Chapter 126. Principles of Peripheral Nerve Stimulation (PNS)

Caroline Buhay, MD

Top

Favorite Table | Download (.pdf) | Print

Electrical Parameters of Peripheral Nerve Stimulation

Variable	Clinical 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

Insulated needle—insulated needle shaft ensures that current dispersion is concentrated at needle tip, allowing specificity of needle tip location
Nerve stimulator device

Favorite Table | Download (.pdf) | Print

Basic Features of Nerve Stimulators

Feature	Function	Settings
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
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

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 ...

GET ACCESS TO THIS RESOURCE

Sign In

Username

Password

Forgot Username? Forgot Password?

Get Free Access Through Your Institution

Contact your institution's library to ask if they subscribe to McGraw-Hill Medical Products.

What is MyAccess?

GET ACCESS TO THIS RESOURCE

Timed Access to all of AccessAnesthesiology

24 Hour

48 Hour

Best Value

$995 USD

Buy Now

View All Subscription Options

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.