EDT Device Setup & Operation 3 — Questions and Answers
Question 1: When calibrating an EMG machine before use, which parameter confirms that the amplifier gain is accurate?
- The sweep speed matches the selected setting on the display
- A known calibration signal produces the expected deflection height on screen (Correct answer)
- The notch filter reduces ambient noise to zero
- The stimulator output matches the displayed mA value only
Correct answer: A known calibration signal produces the expected deflection height on screen
Applying a known calibration pulse and verifying the waveform height confirms that the amplifier gain is set correctly.
Question 2: Which stimulator parameter controls how long the electrical impulse is applied to the nerve during a nerve conduction study?
- Frequency (Hz)
- Pulse duration (µs) (Correct answer)
- Sensitivity (mV/div)
- Sweep speed (ms/div)
Correct answer: Pulse duration (µs)
Pulse duration (typically 0.1–0.2 ms for motor studies) determines the width of the stimulus waveform delivered to the nerve.
Question 3: An EMG technician is performing a needle exam and observes a signal that appears to be cut off at the top of the screen. What is the correct adjustment?
- Increase sweep speed
- Decrease sensitivity (increase mV/div) (Correct answer)
- Enable the notch filter
- Increase stimulator intensity
Correct answer: Decrease sensitivity (increase mV/div)
Decreasing sensitivity (larger mV/div value) expands the vertical scale so tall signals fit within the display window without clipping.
Question 4: For H-reflex recordings in adults, the active electrode is typically placed over which muscle?
- Tibialis anterior
- Soleus (medial gastrocnemius) (Correct answer)
- Extensor digitorum brevis
- Abductor hallucis
Correct answer: Soleus (medial gastrocnemius)
The H-reflex in adults is most reliably recorded from the soleus/medial gastrocnemius via tibial nerve stimulation at the popliteal fossa.
Question 5: What is the recommended inter-electrode distance between E1 and E2 for sensory nerve conduction studies using ring electrodes on a finger?
- 1 cm
- 3–4 cm (Correct answer)
- 10 cm
- 20 cm
Correct answer: 3–4 cm
Ring electrodes on adjacent finger segments are placed approximately 3–4 cm apart to ensure adequate signal separation.
Question 6: During a nerve conduction study, increasing the stimulus intensity beyond the supramaximal level will have what effect on the CMAP amplitude?
- Amplitude continues to increase proportionally
- Amplitude remains stable (plateau) because all fibers are already activated (Correct answer)
- Amplitude decreases due to co-stimulation of inhibitory fibers
- Amplitude fluctuates unpredictably
Correct answer: Amplitude remains stable (plateau) because all fibers are already activated
Once all axons are depolarized at supramaximal stimulation, additional intensity cannot recruit more fibers, so amplitude plateaus.
Question 7: Which of the following best describes the role of the differential amplifier in electrodiagnostic equipment?
- It amplifies only the signal at E1 while ignoring E2
- It amplifies the difference between E1 and E2, rejecting signals common to both (Correct answer)
- It converts analog signals to digital without amplification
- It filters high-frequency noise above 10 kHz
Correct answer: It amplifies the difference between E1 and E2, rejecting signals common to both
A differential amplifier subtracts the E2 signal from E1, amplifying the desired potential while rejecting common-mode interference.
EDT Device Setup & Operation 3
When calibrating an EMG machine before use, which parameter confirms that the amplifier gain is accurate?