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Chapter 126. Principles of Peripheral Nerve Stimulation (PNS)

Caroline Buhay, MD

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  • Ability to stimulate peripheral nerve or plexus depends on:

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Electrical Parameters of Peripheral Nerve Stimulation
VariableClinical significance
Electrical impedance
  • Varies with tissue composition
  • Most stimulators have constant current output generators that automatically compensate for impedance changes
Electrode to nerve distance
  • Ability to stimulate nerve at low current flow (<0.5 mA) indicates close proximity to
  • nerve
  • Target muscle twitch still present at <0.2 mA probably indicates intraneural tip position
  • Increased current flow and pulse duration increase ability to stimulate nerve at a greater distance from stimulating electrode
Current flow (amperage)Presence of appropriate muscle twitch at current 0.23–0.5 mA generally results in safe, reliable block
Position of electrodes
  • Cathode (negative electrode) is attached to insulated needle and anode (positive electrode) is attached to patient skin via an EKG electrode
  • Inverting polarities requires four times current to achieve similar response
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  • Insulated needle—insulated needle shaft ensures that current dispersion is concentrated at needle tip, allowing specificity of needle tip location
  • Nerve stimulator device

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Basic Features of Nerve Stimulators
FeatureFunctionSettings
Constant current outputAutomatically compensates for changes in impedance of tissues, needles, connecting wires, and grounding electrodes during needle placement to ensure consistent delivery of set currentPresent in most modern stimulators
Current meter
  • Displays the current being delivered
  • Make sure it displays in 0.01 mA increments for better accuracy
  • PNS: 0.2–1.5 mA
  • TES: 5 mA
Pulse width/duration
  • Short duration (0.05–0.1 ms) targets A-α motor fibers and avoids stimulation of pain fibers at longer duration
  • Pulse duration required to depolarize pain fibers: A-δ (0.17 ms) and C fibers (0.4 ms)
  • PNS: 0.1 ms
  • Diabetic patients with neuropathy may require a longer pulse duration (0.3 or 1ms) to achieve target twitch
  • TES: 0.2–0.3 ms to achieve motor response at greater distance from nerve
Stimulating frequency
  • Determines how quickly pulses are delivered to elicit twitch (pulses/s)
  • Older models with 1 Hz (1 pulse/s) frequency require slow needle manipulation to avoid missing target twitch
  • Usual setting: 2 Hz allows for faster manipulation of needle tip
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  • Identify landmarks, insertion site, direction, target muscle(s) to elicit response:
    • Due to anatomical variance or large body habitus, the exact course of a nerve can be difficult to identify. In these cases consider doing a surface mapping of the nerve/plexus by transcutaneous electrical stimulation (TES, see below)
  • Ensure cathode (negative electrode, black) is attached to insulated needle and anode (positive electrode, red) is attached to patient
  • Set starting current to about 1.2–1.4 mA. Much higher current can result in patient discomfort
  • Insert needle and advance while observing target muscle group
  • Once target muscle twitch is elicited, slowly dial down current while adjusting needle position to optimize muscle response
  • If twitch disappears at ≥0.5 mA, turn current up until twitch is again present
  • Goal: response present at current 0.2–0.4 mA
  • Hold needle securely to ensure it does not move during injection. Ask assistant to ensure patient does ...

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