Nerve Stimulation for Locating Nerves

• Peripheral nerve stimulation (PNS) enjoys widespread use, with a proven clinical efficacy and safety record.
• Electrical stimulus of the nerve is based on factors such as conductive area of the electrode, resistance to electrical stimulation, distance between skin and nerve, current flow, and pulse duration
• A weak direct current (DC) electrical current is supplied to the stimulating needle by an oscillating (square wave) current generator (the nerve stimulator)
• Total energy (charge) applied to the nerve is the product of the current intensity and the pulse duration
• The actual current output by the stimulator is calculated as voltage output (voltage [V]/impedance [Q] = output [mA]) (Ohm’s law)

• The current is pulsed, typically at a frequency (f) of 1–2 Hz (cycles/s)
• Initial current amplitude (amperage) of 1–2 mA with a pulse duration of 0.1–0.2 ms is then directed to the stimulating needle, which is then inserted through the skin and slowly advanced toward the expected anatomical location of the targeted nerve
• The current is slowly decreased when the appropriate motor response is elicited. Motor contractions that occur at a low current (usually 0.2–0.5 mA) indicate that the needle tip is in close proximity to the nerve
• No motor response will occur if the needle tip is greater than 1 cm from the targeted nerve: PNS is used to refine the search endpoint, guiding the needle through the final 5 mm or so.
• PNS is thus limited in application to mixed peripheral nerves, as a motor response endpoint is desired. Although pure sensory nerves may be stimulated, ultimately obtaining a sensory paresthesia, this is not commonly performed clinically.

Energy

• The amount of electrical energy required to propagate a nerve impulse is the product of the stimulus strength (mA) and current duration (ms).
• For any nerve type, there is a minimum current strength required in order to generate an impulse – this is referred to as the rheobase.
• Below this minimum level, an impulse will not be generated.
• The chronaxie is defined as the stimulus duration needed for impulse generation, when employing a current strength of twice the rheobase.
• Myelinated fibers are much more sensitive and require less electrical energy for stimulation (having shorter chronaxie) than unmyelinated fibers (chronaxie of alpha fibers, 50–100; delta fibers, 170; C fibers, 400).
• The large (alpha) motor fibers are much more readily stimulated than the smaller D (delta) fibers or C fibers responsible for pain sensation. However, when a higher intensity current is used (e.g., greater than 1.0 mA), preferential stimulation of the motor fibers may be lost, and uncomfortable paresthesia-like stimulation often occurs.
• Less current is required to elicit a motor response with a stimulus of longer duration (e.g., >0.5 mA). Thus, using a shorter impulse duration of 0.1 ms will allow for motor nerve stimulation without initiating painful C fiber activity.

Polarity

• Less electrical energy is required if the (negative) cathode is close to the nerve since with a negative stimulating needle, the direction of current flow induces some direct depolarization. The reverse is true with an anodal (positive) needle since the direction of flow induces hyperpolarization of the target nerve. This, in turn, requires a higher current to stimulate the nerve. For these reasons, the needle polarity is designated negative by default. The site of placement of the positive (return) electrode, however, is probably irrelevant, as long as quality grounding electrodes are used and good electrical contact is made.

Distance The relationship between the constant current stimulus intensity and the distance from the nerve is governed by Coulomb’s law: I = K æ Q ö, ç2÷ where I is the current required to stimulate the nerve, K is a constant, Q is the minimal current needed for stimulation, and r is the distance from the stimulus to the nerve. The presence of the inverse square means that a current of very high intensity is required as the needle moves away from the nerve. Conversely, a nerve will only be stimulated when the needle is close to it. The initial current should therefore be set at 1–2 mA with an impulse duration of 0.1 ms. Generally, a motor response is elicited when the needle is within 5–10 mm from the nerve, using a current set at around 0.5 mA. This suggests that the needle tip is 1–2 mm from the motor nerve, signifying that a subsequent block will be satisfactory. If a muscle twitch is generated at a cur- rent strength of less than 0.2 mA, the stimulating needle may have penetrated the epineurium, thus risking a subsequent intraneural injection. It is therefore important to ensure that the muscle twitch disappears at or higher than a current of 0.2 mA. Stimulus Frequency As the needle is advanced, a muscle twitch by the stimulating current indicates that the needle is approaching the target nerve. If the frequency of impulses is too low, the nerve may be inadvertently penetrated. If the frequency is too high, painful muscle twitches (tetany) may be induced. A frequency of 2 Hz (cycles/s) is a good compro- mise as well as a suggested needle advancement speed of approximately 1 mm/s [6]. èrø 7 Equipment and Clinical Practice: Aids to Localization of Peripheral Nerves 181 Summary A peripheral nerve stimulator should provide as a minimum: 1. A square wave impulse with a duration of 0.1 ms. 2. The negative lead connected to the stimulating needle. 3. 2 Hz frequency. 4. Initial current level of 1–2 mA, seeking the nerve. 5. A final current level of 0.3–0.6 mA, positioning the needle tip close to the nerve. 6. Current delivery down to 0.1–0.2 mA, to ensure no intraneural stimulation. Additional safety features include: 1. Accurate current delivery in the range of 0–5.0 mA. 2. Constant current square wave pulse. 3. Display of current flowing into the patient as well as that delivered internally from the device. 4. Open circuit alarm. 5. Excessive impedance alarm. 6. Low battery alarm. 7. Internal malfunction alarm [5].