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INTRODUCTION

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Peripheral nerve stimulation (PNS) is a technique designed to localize plexuses or other larger peripheral nerves. It facilitates the placement of a needle close to the target nerve in order to deposit local anesthetic for conduction blockade (anesthesia and/or analgesia). Until the use of nerve stimulation became widespread in the 1990s, peripheral nerve blocks were traditionally performed using a technique in which subjective paresthesias were intentionally elicited. In contrast, PNS does not depend on patient cooperation for effective nerve localization. Direct electrical stimulation of the nerve produces a reliable and objective endpoint: an evoked motor response (muscle twitch). PNS can be used for both single-injection nerve blocks and insertion of continuous nerve block catheters. Today, it is usually combined with ultrasound guidance for the administration of regional anesthesia.

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ELECTROPHYSIOLOGY

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The neuronal membrane maintains a negative resting potential of approximately 90 mV due to the active transport of sodium and potassium ions. When depolarization of the membrane exceeds a predefined threshold, the flow of sodium ions into the cell triggers an action potential which propagates along the nerve fiber. Depolarization occurs from within the nervous system. PNS utilizes extracellular stimulation to depolarize the nerve fiber from outside the neuron. The negative polarity of the electrical stimulus (needle) removes positive charges from outside the neuronal membrane. This extracellular change enables the membrane potential to decrease towards the threshold level for generating an action potential.

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A peripheral nerve stimulator delivers square pulses of current rather than a single prolonged current. The total electrical charge (Q) applied to a nerve is the product of the current intensity (I) and current duration (t): Q = I × t. Therefore, in order to cause depolarization, the stimulus current must have both sufficient strength and duration. A weak or brief current will not generate an action potential.

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The current intensity (I) required to stimulate the nerve depends on three variables: rheobase (Ir), chronaxie (C), and stimulus duration (t). These variables are related by the following equation:

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  1. Rheobase—To reach the threshold potential for nerve excitation, a certain minimum level of current intensity is necessary. The rheobase is the minimum threshold current (in amperes) required to stimulate a nerve when using long pulse durations. Current intensities below rheobase will not generate an action potential even if administered for a long time.

  2. Chronaxie—The chronaxie time is the minimum duration (in milliseconds) required to generate an action potential when the current is applied at two times the rheobase level. A current applied at twice the rheobase value needs less time to achieve stimulation. Reducing pulse duration to very short times diminishes current dispersion from the needle tip, thereby requiring the tip to be placed very close to the nerve.

    Chronaxie times reflect the relative excitability of different peripheral nerve ...

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