EDT EDT Quality Assurance & Documentation 2 ā Questions and Answers
Question 1: When a patient's CMAP or SNAP amplitude is unexpectedly low, the first technical factor to assess is:
- Whether the sweep speed is set correctly
- Whether the recording electrodes have adequate skin contact and low impedance (Correct answer)
- Whether the repetition rate is too fast
- Whether the filter bandwidth is set to 20 Hzā10 kHz
Correct answer: Whether the recording electrodes have adequate skin contact and low impedance
High electrode impedance due to poor skin contact is one of the most common technical causes of reduced CMAP or SNAP amplitude and should be checked before concluding pathology.
Question 2: Stimulus artifact in NCS is best minimized by:
- Increasing stimulus intensity to supramaximal levels
- Ensuring the ground electrode is placed between the stimulator and recording electrodes (Correct answer)
- Decreasing the low-frequency filter setting
- Moving recording electrodes closer to the stimulator
Correct answer: Ensuring the ground electrode is placed between the stimulator and recording electrodes
Placing the ground electrode between the stimulator and recording electrodes provides a current path to ground that reduces stimulus artifact contaminating the recording.
Question 3: 60 Hz power line interference on the EMG tracing is best eliminated by:
- Increasing stimulus intensity
- Optimizing skin preparation, reducing electrode impedance, and ensuring proper grounding (Correct answer)
- Increasing sweep speed to compress the signal
- Decreasing amplifier gain
Correct answer: Optimizing skin preparation, reducing electrode impedance, and ensuring proper grounding
60 Hz interference is primarily caused by high electrode impedance; good skin preparation (abrasion and cleaning) and proper ground electrode placement reduce this artifact.
Question 4: The recommended stimulation rate for standard nerve conduction studies is:
- 0.5ā1 Hz
- 1ā3 Hz (Correct answer)
- 10ā20 Hz
- 50 Hz
Correct answer: 1ā3 Hz
Stimulation at 1ā3 Hz allows sufficient recovery time between stimuli to prevent rate-dependent changes in CMAP or SNAP amplitude.
Question 5: Why is supramaximal stimulation required for accurate nerve conduction studies?
- To prevent patient discomfort
- To ensure all axons in the nerve are activated, producing a maximal and reproducible evoked potential (Correct answer)
- To test the strength of the stimulator
- To achieve the fastest nerve conduction velocity
Correct answer: To ensure all axons in the nerve are activated, producing a maximal and reproducible evoked potential
Supramaximal stimulation (typically 20ā30% above the level that produces maximal response) ensures all axons are activated, making amplitude and velocity measurements reliable.
Question 6: Volume conduction artifacts on SNAP recordings can be reduced by:
- Increasing the distance between active and reference electrodes
- Using near-nerve needle electrodes or ring electrodes with standardized technique (Correct answer)
- Applying maximal stimulus intensity
- Increasing the high-frequency filter cutoff
Correct answer: Using near-nerve needle electrodes or ring electrodes with standardized technique
Near-nerve needle electrodes or ring electrodes with proper placement minimize volume-conducted signals from adjacent nerves and improve SNAP recording specificity.
EDT EDT Quality Assurance & Documentation 2
When a patient's CMAP or SNAP amplitude is unexpectedly low, the first technical factor to assess is: