- Ability to stimulate peripheral nerve or plexus depends on:
Electrical Parameters of Peripheral Nerve Stimulation
- 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
- 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
- Insulated needle—insulated needle shaft ensures that current dispersion is concentrated at needle tip, allowing specificity of needle tip location
- Nerve stimulator device
Basic Features of Nerve Stimulators
|Constant current output||Automatically compensates for changes in impedance of tissues, needles, connecting wires, and grounding electrodes during needle placement to ensure consistent delivery of set current||Present in most modern stimulators|
- Displays the current being delivered
- Make sure it displays in 0.01 mA increments for better accuracy
- 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
- 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
- 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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