Electrical Nerve Stimulators and Localization of Peripheral Nerves

1780: Galvani (1) was the first to describe the effect of electrical neuromuscular stimulation
1912: Perthes (2) developed and described an electrical nerve stimulator
1955: Pearson (3) introduced the concept of insulated needles for nerve location
1962: Greenblatt and Denson (4) introduced a portable solidstate nerve stimulator with variable current output and described its use for nerve location
1973: Montgomery et al (5) demonstrated that noninsulated needles require significantly higher current amplitudes than the insulated needles
1984: Ford et al (6) reported a lack of accuracy with noninsulated needles once the needle tip passed the target nerve Ford et al suggested the use of nerve stimulators with a constant current source, based on the comparison of the electrical characteristics of peripheral nerve stimulators

The use of nerve stimulation became commonplace in clinical practice only in the mid- to late 1990s. Research on the needle-nerve relationship and the effect of stimulus duration ensued. (9-11) More recently, the principles of electrical nerve stimulation were applied to surface mapping of peripheral nerves using percutaneous electrode guidance (PEG) (12-15) for confirmation and epidural catheter placement (16-18) and peripheral catheter placement (19). This chapter discusses the electrophysiology of nerve stimulation, electrical nerve stimulators, various modes of localization of peripheral nerves, and integration of the technology into the realm of modern regional anesthesia.

Nerve stimulation is a commonly used method for localizing nerves before the injection of local anesthetic. Electrical nerve stimulation in regional anesthesia is a method of using a low-intensity (up to 5 mA) and short-duration (0.05-1 ms) electrical stimulus (at 1-2 Hz repetition rate) to obtain a defined response (muscle twitch or sensation) to locate a peripheral nerve or nerve plexus with an (insulated) needle. The goal is to inject a certain amount of local anesthetic in close proximity to the nerve to block nerve conduction and provide a sensory and motor block for surgery and/or, eventually, analgesia for pain management. The use of nerve stimulation can also help to avoid an intraneural intrafascicular injection and, consequently, nerve injury.

Electrical nerve stimulation can be used for a single-injection technique, as well as for guidance during the insertion of continuous nerve block catheters. More recently, ultrasound (US) guidance and, in particular, the so-called dual guidance technique in which both techniques (peripheral nerve stimulation [PNS] and US) are combined, has become a common practice in many institutions.

In principle, almost all plexuses or other larger peripheral nerves can be located using PNS. (20) The goal of nerve stimulation is to place the tip of the needle (more specifically, its orifice for injection) in close proximity to the target nerve to inject the local anesthetic in the vicinity of the nerve. The motor response (twitch) to PNS is objective and reliable and independent from the patient's (subjective) response. Nerve stimulation is often helpful to confirm that the structure imaged with ultrasound (US) is actually the nerve that is sought. This is because the needle-nerve relationship may not always be visualized on US; an unexpected motor response can occur, alerting the operator that the needle tip is already in close proximity to the nerve. Likewise, the occurrence of a motor response at a current intensity of

The disadvantages of PNS are the need for additional equipment (nerve stimulator and insulated needles), the greater cost of insulated needles, and abnormal physiology or anatomy where it may be difficult to elicit a motor response. 

Basics of Neurophysiology and Electrophysiology

Membrane Potential, Resting Potential, Depolarization, Action Potential, and impulse Propagation

Figure 1: A) Schematic anatomic and electrophysiologic structure of nerve fibers of myelinated and (B) unmyelinated nerve fibers.

All living cells have a membrane potential (a voltage potential across their membrane, measured from the outside to the inside), which varies (depending on the species and the cell type) from about -60 mV to -100 mV. Nerve and muscle cells in mammals typically have a membrane potential (resting potential) of about -90 mV.

Only nerve and muscle cells have the capability of producing uniform electrical pulses, the so-called action potentials (also called spikes), which are propagated along their membranes, especially along the long extensions of nerve cells (nerve fibers, axons). A decrease in the electric potential difference (e.g., from -90 mV to -55 mV, or depolarization) elicits an action potential. If the depolarization exceeds a certain threshold, an action potential or a series of action potentials is generated by the nerve membrane (also called firing) according to the all-or-nothing rule, resulting in propagation of the action potential along the nerve fiber (axon). To depolarize the nerve membrane from outside the cell (extracellular stimulation), the negative polarity of the electrical stimulus is more effective in removing the positive charge from the outside of the membrane. This in turn decreases the potential across the membrane toward the threshold level.

There are several types of nerve fibers. Each fiber type can be distinguished anatomically by their diameter and degree of myelinization. Myelinization is formed by an insulating layer of Schwann cells wrapped around the nerve fibers. These characteristics largely determine the electrophysiologic behavior of different nerve fibers, that is, the speed of impulse propagation of action potentials and the threshold of excitability. Most commonly, the distinguishing features are motor fibers (e.g., A, Aβ) and pain fibers (C). The Aα motor fibers have the largest diameter and highest degree of myelinization and therefore the highest speed of impulse propagation and a relatively low threshold level to external stimulation. C-fibers (which transmit severe, dull pain) have very little to no myelinization and are of smaller diameter. Consequently, the speed of propagation in these fibers is relatively low, and the threshold levels to external stimulation, in general, are higher.

There are several other, efferent fibers, which transmit responses from various skin receptors or muscle spindles (Ad). These are thinner than Aα fibers and have less myelinization. Some of these (afferent) sensory fibers, having a relatively low threshold level, transmit the typical tingling sensation associated with a lower level of pain sensation when electrically stimulated. Such sensation can occur during transcutaneous stimulation before a motor response is elicited.

The basic anatomic structure of myelinated Aα fibers (motor) and nonmyelinated C fibers (pain) is shown schematically in Figure 1. The relationship between different stimuli and the triggering of the action potential in motor and pain fibers is illustrated in Figures 2A, B.

A certain minimum current intensity is necessary at a given pulse duration to reach the threshold level of excitation. The lowest threshold current (at infinitely long pulse durations) is called rheobase. The pulse duration (pulse width) at double the rheobase current is called chronaxy. Electrical pulses with the duration of the chronaxy are most effective (at relatively low amplitudes) to elicit action potentials. This is the reason why motor response can be elicited at such short pulse duration (e.g., 0.1 ms) at relatively low current amplitudes while avoiding the stimulation of C-type pain fibers. Typical chronaxy figures are 50 to 100 μs (Aα fibers), 170 μs (AΔ fibers), and ≥400 μs (C fibers). Figure 3 illustrates the relationship of the rheobase to chronaxy for motor fibers versus pain nerve fibers.

Impedance, Impulse Duration, and Constant Current

The electrical circuit is formed by the nerve stimulator, the nerve block needle and its tip, the tissue characteristics of the patient, the skin, the skin electrode (grounding electrode), and the cables connecting all the elements. The resistance of this circuit is not just a simple Ohm's resistor equation because of the specific capacitances of the tissue, the electrocardiogram (ECG) electrode to skin interface, and the needle tip, which influence the overall resistance. The capacitance in the described circuit varies with the frequency content of the stimulation current, and it is called impedance, or a so-called complex resistance, which depends on the frequency content of the stimulus. In general, the shorter the impulse, the higher its frequency content, and, consequently, the lower the impedance of a circuit with a given capacitance. Conversely, a longer pulse duration has a lower frequency content. As an example, for a 0.1-ms stimulus, the main frequency content is 10 kHz plus its harmonics; whereas for a 1.0-ms impulse, the main frequency content is 1 kHz plus harmonics. In reality, the impedance of the needle tip and the electrode to skin impedance have the highest impact. The impedance of the needle tip largely depends on the geometry and insulation (conductive area). The electrode to skin impedance can vary considerably between individuals (e.g., type of skin, hydration status) and can be influenced by the quality of the ECG electrode used.

Because of the variable impedance in the circuit, created primarily by the needle tip and electrode to skin interface, a nerve stimulator with a constant current source and sufficient (voltage) output power is important to use to compensate for the wide range of impedances encountered clinically.

The following are a few important aspects for successful electrolocalization of the peripheral nerves using PNS:

- Use a high-quality nerve stimulator and a high accuracy constant current source

- Use insulated nerve stimulation needles with a small conductive area at the tip. The smaller the conductive area,    the higher the current density is at the tip, and the greater spatial discrimination in the near field.

- Use high-quality skin electrodes with a low impedance

- Before starting the procedure, check for the proper functioning of the nerve stimulator and the connecting cables.

- During nerve stimulator-assisted nerve localization, the negative pole (cathode) should be connected to the    stimulating electrode (needle) and the positive pole (anode) to the patient's skin.

- The design of the connectors should prevent a faulty polarity connection.

- Connect the nerve stimulation needle to the nerve stimulator (which should be turned on), and set the current    amplitude and duration to the desired levels.

- For superficial blocks, select 1.0 mA as a starting current intensity.

- For deep blocks, select 1.5 mA as a starting current intensity.

- Select between 0.1 and 0.3 ms of current duration for most purposes.

- For more technical details and how to operate a specific nerve stimulator, refer to the instructions for use supplied    with the stimulator.

Figure 4: Tip configuration of several commercially available nerve mapping peripheral nerve stimulators. From left to right: Stimuplex Pen, B. Braun Melsungen (Germany); nerve mapping pen, Pajunk (Germany); NeuroMap, HDC (USA).c toxicity

Electrolocalization of peripheral nerves is typically accomplished by inserting a needle into the tissue and advancing the needle toward the expected location of the nerve(s) of interest. However, a nerve mapping pen can be used to locate superficial nerves (up to a maximum depth of approximately 3 cm) with transcutaneous nerve stimulation before the nerve block needle is inserted. Transcutaneous nerve mapping is particularly useful when identifying the best site for needle insertion in patients with difficult anatomy or when the landmarks prove difficult to indentify. Figure 4 shows three examples of commercially available nerve mapping pens.

Nerve mapping is also very useful when training anesthesia residents. It should be noted that longer stimulus duration (e.g., 1 ms) is needed to accomplish transcutaneous nerve stimulation, because the energy required to stimulate transcutaneously is larger. The electrode tip of the pen should have an atraumatic ball-shaped tip. The conductive tip diameter should not be larger than approximately 3 mm to provide sufficient current density and spatial discrimination, which may not be the case with larger tip diameters. Some nerve stimulators do not provide the required impulse duration of 1 ms or a strong enough constant current source (5 mA at minimum 12-kOhm output load) to perform nerve mapping. Therefore, it is recommended that the mapping pen and the nerve stimulator be paired, ideally by acquiring them from the same manufacturer.

The transcutaneous stimulation often results in a sensation reported by the patient as tingling, pinprick, or a slight burning sensation. The perception varies greatly among individuals. Most people tolerate transcutaneous stimulation with a nerve mapping pen very well; however, some individuals describe it as uncomfortable or even painful (depending on the stimulus amplitude and duration). However, the amount of energy delivered by nerve stimulators with a maximum output of 5 mA at 1 msec pulse duration is far too low to create any injury of the skin or the nerves. A moderate premedication is usually sufficient to make the procedure well tolerated by patients.

Figure 5: Percutaneous electrode guidance technique using Stimuplex Guide (B. Braun Melsungen, Germany) during a vertical infraclavicular block procedure.

PEG (10,11) combines the transcutaneous nerve stimulation (nerve mapping) with nerve block needle guidance (Figure 5). In essence, a small aiming device is mounted and locked onto a conventional nerve block needle, which allows the conductive needle tip to make contact with the skin without scratching or penetrating the skin. Once the best response is obtained, the needle is advanced through the skin in the usual fashion and the remainder of the apparatus continues to stabilize the needle and guide it toward the target. The device also allows the operator to make indentations in the skin and tissue so the initial distance between the needle tip at the skin level and the target nerve is reduced and the nerve block needle has less distance to travel through tissue. The technique allows for prelocation of the target nerve(s) before skin puncture.

The starting amplitude used for nerve stimulation depends on the local practice and the projected skin-nerve depth. For superficial nerves, amplitude of 1 mA at 0.1 (or 0.3) ms impulse duration to start is chosen in most cases. For deeper nerves, it may be necessary to increase the initial current amplitude between 1.5 and 3 mA until a muscle response is elicited at a safe distance from the nerve. Too high current intensity, however, can lead to direct muscle stimulation or discomfort for the patient, both of which are undesirable.

Once the sought-after muscle response is obtained, the current intensity amplitude is gradually reduced and the needle is advanced further slowly. The needle must be advanced slowly to avoid too rapid advancement between the stimuli. Advancement of the needle and current reduction are continued until the desired motor response is achieved with a current of 0.2-0.5 mA at 0.1 ms stimulus duration. The threshold level and duration of the stimulus are interdependent; in general, a short pulse duration is a better discriminator of the distance between the needle and the nerve. (20) When the motor twitch is lost during needle advancement, the stimulus intensity first should be increased to regain the muscle twitch rather than move the needle blindly. Once a proper motor response is obtained with a current of 0.2-0.5 mA (most nerve blocks), the needle is positioned correctly for an injection of local anesthetic. A small test dose of local anesthetic is injected, which abolishes the muscle twitch. Then the total amount of local anesthetic appropriate for the desired nerve block is injected. Of note, the highly conductive injectate (e.g., local anesthetic or normal saline solution) short-circuits the current to the surrounding tissue, effectively abolishing the motor response. In such situations, increasing the amplitude may not bring back the muscle twitch. Tsui and Kropelin (21) demonstrated that injection of dextrose 5% in water (D5W) (which has a low conductivity) does not lead to loss of the muscle twitch if the needle position is not changed.

It should be remembered that the absence of the motor response with a stimulating current even up to 1.5 mA does not rule out an intraneural needle placement (low sensitivity). However, the presence of a motor response with a low-intensity current (

To avoid or minimize discomfort for the patient during the nerve location procedure, it is recommended that a too high stimulating current be avoided. The needle should not be advanced too fast because it can increase the risk of injuries and the evoked motor response may be missed.

Figure 6: (A) Stimulating needle at a distance to the nerve and high stimulus current elicits a weak evoked motor response. (B) Stimulation needle close to the nerve and high stimulus current eliciting a strong muscle twitch. (C) Stimulating needle close to the nerve and low (near threshold) stimulus current elicits a strong evoked motor response.

Measurement of the impedance can provide additional information if the electrical circuit is optimal. Theoretically, impedance can identify an intraneural or intravascular placement of the needle tip. Tsui and colleagues (22) reported that the electrical impedance nearly doubles (12.1-23.2 kOhm), which is significant, when the needle is advanced from an extraneural to intraneural position in a porcine sciatic nerve. Likewise, injection of a small amount of (D5W), which has a high impedance, results in a significantly higher increase of impedance in the perineural tissue than it does within the intravascular space. (23) Thus measurement of the impedance before and after dextrose injection can potentially detect intravascular placement of the needle tip, thus identifying the placement before the injection of local anesthetic. In this report, the perineural baseline impedance [25.3 (± 2.0) kOhm] was significantly higher than the intravascular [17.2 (± 1.8) kOhm]. Upon injection of 3 mL of D5W, the perineural impedance increased by 22.1 (± 6.7) kOhm to reach a peak of 50.2 (± 7.6) kOhm and remained almost constant at about 42 kOhm during the 30-second injection time. By contrast, intravascular impedance increased only by 2.5 (± 0.9) kOhm, which is significantly less compared with the perineural needle position. At the present time, however, more data are needed before these findings can be incorporated as an additional safely monitoring method in clinical practice.

Current nerve stimulation uses stimuli of identical duration (typically 0.1 ms), usually at 1 or 2 Hz repetition frequency. A common problem during nerve stimulation is that the evoked motor response is often lost while moving the needle to optimize its position. In such cases, it its recommended that the operator either increase the stimulus amplitude (mA) or increase the impulse duration (ms), the latter of which may not be possible. Alternatively, the operator can take a couple of steps, depending on type of the nerve stimulator used. The SENSe (sequential electrical nerve stimulation) technique incorporates an additional stimulus with a longer pulse duration after two regular impulses at 0.1 msec duration, creating a 3 Hz stimulation frequency. (24) The third longer impulse delivers more charge than the first two and therefore has a longer reach into the tissue. Consequently, an evoked motor response often is elicited at 1 Hz, even when the needle is distant from the nerve. Once the needle tip is positioned closer to the nerve, muscle twitches are seen at 3 Hz. The advantage of the SENSe is that a motor response (at 1/second) is maintained even when the motor response previously elicited by the first target threshold amplitude remains the same as usual (about 0.3 mA) but at 3 stimuli per second. With the SENSe technique, a motor response at only 1/second indicates that the needle is not yet placed correctly.

- An adjustable constant current source with an operating range of 10 kOhm, minimally, output load (impedance) and    ideally at ≥15 kOhm.
- A precisely adjustable stimulus amplitude (0-5 mA): An analog control dial is preferred over up/down keys.
- A large and easy-to-read digital display of actual current flowing to maintain precise control of the stimulus.
- A selectable pulse duration (width), at least between 0.1 ms and 1.0 ms, to allow the operator to selectively    stimulate motor fibers (0.1 ms) and to stimulate sensory fibers as well (1.0 ms).
- A stimulus frequency between 1 and 3 Hz (meaning 1-3 pulses per second) because the use of a too low frequency can lead to "blind" advancement of the needle in between stimuli. Use of a too high frequency will lead to superimposing of muscle twitches, which makes the detection of twitches more difficult.
- A monophasic rectangular output pulse to provide reproducible stimuli.
- Configurable start-up parameters so the machine will comply with the hospital protocol and to avoid mistakes when multiple users are working with the same device.
- A display of the circuit impedance (kOhm) is recommended to allow the operator to check the integrity of the electrical circuit and to detect a potential intraneural or intravascular placement of the needle tip.
- An automatic self-check process of the internal functioning of the unit with a warning message if something is wrong.
- An optional remote control (handheld remote control or foot pedal).

Safety Features

- Easy and intuitive use
- A large and easy-to-read display
- Limited current range (0-5 mA) because a too high amplitude may be uncomfortable to the patient
- A display of all relevant parameters such as Amplitude (mA) [alternatively stimulus charge (nC)], stimulus duration (ms), stimulus frequency (Hz), impedance (kOhm), and battery status
- Clear identification of output polarity (negative polarity at the needle)
- Meaningful instructions for use, with lists of operating ranges and allowed tolerances
- Battery operation of the nerve stimulator, as opposed to electrical operation, provides intrinsic safety: no risk of serious electric shock or burns caused by a short circuit to main supply of electricity
- The maximum energy delivered by a nerve stimulator with 5 mA and 95 V output signal at 1 ms impulse duration is only 0.475 mWs (see Section 7.3).
- Combined units for peripheral (for PNB) and transcutaneous (for muscle relaxation measurement) electrical nerve stimulation are not to be used because the transcutaneous function produces an unwanted high energy charge

Alarms/Warnings

- Open circuit/disconnection alarm (optical and acoustical)
- Warning/indication if impedance is too high; that is, the desired current is not delivered
- The display of actual impedance appears to be useful and recommendable
- Near threshold amplitude indication or alarm Low battery alarm
- Internal malfunction alarm

Table 2 provides a comparison of the most important features of commonly used nerve stimulators.

Stimulating Needles

Needle

A modern stimulating needle should have the following characteristics:
- A fully insulated needle hub and shaft to avoid current leakage
- The conductive electrode area should be able to accomplish higher current density at the tip for precise nerve location
- Depth markings for easy identification and documentation of the needle insertion depth

Figures 8A and B show a comparison of the electrical characteristics of noninsulated and insulated needles with uncoated bevel (Figure 8A) and fully coated needles with a pinpoint electrode (Figure 8B). Even though a noninsulated needle provides for discrimination (change in threshold amplitude) while approaching the nerve, there is virtually no ability to discriminate once the needle tip has passed the nerve. The discrimination near the nerve is more precise in needles with a pinpoint electrode tip (Figure 8B) compared with needles with an uncoated bevel (Figure 8A).

Connectors

Connectors and cables should be fully insulated and include a safety connector to prevent current leakage as well as the risk of electric charge if the needle is not connected to the stimulator. Extension tubing with a Luer lock connector should be present for immobile needle techniques.

Visualization of the Needle Under Ultrasound Imaging

Because US imaging is more in use (in particular with the use of the "dual guidance" technique), the importance of good visualization of the nerve block needle is becoming an additional important feature. The visibility (distinct reflection signal) of the needle tip certainly is the most important aspect because this is the part of the needle that is placed in the target area next to the nerve. However, in particular when using the in-plane approach, the visibility of the needle shaft is of interest as well because it helps to align the needle properly with the US beam and to visualize its entire length up to the target nerve.

Stimulating Catheters

In principle, stimulating catheters function like insulated needles. The catheter body is made from insulating plastic material and usually contains a metallic wire inside, which conducts the current to its exposed tip electrode. Such stimulating catheters are usually placed using a continuous nerve block needle, which is placed by first using nerve stimulation as described and acts as an introducer needle for the catheter. Once this needle is placed close to the nerve or plexus to be blocked, the stimulating catheter is introduced through it and the nerve stimulator is connected to the catheter. Stimulation through the catheter should reconfirm that the catheter tip is positioned in close proximity to the target nerve(s). However, it must be noted that the threshold currents with stimulating catheters may be considerably higher. Injection of local anesthetic or saline (which is frequently used to widen the space for threading the catheter more easily) should be avoided because this may increase the threshold current considerably and can even prevent a motor response. D5W can be used to avoid losing a motor response. (21) Since the ultimate test for the properly positioned catheter is the distribution of the local anesthetic, rather than evoked motor response, the role of the catheter stimulating with US-guided blocks is not clear.

Recommendations for Best Practice

- Adequate knowledge of anatomy
- Correct patient positioning
- Proper technique and equipment

Standard nerve stimulator settings for peripheral nerve blocks:

- Stimulus duration: 0.1 ms for mixed nerves
- Amplitude range: 0-5 mA or 0-1 mA (sufficient for superficial nerves)
- Stimulus frequency: 2 or 3 Hz, or SENSe

Nerve stimulator check:

- Check battery status
- Check that all connections are placed properly (cable, needle, skin electrode)
- Check the entire nerve stimulator function using a test resistor (this automatically checks connectors and cable as well)

Needles:

- Use fully insulated nerve block needles, Figure 8.
- Use the appropriate gauge size and length (avoid too long needles; see Table 3)

End point of nerve stimulation:

- Threshold current 0.2 to 0.3 mA (at 0.1 ms)
- Higher threshold current (≥0.5 mA) means the needle tip is too far from the nerve end point and block failure becomes more likely
- Lower threshold current (≤0.2 mA) signals a risk of intraneural/fascicular injection, and consequently, the risk for neural damage increases

To avoid discomfort for the patient and take precautions for safety:

- Use a low-intensity current nerve stimulation
- Limit the stimulus energy by limiting the initial stimulus current amplitude: 1 mA (superficial blocks), 2 mA (deeper blocks), maximum 5 mA (e.g., psoas compartment and deep sciatic blocks), and the stimulus duration; do not use long stimulus duration (1 ms) if it is not needed
- Do not inject at exceedingly high pressure or if the threshold current is - Apply appropriate anesthesia technique (e.g., infiltration of puncture site, light sedation)

Figure 8: (A) Threshold amplitude achieved with an uncoated needle and a coated needle with an uncoated bevel. (B) Threshold amplitude achieved with a fully coated needle and a pinpoint electrode.

Appendix: Glossary of Physical Parameters

Voltage, Potential, Current, Resistance/Impedance

This means the current (or charge) that reaches the nerve decreases by a factor of 4 if the distance to the nerve is doubled, or conversely, it increases by a factor of 4 if the distance is divided in half (ideal conditions assumed).

Energy of Electrical Impulses Delivered by Nerve Stimulators and Related Temperature Effects

According to a worst-case scenario calculation, the temperature increase caused by a stimulus of 5 mA current and 1 ms duration, at a maximum output voltage of 95 V, would be

The maximum energy (E) of the electrical impulse deliv- ered by a common nerve stimulator would be:

The caloric equivalent for water is cw = 4.19 J g-1 K-1 One stimulus creates a temperature difference ΔT within 1 mm3 of tissue around the tip of a nerve stimulation needle. For the calculation that follows, it is assumed that tissue contains a minimum of 50% water and the mass (M) of 1 mm3 of tissue is 1 mg.

That is, the maximum temperature increase in this worstcase scenario calculation is