Lesson 2 – Stimulation Electrode Precautions

This lesson guides you through all the electrode placement precautions

Setup Requirements

To complete this lesson, you will need:

  1. Stimpod NMS450X Neuromuscular Monitor
  2. NMT Monitoring Cable EMG or NMT Monitoring Cable AMG
  3. EMG Electrode or Xavant NMT Electrode or two ECG electrodes

General Electrode Placement Precautions

The following general guidelines can assist to avoid common mistakes whilst conducting correct electrode setup
  • Ensure the electrodes properly adhere to the patient. Proper skin preparation may be required.
  • Ensure electrodes are of good quality. Poor quality electrodes may have a negative effect.
  • When using acceleromyography
    • Ensure the clips are secured effectively prior to tucking or wrapping the patient’s arm for surgery. This way they won’t pop off.
    • An Open circuit error may occur if the clips are not making proper contact with the electrodes
  • When using electromyography
    • Ensure the electrode sleeve is still sealed prior to use
    • An Open circuit error may occur if either the stimulating or sensing electrodes are not correctly attached.

Correct vs Incorrect Electrode Placement

Correct electrode placement is essential in order to stimulate the target motor nerve while avoiding a direct stimulation of the target muscle.

If the muscle is stimulated directly the effect of the NMBA (Neuromuscular Blocking Agent) in the neuromuscular junction cannot be assessed and contractions will occur even during a deep or profound block.

Electrode Polarity

It is important that the cathode rather than the anode is placed on the most superficial nerve site in order to cause depolarization as opposed to hyperpolarization of the nerve.

Depolarization of the nerve requires less current than hyperpolarization and therefore producing a supramaximal current is far easier to achieve as opposed to if the polarities were reversed. 

Electrode Skin Interface

It is important to verify that the current setting is more or less the same as the current delivered. If the current delivered is much less than the current setting, there is a problem with the electrode skin interface. Possible causes can include a need for skin preparation or a defective electrode.

Possible Impedance Factors:

  • Skin temperature
  • Drug-induced Changes
  • Electrode movement due to patient movement
  • Defective electrode due to it being too dry
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