Peripheral nerve block materials

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CHAPTER 6 Peripheral nerve block materials

Frank Loughnane

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

	Nerve fiber type	Chronaxie
Cat sural nerve	Aα	50–100 µs¹³
Cat sural nerve	Aδ	170 µs¹⁴
Cat saphenous nerve	C	400 µs¹⁵

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.

Table 6.3 Polarity of stimulation

Anodal vs cathodal current required to stimulate peripheral nerve	Reference
∞ 4.57	BeMent & Ranck, 1969¹⁶
∞ 4.3	Ford et al, 1984¹⁷

(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.)