William F. Urmey
Carlos A. Bollini
Nerve mapping in adults refers to the use of a surface stimulating electrode to outline the course of an underlying peripheral nerve or neural plexus. Nerve mapping can serve as a surrogate method to the use of conventional anatomical surface landmarks for the performance of peripheral nerve blocks. Conventionally, blocks of peripheral neural or plexuses have been performed following the identification of previously described or published surface landmarks, which serve as approximate starting points for invasive exploration with a block needle. Exploration with the block needle proceeds until the identification of an appropriate endpoint, following which injection of local anesthetic results in a high rate of success. Two types of such endpoints exist: 1) anatomical endpoints that rely on the identification of anatomical structures that are closely related to the nerve, and 2) functional endpoints that rely on neural function or response to mechanical or electrical stimulation by the block needle. Functional endpoints include mechanical paresthesia (sensory response to mechanical stimulation) as well as motor response to electrical stimulation, comprising the two most frequently utilized endpoints in peripheral or plexus blockade.
Designated anatomical landmarks simply serve as a starting point for invasive needle exploration, independent of the technique used. Anatomical landmarks have limitations. They vary from patient to patient, and are dependent on patient size or body habitus. Published anatomical landmarks often include measured angles and distances in centimeters that are not normalized or adapted to patient size or variations in anatomy. Such landmarks can only serve, at best, as an approximation.
Nerve mapping, in contrast to previously described anatomical landmarks, involves the use of electrical nerve stimulation and thus yields useful information prior to needle insertion. This allows for prelocalization of the targeted nerve or plexus prior to invasive search with the needle. Nerve mapping has in common with ultrasonography or other imaging techniques the ability to yield information noninvasively to facilitate nerve location. This chapter focuses on the science of nerve stimulation, in particular transcutaneous nerve stimulation for nerve mapping or prelocation of nerves in adults. The recently published technique of indentation percutaneous electrode guidance (PEG) of the block needle will be discussed as it applies to enhancement of the concept of nerve mapping and subsequent final location and injection of the targeted nerve or plexus.
Luigi Galvani first demonstrated that an electrical charge could result in an electrical stimulation and resulting muscular contraction in 1780. However it was not until 1850 when the underlying physiology was studied in depth by Von Helmholtz, who performed numerous experiments using isolated nerve/muscle preparations. In 1912, Perthes described the technique of electrical stimulation using a selective peripheral nerve stimulator with a nickel-insulated needle to assist in regional anesthetic neural blockade. Although there was strong interest in regional anesthesia and anesthetic techniques in the beginning of the twentieth century, this interest faded for a period. A resurgence in regional anesthetic techniques occurred later in the century, which has increased dramatically to the present day. In 1955, Pearson was the first to use an insulated needle to successfully locate motor nerves by electrical stimulation. In 1962, Greenblatt and Denson used a self-built electrical nerve stimulator to assist in peripheral nerve and plexus blocks. They demonstrated that the motor component of mixed nerves could be stimulated without causing pain. In 1969, Magora, using an electrical stimulator with an ammeter, determined that 0.5 mA was a suitable stimulation threshold for successful blockade of the obturator nerve. In 1980 Raj et al. re-introduced the idea of nerve stimulation to assist in the performance of peripheral nerve blocks, ushering in the modern era of the electrical nerve stimulator. A review article was published in 1985 by Pither, et al. that covered the experience to that date of the use of electrical nerve stimulators in regional anesthesia, focusing on the characteristics of nerve stimulators, needles, basic science, clinical technique, and applications of the techniques.
In 1993, Ganta and colleagues reported on the use of a modified electrocardiographic electrode with coupling gel that was used to assist in the performance of interscalene block by prelocation of the brachial plexus using transcutaneous stimulation. In 2002, Urmey and Grossi first described the technique of percutaneous electrode guidance. These investigators used a cylindrical electrically shielded cutaneous electrode to map nerves by transcutaneous stimulation as well as to guide the block needle to the targeted nerve by indentation of the skin using the cylindrical electrode. Also in 2002, Bosenberg and colleagues published a report that many superficial peripheral nerves can be “mapped” prior to skin penetration by transcutaneous stimulation in the 2 to 3.5 mA range. In 2003, Hadzic, et al. analyzed the characteristics of a large number of various commercially available nerve stimulators using an oscilloscopic analysis of the resulting square waves. In 2004, the same investigators examined the significance of anatomical placement of the ground lead as well as the relationship between the electrical pulse amplitude (amperage) and pulse duration. In 2006, Urmey and Grossi published the technique of sequential electrical nerve stimulation, which used a series of alternating pulse durations to increase feedback or information—motor responses—at a distance from the nerve.
Scientific Principles of Electrical Nerve Stimulation
The method of peripheral nerve stimulation for nerve location involves the use of a peripheral nerve stimulator; that is, a direct current (DC) “square-wave” impulse generator. Peripheral nerve stimulators typically supply a constant electrical current, the frequency (Hz), amplitude (mA), and pulse duration (ms) of which can be manipulated in order to assist in location of motor or mixed motor/sensory nerves. Depolarization of the nerve depends on the distance from the electrical field generated at the tip of the stimulating microelectrode needle, the electrical charge, and the stimulation threshold of the targeted nerve. Depolarization and the resulting action potentials will elicit a motor response and movement of varying intensity.
The peripheral nerve stimulator (PNS) consists of four essential components: an oscillator, a constant current generator, a display, and capabilities to control stimulus, intensity, duration, and frequency. The most modern stimulators involve a microprocessor that is programmed to control these parameters and ensure their accuracy. Most modern units are constant current generators that ensure accurate delivery of a constant current in the face of changes in electrical impedance that occur between anode and cathode.
In order to stimulate a nerve a certain charge threshold must be achieved. The electrical charge applied to the nerve is a product of the current amplitude (mA) and the pulse duration in milliseconds (ms). The threshold for stimulation of the nerve is quantified by the rheobase and chronaxie of the nerve. The characteristic rheobase of a nerve (Fig. 4-1) is the lowest current amplitude with long or indefinite pulse duration applied to depolarize a nerve. Chronaxie is the pulse duration at which the threshold current amplitude is twice that of the rheobase. A pulse duration longer than the chronaxie is not desired because current consumption is increased without decreasing the threshold significantly.
I = Ir (1 + C/t)
where I = current amplitude, Ir = rheobase, C = chronaxie, and t = pulse duration, illustrates that the current amplitude necessary for nerve stimulation is very dependent on the pulse duration of the stimulus. Larger fibers are more easily stimulated than smaller fibers. Large motor fibers can be stimulated with pulse durations as low as 0.05 ms with very little discomfort.
Figure 4-1. Approximate distances between needle microelectrode and the targeted axon in practice when a pulse duration of 0.1 ms is utilized. Actual distance and current amplitude varies to some degree depending on actual electrical impedances of the tissues between the stimulating electrode and the nerve. Such variations are minimized by use of a constant current generator. (Reprinted with permission from Bollini C, Cacheiro F. Peripheral nerve stimulation. Tech Reg Anesth Pain Manage 2006;10:79–88.)
Nerve localization has been achieved by following previously described surface anatomical landmarks. Such landmarks have been widely published and underlie the various techniques used for regional anesthetic blocks. Surface landmarks have been described for many different peripheral nerves or plexuses of nerves. Following identification of landmarks, a block needle has been used to search for a distinct endpoint with the objective of putting the needle tip in the immediate vicinity of the targeted nerve or nerves. Two categories of endpoints exist in practice. Anatomical endpoints are dependent on intimate anatomical relationships of other structures to the targeted nerve or nerves. Examples of regional anesthetic techniques that utilize anatomical endpoints include transarterial or periarterial techniques for brachial plexus block, field blocks, or the use of imaging techniques such as ultrasonographic guidance. Functional endpoints, by contrast, require neural function; that is, a neural response to mechanical or electrical stimulation. The two major functional endpoints that have been used in clinical practice include: 1) a sensory response to mechanical stimulation (i.e., a mechanical paresthesia), and 2) a motor response to electrical stimulation (i.e., a muscular twitch).
The use of motor responses to electrical stimulation has dominated the field of regional anesthesia in recent years. The major difference between the use of a motor response to electrical stimulation and a mechanical paresthesia is that the motor response is a graded phenomenon that yields information about nerve location from a distance, whereas a mechanical paresthesia is an all-or-nothing response requiring contact with the nerve. A mechanical paresthesia supplies no information at distance from the nerve.
Optimal Use of A Peripheral Nerve Stimulator for Nerve Location
A peripheral nerve stimulator utilizes an oscillating rectangular wave current generator. By altering the time base on an oscilloscope, these waves can be graphed as “square-waves”; therefore, nerve stimulators are referred to as square-wave generators. These square waves or “pulses” are programmed to occur at a given frequency, typically 1 to 2 Hz (1 to 2 cycles/sec). Most commercially available nerve stimulators today are constant current generators that deliver accurate pulses of electrical current in the face of varying tissue impedances (resistances, capacitances, and inductances). The newest commercially available peripheral nerve stimulators are capable of producing electrical pulses of accurate duration in the 0.1 to 1.0 ms range. They have the capability of continuously and accurately controlling electrical current amplitude in the range of 0 to 5 mA. Armed with modern peripheral nerve stimulators, the regional anesthesia practitioner is able to control the following variables during nerve location: 1) electrical pulse frequency, 2) current amplitude (amperage), 3) electrode conductive area, 4) electrical pulse duration, and 5) tissue electrical impedance.
Electrical Pulse Frequency
The most commonly utilized electrical pulse frequency during peripheral nerve stimulation is 2 Hz. This frequency is the same as that used during train-of-four stimulation used to monitor the degree of motor blockade during general anesthesia. Although most stimulators allow 1 Hz stimulation, 2 Hz stimulation results in more rapid feedback, thus serving to decrease the amount of time required for nerve location. Frequency can be increased above 2 Hz, but above 4 Hz there is inadequate time for the relaxation phase of the action potential, which results in sustained tetanus. Therefore, 1, 2, or 3 Hz are acceptable frequencies during neural location.
Electrical current transmission between 2 electrodes in a homogeneous medium of constant impedance follows what is commonly referred to as the “inverse square law.” The relationship between required electrical current was first understood and described by Coulomb in the 1780s. The relationship between the required electrical current to stimulate a nerve and the distance to the nerve follows Coulomb's law:
Figure 4-2. As shown in this computer model of an electrical field surrounding a block needle used in nerve location, electrical current dissipates very quickly from the tip of the needle to the inverse square of the distance from the needle tip. Movement of the needle tip just a few millimeters away from the nerve may require several-fold current increases to achieve similar motor response to electrical stimulation. (Reprinted with permission from Johnson CR, Barr RC, Klein SM. A computer model of electrical stimulation of peripheral nerves in regional anesthesia. Anesthesiology 2007;106:323–330.)
E = K(Q/r2)
where E = required stimulating current, K = constant, Q = minimal required stimulation current, and r = distance between electrode and nerve. According to this law, or as can be seen from this relationship as described by Coulomb's law, electrical current dissipates very rapidly as a function of the distance from the tip of the stimulating electrode (to the inverse square of the distance between the needle and the nerve). This relationship was recently analyzed by computer modeling by Johnson, et al. and is illustrated in Figure 4-2. As a stimulating electrode moves away from a peripheral nerve, the amount of current necessary to stimulate the nerve increases significantly. The converse is the underlying principle of nerve location utilizing electrical stimulation; that is, the ability to elicit a motor response at very low amperage (e.g., ≤0.5 mA) and low pulse duration (e.g., 0.1 ms) indicates extremely close proximity of the needle tip electrode to the nerve. Sung showed in studies of rabbit sciatic nerve that such stimulation correlated with a distance of approximately -1 mm to +1 mm in relation to the targeted sciatic nerve. This principle is responsible for the very high success rates published in clinical studies of peripheral nerve stimulation. Higher current amplitude (e.g., 0.5 to 5 mA) allows for stimulation and therefore visual cues at greater distance from the nerve or through the skin. Therefore, higher current amplitudes can be used for transcutaneous stimulation and nerve mapping at a distance from the targeted nerve or neural plexuses.
Stimulating Needle Electrode
Resistance to current flow and electrical conduction is inversely related to an electrode's conductive area. This relationship is described by Ohm's law:
where R = resistance, ρ = tissue resistivity, L = distance to the nerve, and A = conductive area. Therefore, when the goal is to minimize resistance to electrical current, as is the case with ventricular defibrillation by external chest paddles, the electrode surface is designed to be very large. For electrocardiographic recordings, electrodes are typically approximately 1 cm in diameter to minimize resistance. By contrast, needle-tip microelectrodes are used during nerve location for peripheral nerve block in order to limit electrical dispersion to a small microsphere at the needle tip. This ensures that the needle tip must be very close to the nerve to result in stimulation and motor response, thereby enhancing specificity and accuracy of nerve location. This property has lead to shielding of stimulating needles used in regional anesthesia. Bashein studied the difference between shielded microelectrode needles and metal needles that were not electrically shielded by the use of electrophoresis. These investigators found that the current dispersion was limited to the tip of the electrically shielded needle, but the unshielded needles conducted along the shaft as well as the tip.
Electrical Pulse Duration
Electrical pulse duration is the duration of the square wave pulse generated by a nerve stimulator at a given frequency. Short pulse durations of 0.05 to 1 ms have been used clinically for nerve location during regional anesthetic techniques. The 0.1 ms duration is the most commonly used clinically for ultimate nerve location prior to injection of local anesthetic. The total electrical charge of an electrical pulse is the product of the current amplitude (amperage) and pulse duration. Increasing electrical pulse duration increases the charge by allowing a greater flow of electrons to occur during the pulse. The total electrical energy is proportional to the calculated area under the curve of the rectangular pulse wave (Fig. 4-3). Therefore, simply by increasing pulse duration there is an increased ability to stimulate the nerve at any given amperage if all other parameters remain the same. Hadzic et al. showed that higher current amplitude was needed to elicit a similar motor response at lower pulse duration in a study of volunteers. Increasing electrical pulse duration also increases the ability to stimulate the nerve at a distance similar to what occurs when increasing amperage. Therefore, an increase in pulse duration results in an increase in sensitivity or ability to elicit a motor response during electrical nerve stimulation. Conversely, use of a higher pulse duration is less specific for final nerve location; that is, the ability to stimulate at ≤ 0.5 mA using pulse durations of, for example, 0.3 ms or 0.5 ms, at the same amperage would not be expected to yield the same high success rates that have been found at ≤ 0.5 mA, using a 0.1 ms pulse duration.
Figure 4-3. Actual oscilloscopic tracings of three square-wave pulses of 1 mA current amplitude from a commercially available peripheral nerve stimulator (B. Braun HNS 11). Current is shown in negative milliamperes (mA, y-axis) versus time base in milliseconds (ms, x-axis). Increasing pulse duration to 0.3 ms or 1.0 ms progressively increases the calculated area under the curve representing a larger flow of electrons for the same current amplitude. Larger pulse durations result in an increased ability to stimulate the nerve at a distance or through the skin without patient discomfort. (Adapted with permission from Urmey W, Grossi P. Use of sequential electrical nerve stimuli [SENS] for location of the sciatic nerve and lumbar plexus. Reg Anesth Pain Med 2006;31:463–469.)
Tissue Electrical Impedance
Electrical impedance of biological tissues is the inverse of tissue conductance. Flow of electrical current is inhibited by tissues of higher impedance. The electrical impedance of a tissue is a function of the tissue's resistance, capacitance, and inductance. However, impedance mainly represents electrical resistance in biological tissues. In general, the higher the lipid/water ratio, the higher the tissue resistance or impedance. Tissues have varying degrees of electrical impedance. Impedance varies, in general, according to tissue type as indicated below:
Most conductive → Least conductive
Nerves > Blood Vessels > Muscle > Skin > Fat > Bone
Transcutaneous Nerve Mapping in Adults
Transcutaneous nerve mapping is useful in determining the course of peripheral nerves or elongated nerve plexuses. Transcutaneous stimulation has been described to map nerves in children and adults. Prelocation of nerves or nerve plexuses gives information noninvasively to aid in nerve location. In theory, nerve mapping serves to decrease the number of invasive needle passes when searching for nerves.
Transcutaneous stimulation can be performed through tissues with the exception of underlying bone or lung. Conventional surface landmarks are useful starting points for mapping the course of the neural plexus or peripheral nerve. Since nerves are composed of specialized conductive tissue, they are characterized by very low electrical resistance. Other tissues have varying electrical resistances. By indentation of these other tissues toward the nerve, resistance is diminished and nerves can be mapped more easily and at lower current amplitudes. Use of smaller (i.e., ≤1 mm) diameter stimulating electrodes results in increased accuracy in outlining the course of the nerve or neural plexus.
Mapping of Specific Nerves and Plexuses
The brachial plexus is characterized by an almost linear course from the entry point for the conventional interscalene block by the Winnie technique to the needle entry point for the conventional axillary block. This course has been discussed in publications by Grossi, who coined the term the “Anesthetic Line” to describe it. Transcutaneous stimulation of the brachial plexus with a peripheral nerve stimulator can be easily achieved anywhere above the clavicle at low amperage by indentation of the overlying skin and subcutaneous tissues (Fig. 4-4). The brachial plexus courses below the clavicle, the infraclavicular area where needle entry points for conventional infraclavicular or coracoid approaches occur, posing more of a problem for successful transcutaneous stimulation. In thin adults, transcutaneous stimulation can be achieved with indentation of the overlying skin, subcutaneous tissues, and muscle. Since the plexus is deeper at this point, it is not always possible to stimulate transcutaneously at low milliamperage in larger adults. It is easier to successfully stimulate the brachial plexus at the point described for needle entry for vertical infraclavicular block (VIB) than for infraclavicular block sites that are more lateral. Moving to the axilla, axillary transcutaneous mapping of the brachial plexus can be easily and separately done for each of the four terminal nerves of the brachial plexus—median, radial, ulnar, and musculocutaenous.
Figure 4-4. Typical transcutaneous stimulation points at <5 mA outline the “anesthetic line of Grossi.” Nerve mapping all along the brachial plexus can be performed except over bony prominences.
Midhumeral transcutaneous stimulation can be performed anywhere from the axilla to the elbow for the median nerve which crosses the brachial artery at approximately the midhumeral level. The artery can serve as a landmark. The ulnar nerve can be similarly mapped very easily from axilla to elbow (Fig. 4-5). The radial nerve by contrast is easily mapped by transcutaneous stimulation at the axilla by indentation of the skin from above or below the artery toward the posterior aspect of the axillary artery. As the nerve courses more distally, it moves deeper and more posteriorly. Nevertheless, it can be mapped by indentation of the skin under the biceps toward the posterior aspect of the arm. The musculocutaneous nerve results in biceps contraction. It is best mapped only at the axilla, at the origin of the biceps. The musculocutaneous nerve cannot be reliably mapped in the midhumeral region since direct stimulation of the biceps muscle cannot be distinguished from stimulation of the nerve, itself. At the wrist, the median and ulnar nerves can each be stimulated transcutaneously.
In the lower extremity the femoral nerve can be stimulated transcutaneously by indentation of the skin and underlying tissues at the level of the femoral crease. The femoral nerve is very superficial at this point, even in large or obese adults. Because of the large complement of motor nerve fascicles to the quadriceps, achievement of a motor response is very easy. The fibers to the adductors are superficial and/or medial to the fibers enervating the vastus muscles of the quadriceps. Therefore in practice, if motor response to the adductor is all that can be achieved, the quadriceps motor response will be achieved by going through these fibers with the needle. This occurs in approximately 50% of adults. Thus, the ability to stimulate only the adductors can be classified as successful nerve mapping of the femoral nerve.
The sciatic nerve lies much deeper in the posterior thigh where the conventional Labatt approach occurs. At this point, transcutaneous stimulation is not normally possible in the adult. By contrast, stimulation can occur in thin adults at the popliteal block point, approximately 7 cm above the popliteal crease. Alternatively, the components of the distal sciatic nerve, the common tibial, and common peroneal nerve can each be stimulated separately and easily a few centimeters above the popliteal crease (Fig. 4-6).
Figure 4-5. Course of the ulnar nerve from axilla to elbow can be determined by transcutaneous stimulation at approximately 2 mA. In contrast to the median nerve, the ulnar nerve should not be blocked at the elbow near or at the ulnar groove.
Moving down the lower extremity, to the ankle, the posterior tibial nerve can be easily stimulated transcutaneously just behind the medial malleolus to assist with ankle block. Stimulation at this point causes plantar flexion of the foot by means of the intrinsic musculature of the foot. This is a slightly different, less intense motor response than results from stimulation of the tibial nerve or sciatic nerve more proximally, where a motor response includes extrinsic as well as the intrinsic muscles.
Percutaneous Electrode Guidance of the Block Needle
The technique of percutaneous electrode guidance of a block needle refers to transcutaneous stimulation followed by physical guidance of the needle tip toward the nerve by the transcutaneous electrode itself. This was first described by Urmey and Grossi in 2002. In their original report, they described the use of a cylindrical electrode with a 1 mm conductive tip. This was used to stimulate the nerve transcutaneously, following which a separate needle electrode was guided toward the nerve. This technique was later modified by the investigators. The newer technique utilized the stimulator needle electrode tip itself for transcutaneous stimulation. The needle electrode was protected by being encased in smooth rounded plastic. This allowed for indentation percutaneous electrode guidance of the needle tip by use of a single electrode and nerve stimulator to achieve transcutaneous as well as invasive nerve stimulation. This technique can be used on any of the nerves or plexuses that can be mapped as described above. Following the initial reports, Capdevila et al. used the needle electrode itself, without indentation of the skin, to successfully transcutaneously and subsequently invasively stimulate the underlying nerves of the axillary brachial plexus. In comparison with the above techniques of indentation percutaneous electrode guidance, use of the needle alone is limited to very superficial nerves and requires much higher current amplitude than when skin and underlying tissues are indented toward the nerve.
Figure 4-6. The common tibial (T) or common peroneal (P) nerves can be stimulated close to the popliteal crease at <0.5 mA.
Technique of Indentation Percutaneous Electrode Guidance
Performance of indentation percutaneous electrode guidance is done by first loading a standard 22-gauge 50- to 100-mm insulated block needle into a Stimuplex Guide (Fig. 4-7), by dropping the needle vertically into the Stimuplex Guide. The needle tip is aligned by means of a removable alignment cap. An adjustable dial is then turned to secure the needle and maintain the alignment when the alignment cap is removed. This in effect converts the sharp block needle into a smooth transcutaneous microelectrode that can be used to indent the skin for prelocation of the nerve or plexus without scratching or injuring the skin or overlying tissues. The point at which stimulation can occur transcutaneously at lowest amperage utilizing a 1 ms pulse duration is determined, following which the amperage is lowered to 1 mA and pulse duration to 0.1 ms. The Stimuplex Guide dial is then turned to release the needle thus allowing the needle to be freely advanced toward the targeted nerve (Fig. 4-8). The remainder of the technique is the same as with conventional invasive nerve stimulation techniques.
Figure 4-7. Photograph of Stimuplex Guide used for indentation percutaneous electrode guidance (PEG) with alignment cap in place. Adjustable dial at needle shaft is indicated. Adjustable dial is tightened by turning in a clockwise direction as viewed from above to secure needle within Stimuplex Guide. (Reproduced with permission from Jankovic D. Regional Nerve Blocks and Infiltration Therapy: Textbook and Color Atlas. 3rd Edition. Germany: Wiley-Blackwell, 2004.)
Figure 4-8. Stimuplex Guide used for indentation percutaneous electrode guidance (PEG). Here, skin is indented toward the plexus, the adjustable dial is released, allowing the needle to be inserted to the neural plexus. (Reproduced with permission from Jankovic D. Regional Nerve Blocks and Infiltration Therapy: Textbook and Color Atlas. 3rd Edition. Germany: Wiley-Blackwell, 2004.)
Peripheral nerve stimulation for nerve location is based on a scientific understanding of several variables that can be manipulated to achieve both sensitivity and specificity during the search for the targeted nerve. Use of accurate constant current generators and electrically insulated microelectrode needles achieves the required high specificity for ultimate nerve location. Variables utilized during peripheral nerve stimulation techniques include current amplitude, pulse duration, microelectrode size, and the distance between the electrode and the targeted nerve. Nerve mapping and prelocation of the course of a neural plexus or peripheral nerve can be used to assist or facilitate subsequent invasive needle exploration. Greater sensitivity is achieved for nerve mapping and prelocation by increasing current amplitude and pulse duration, as well as by indenting the overlying skin and subcutaneous tissues toward the targeted nerve, which serves to decrease the distance to the nerve as well as the impedance or resistance of the overlying tissues. Indentation percutaneous electrode guidance serves to combine nerve mapping or prelocation with ultimate invasive nerve location.
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