• Admir Hadzic, MD

        INTRODUCTION

Eliciting paresthesia or nerve stimulation are commonly used methods for localizing nerves prior to the injection of local anesthetic. Paresthesia is thought to result from mechanical stimulation of the nerve, resulting in a sensory feeling described as “an electric current” or “shock” in the sensory distribution of the nerve that is being touched. As such, paresthesia can indicate that the needle is in close proximity to the nerve and may be a warning sign of impending mechanical injury, should the needle be further advanced. In contrast, nerve stimulation techniques rely on the use of electric current to elicit motor stimulation of nerves and confirm the proximity of the needle to the nerve.

        Electrical nerve stimulation is currently the most common technique for localizing nerves prior to the injection of local anesthetic. Depolarizing the nerve membrane results in contraction of the effector muscles (motor fibers) or in paresthesias (sensory fibers) in the distribution of the nerve. These responses can be used to confirm the proximity of a needle or catheter to the nerve. This localization technique for nerve blocks was first described by von Perthes in 1912; however, it has only gained wider acceptance in regional anesthesia over the last two decades.¹ Subsequently, a number of researchers have further improved and developed this technique. Pearson introduced the concept of using an insulated needle for the localization of nerves;² however, Montgomery and colleagues later demonstrated that ordinary uninsulated needles could also be used to localize nerves, albeit with a higher current.³ The use of a portable transistorized nerve stimulator with a variable current output was first introduced by Greenblatt and Denson.⁴ Ford and associates further emphasized the important characteristics of electrical nerve stimulators and the differences between insulated and uninsulated needles.^5,6 In recent years, the same electrical stimulation principles have been applied for new uses such as percutaneous electrode guidance (PEG),^7–10 confirmation of epidural catheter placement,^11–13 and peripheral catheter placement for continuous regional anesthesia.¹⁴ The clinical relevance of the duration of the stimulating current and optimal placement of the return (skin) electrode have also been determined.¹⁵

Figure 5-1.Strength-duration curve. The curve illustrates the relationship between the threshold current intensity and pulse duration. (Adapted, with permission, from Pither CE, Raj PP, Ford DJ: The use of peripheral nerve stimulators for regional anaesthesia: A review of experimental characteristics, technique, and clinical applications. Reg Anesth 1985;10:49-58).

        In order to use nerve stimulation effectively a basic knowledge of the electrophysiologic principles is necessary. The following discussion is based on the commonly accepted theoretical and practical concepts of nerve stimulation. However, our understanding of the mechanisms of nerve stimulation is still incomplete. Thus, the reader should remain cognizant of the fact that current literature still has conflicting concepts and recommendations regarding several aspects of nerve stimulation.¹⁶

        CHARACTERISTICS OF ELECTRICAL IMPULSE: INTENSITY, DURATION, & RATE OF CHANGE

Clinical Pearls

        Electrical impulses excite nerves by inducing a flow of ions through the nerve membrane, which then initiates an action potential. The characteristics of the electrical impulse affect its ability to stimulate motor and sensory nerve fibers. The quality of stimulation is also influenced by the polarity and the type of electrode, the distance between the stimulating needle and the nerve, and the interactions at the tissue-needle interface.

Intensity

A total charged (Q) applied to a nerve is equal to the product of the intensity (I) of the applied current and the duration (f) of the square pulse of the current:

        The minimum current intensity (7) required to produce an action potential can be expressed by the relationship I = Ir (1 + C/t), with three important parameters (Figure 5-1). In this equation: (t) is the duration of the applied pulse (Ir), or rheobase, is the minimum current intensity required to depolarize the nerve and (C), or chronaxy is the minimum duration of the pulse required to depolarize the nerve when the current intensity is twice the rheobase.

        Chronaxy measures the stimulation threshold of the different types of nerve fibers.¹⁷ The larger the fiber, the shorter its chronaxy and the easier it is to stimulate.^18,19 Varying the electrical pulse width can stimulate different types of nerve fibers. Large A-a motor fibers with chronaxies of 50-100 μsec can be stimulated without stimulating the smaller A-δ and C sensory fibers with chronaxies of about 150 μsec and 400 μsec, respectively.^20,21 A painless motor response to nerve stimulation can be elicited using a lowintensity stimulating current and a short pulse width because sensory fibers require a longer pulse for stimulation. The current intensity required to obtain a clinically discernable motor response is shown in Figure 5-2. However, recent data have demonstrated that choosing the pulse width to achieve sensory-motor differentiation and avoid discomfort to the patient during peripheral nerve stimulation may not be applicable in clinical practice as previously thought.^22,23 Rather, the main causes of discomfort are related to the withdrawal and repositioning of the stimulating needle,²⁴ maintain the strength of elicited muscle contractions, and the high-intensity of the stimulating current.²² The discomfort on nerve stimulation occurs mainly when an exaggerated (violent) motor response to motor response is elicited, regardless of the current intensity, Figure 5-3.¹⁵ Therefore, when the current intensity is properly selected (and appropriately low current intensity is used), one can avoid discomfort on nerve stimulation with current of any duration (0.05-1.0 msec). By and large, using a longer pulse width of at least 300 μsec to 1 msec and low-intensity current, one may still preferentially stimulate sensory nerves and elicit a radiating paresthesia in the distribution of the nerve with every pulse.^20,24

Figure 5-2. Intensity of the stimulating current required to obtain specific visible motor response during interscalene (biceps contractions) and femoral block (quadriceps contractions). Current of longer duration elicits motor response at significantly lower current intensity.

        A narrow pulse width may be superior to a long pulse width in estimating the relative distance between needle and nerve.⁵ This is illustrated in Figure 5-4 where the current intensity necessary to produce a twitch is plotted against the pulse width. The narrower the pulse width, the greater the intensity required. When the needle tip is in contact with the nerve, the pulse width of the stimulus has only a moderate effect on the minimal intensity required. However, when the needle tip is far away from the nerve, the influence of the pulse duration becomes a more important variable. For instance, when the needle tip is 1 cm away from the nerve, compared with the needle directly contacting the nerve, there is a 10-fold increase in the threshold current with a 40-μsec pulse. If the pulse width is lengthened to 1 msec, the required current increase is only two fold.

Rate of Change

A prolonged subthreshold stimulus or a slowly rising current may reduce nerve excitability by inactivating sodium conductance before the depolarization reaches its threshold.²⁵ Under these circumstances, it may be impossible to stimulate a nerve fiber even with a strong stimulus, if it is applied too slowly. This phenomenon is described as the “accommodation” of nerve fibers. To avoid accommodation in clinical practice, a square wave of current with a sharp rising time is typically used.

        POLARITY OF STIMULATING & RETURNING ELECTRODES

Clinical Pearls

Figure 5-3. Intensity of the stimulating current at which a painful motor response (VAS >> 3) is elicited during interscalene brachial plexus (biceps muscle) and femoral nerve (quadriceps muscle) stimulation is obtained. The discomfort on stimulation occurs at a significantly lower current intensity with currents of longer rather than shorter duration. However, the discomfort occurs only with the exaggerated (violent) motor response, regardless of the current intensity. Therefore, the main factor in determining the discomfort during nerve stimulation is the energy (E = I(mA) × t(sec)) delivered to the tissue.

Figure 5-4. Distance-pulse duration curve. A short pulse duration is a better discriminator of the distance between needle and nerve. (Adapted, with permission, from Pither CE, Raj PP, Ford DJ: The use of peripheral nerve stimulators for regional anaesthesia: A review of experimental characteristics, technique, and clinical applications. Reg Anesth 1985;10:49–58.)

        The polarity of the stimulating current is an important aspect of nerve stimulation. Preferential cathodal stimulation refers to the fact that when the cathode (negative electrode) is used as the stimulating electrode, rather than the anode (positive electrode), significantly less current (three to four times) is required to elicit a motor response.^5,21,26,27 Thus, the negative electrode is typically connected to the stimulating needle/catheter and the positive electrode to the patient’s skin as the returning electrode. The negative current from the cathode reduces the voltage immediately outside the membrane. As a result, the voltage gradient across the membrane is decreased, causing an area of depolarization and resulting in an action potential (Figure 5-5). Hyperpolarization and a decrease in excitability occurs if the polarity of the stimulating and returning electrodes are reversed. It was previously suggested that the returning electrode (anode) had to be positioned at least 20 cm away from the site of stimulation in order to prevent the direct stimulation of muscles via a local flow of the current.²⁸ However, recent reports have demonstrated that the anode site is not critical when using a constant-current output nerve stimulator.²¹

Figure 5-5. Preferential cathodal stimulation. A. Cathode stimulation favors depolarization B. Anode stimulation results in hyperpolarization and requires higher current for depolarization. (Adapted, with permission, from Pither CE, Raj PP, Ford DJ: The use of peripheral nerve stimulators for regional anaesthesia: A review of experimental characteristics, technique, and clinical applications. Reg Anesth 1985;10:49–58.)

        DISTANCE-CURRENT INTENSITY RELATIONSHIP

Clinical Pearls

Peripheral Nerve Blocks

The relationship between the current intensity required for excitation and the distance from the nerve is governed by Coulomb’s law:

where (I) is the current required, (k) is a constant, (i) is the minimal current, and (r) is the distance from the nerve. Since the current relationship is the inverse of the square of the distance, a very high stimulus current is required as the stimulating electrode moves away from the nerve (Figure 5-6).²⁰ In clinical practice, an initial stimulating current of 1-2 mA with a pulse of 100 to 200 μsec is used to elicit a response. The stimulating needle is then advanced until it reaches a distance close enough to the nerve to elicit contractions of the appropriate muscle group at a threshold current less than 0.5 mA. The most common acceptable current range with a clear motor response is between 0.2 and 0.5 mA.²⁹ It has been postulated that stimulation at currents higher than 0.5 mA may result in block failure because the needle tip may be too far from the nerve,³⁰ whereas stimulation at currents lower than 0.2 mA may increase the risk of intraneural injection.²¹ It has been also suggested that it was unnecessary to search for a nerve response at currents lower than 0.2 mA (100 μsec) because the minimal current required to produce an easily visualized twitch was about 0.3 mA with a 100-μsec duration.²⁰ These threshold currents may not apply to all patients, particularly elderly patients or those patients with underlying neuropathies or diabetes, as these patients may have slower nerve conduction velocities and lower motor response amplitudes.^31,32

Figure 5-6. Distance-current relationship for insulated and uninsulated needles. (Adapted, with permission, from Pither CE, Raj PP, Ford DJ: The use of peripheral nerve stimulators for regional anaesthesia: A review of experimental characteristics, technique, and clinical applications. Reg Anesth 1985;10:49–58.)

Percutaneous Electrode Guidance

Clinical Pearls

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Ultrasound-Assisted Nerve Blocks in Adults. Cutaneous Blocks for the Upper Extremity. Cervical Plexus Block.