specified current.
Current meter—It is important that the current display indicates both the current delivered by the nerve stimulator and the actual current delivered to the patient. The currents should be similar.
Current intensity control—Whatever means to control the current are used, it is important that the current can be easily and conveniently adjusted during block performance. Some newer units, which incorporate a remote control of the current, are also being introduced. This feature allows a single operator to perform the block by controlling the intensity of the current using a foot pedal or a hand-controlled device.
2-Hz stimulating frequency—While many nerve stimulators currently in clinical use feature 1-Hz stimulation, 2-Hz stimulation is clinically much more advantageous, since it allows faster manipulation of the needle.
Disconnect indicator—This feature alerts the operator when the stimulus is not being delivered due to a disconnection or unit problem.
Digital display—This feature allows the operator to monitor the intensity of the current delivered by the nerve stimulator to the patient, and the frequency and the pulse duration used.

Needles

Insulated beveled needles are commonly used in combination with a nerve stimulator for single nerve blocks (Fig. 2-2). The negative electrode of the nerve stimulator is connected to the insulated needle while the positive electrode of the nerve stimulator is connected to an electrocardiogram electrode serving as a ground electrode. There are different sizes of needles and, for some sizes, different gauges. Although the most appropriate angle at the tip remains the object of some debate (15° vs. 30°), the size and the gauge of the needle for a given block should be chosen according to the approach and the patient population (e.g., adult vs. pediatric, larger patients vs. smaller patients). Thus a 22-gauge, 2.5-cm needle is indicated for an interscalene block in adults, whereas in children a 25-gauge, 2.5-cm

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needle is preferred for the same block. The use of a longer needle (up to 5 cm) may also be indicated in morbidly obese or very muscular patients. Although some experts prefer the use of a 5-cm needle to perform an interscalene block, as a rule of thumb, for less-experienced practitioners, the shortest recommended needle is generally safest and should be preferred. Insulated needles are usually available in lengths from 2.5 to 15 cm. In addition, for the placement of perineural catheters for continuous nerve block techniques the use of an insulated introducer Tuohy needle is frequently preferred (Fig. 2-3). Although the use of a stimulating stylet for the placement of a perineural catheter was described as early as 1951, the use of a stimulating catheter (Fig. 2-4) has only recently been introduced clinically.

Figure 2-2. Insulated beveled needles commonly used in combination with a nerve stimulator for single nerve blocks.

Figure 2-3. Insulated introducer Tuohy needle.

In a few cases, it is possible to use noninsulated needles to perform peripheral nerve blocks, including transarterial (axillary) and paresthesia techniques (interscalene and axillary), and field blocks (median and radial nerve blocks at the wrist and fascia iliaca block). For the fascia iliaca block, the use of a Tuohy needle in adult patients facilitates the performance of the block by allowing a better feeling of the needle going through the fasciae lata and iliaca.

Figure 2-4. Stimulating catheter.

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