Nerve stimulators

In 1911, Stoffel demonstrated how a galvanic current could be applied to identify nerve fibers.¹ A year later, Perthes described how the use of electrical stimulation could improve the safety of neural block in the practice of anesthesia.²

Nerve stimulation is a popular technique for the location and identification of nerve fibers, particularly in Europe.³ It was introduced into contemporary practice in 1973 by Montgomery and Raj against considerable opposition, particularly in the USA, where many practitioners advocated the dictum ‘no paresthesia, no anesthesia’.^4,5 Nerve stimulation, through the intentional avoidance of direct contact with the nerve fiber, aims to reduce the risk of neurologic complications. However, the relations between stimulating current, motor and sensory responses, success rates, and needle–nerve distances are far from clear in the clinical setting.^6–8 The nerve stimulation method produces peripheral nerve injury in up to three cases in 10 000.⁹ In contrast, the transarterial approach to brachial plexus anesthesia produces nerve lesions in 0.8% of cases and the paresthesia approach in 2.8%.^10,11 The following is a discussion on the theoretical as well as practical aspects of nerve stimulation and the equipment commonly used to locate nerves. The reader should remain cognizant of the fact that no definitive study outlining the exact nature of the relationship between the stimulating current and the observed responses in clinical practice exists to date.

Electrophysiology

The electrochemical nature of nerve fiber conduction renders it amenable to electrical stimulation. The strength–duration curve demonstrates the relation between the intensity and duration of current in peripheral nerve stimulation (Fig. 6.1).

Figure 6.1 Strength–duration curve, cat sciatic nerve. The rheobase is the smallest current to stimulate the nerve with a long pulse width. The chronaxie is the pulse duration at a stimulus strength twice the rheobase. The curve was obtained from a cat sciatic nerve with the stimulating needle touching the nerve.

(From Pither C, Prithvi R, Ford D. The use of peripheral nerve stimulators for regional anesthesia. A review of experimental characteristics, techniques and clinical applications. Reg Anesth 1985; 10; 49–58, with permission from the American Society of Regional Anesthesia and Pain Medicine.)

The total charge applied to the nerve is a product of the current intensity and the duration of the pulse. The minimum in vitro quantity of current necessary to generate an action potential can be calculated from the equation

I is the current required, Ir the rheobase, C the chronaxie, and t the duration of stimulus. The rheobase is the minimum current required to depolarize a nerve when applied for a long period. The chronaxie is the duration of impulse necessary to stimulate at twice the rheobase.

The chronaxie of a motor nerve is less than that of a sensory nerve. In the clinical setting, therefore, a motor response may be elicited without stimulating pain fibers if the duration of impulse is short. Sensory nerves may also be identified using a nerve stimulator if the pulse duration is greater than 400 µs (Table 6.1).

Table 6.1 Chronaxies of mammalian peripheral nerves

Coulomb’s law:

governs the relation between the stimulus intensity and the distance from the nerve. E is the current required, K a constant, Q the minimal current, and r the distance. The significance lies in the squaring of the distance. While one may thus approach the nerve through the progressive diminution of current, at distances greater than 0.5 cm from the nerve large currents are required; at greater than 2 cm, currents of up to 50 mA may be generated. These currents produce pain locally and require that appropriate care be taken in patients with intracardiac electrodes (Table 6.2).

Table 6.2 Calculated values for current required to stimulate nerve at various distances from the nerve

Ohm’s law describes the relation between potential difference (U), resistance (R), and intensity (I):

In practice, U corresponds to the potential difference between the poles of the nerve stimulator; R corresponds to the internal resistance of the patient and the resistance of the cables. The negative electrode is connected to the needle and the positive to the patient’s skin via a gel electrode. Because the interior of a nerve at rest is negatively charged relative to the exterior, if the poles are reversed hyperpolarization of the nerve occurs; it is then necessary to apply a current of greater intensity to achieve the same motor response. These currents may be uncomfortable for the patient (Fig. 6.2, Table 6.3).

Figure 6.2 Preferential cathodal stimulation. With the needle as the cathode (A), electron flow is toward the needle, causing an area of depolarization around the needle tip. With the needle as anode (B), the area adjacent to the nerve is hypopolarized, with a zone of depolarization in a ring distant to the needle, an arrangement that requires more current to stimulate the nerve.

(From Pither C, Prithvi R, Ford D. The use of peripheral nerve stimulators for regional anesthesia. A review of experimental characteristics, techniques and clinical applications. Reg Anesth 1985; 10; 49–58, with permission from the American Society of Regional Anesthesia and Pain Medicine.)

Table 6.3 Polarity of stimulation

(From¹² Pither C, Prithvi R, Ford D. The use of peripheral nerve stimulators for regional anesthesia. A review of experimental characteristics, techniques and clinical applications. Reg Anesth 1985; 10; 49–58, with permission from the American Society of Regional Anesthesia and Pain Medicine.)

Characteristics

The characteristics considered desirable in a nerve stimulator are constant current output; digital display; square-shaped, monophasic, negative impulse; variable output control; linear output; clearly marked polarity; short pulse width; variable stimulation frequency of 1 or 2 Hz; high-quality cables and connections; and indicators of power failure, circuit closure, high circuit resistance, and device malfunction.^17,18

The resistance of the human body, cables, connections, etc., may vary between 1000 and 20 000 ohms. It is important that the current should not vary with these changes in resistance, i.e. the device should have a constant current output. As U = R∞I (U being the potential difference between the poles of the device, R the impedance of the external electrical circuit, and I the current intensity), the nerve stimulator must be able to deliver a high output load to avoid a possible 20-fold change in the current delivered.

A digital display of the current intensity delivered is important as one approaches the nerve with very small currents. Knowledge of the precise intensity is vital for accurate nerve location. A final current intensity of 0.5 mA or less is associated with a high success rate in brachial plexus anesthesia.¹⁹

The current impulse needs to be square-shaped, monophasic, and negative. The amplitude corresponds to the intensity of the electric current and is expressed in milliamperes (mA); the duration is measured in ms or µs. It is important to have a short ascent and descent time to the impulse because the charge applied to the nerve is a product of the current and the duration. Therefore the more square-shaped the signal, the greater the precision of the instrument.

To be able to choose between several pulse widths is equally of value. A short pulse width of 50–100 µs is necessary because this corresponds to the chronaxies of mammalian Aα fibers (see Table 6.1). According to Coulomb’s law, the electrical field produced for a current intensity of constant duration is inversely proportional to the square of the distance:

(see previous section, Electrophysiology). Therefore one may bring the needle tip closer to the nerve through the progressive diminution of current intensity. Conversely, as one moves away from the nerve, currents of high intensity are required to stimulate the nerve.

The nerve stimulator should have a variable output control that operates on a linear scale. This means that the output of the device alters in proportion to the movement of the dial.

The negative lead must be attached to the needle for reasons already outlined. By convention, the negative lead is colored black and the positive red. To avoid confusion, clear labeling or non-interchangeable connections are required.