Technical Factors EMG/NCS Flashcards
High frequency filters do what?
- Exclude signals above a set frequency
- Allow lower frequency signals to pass through (low pass)
Low frequency filters do what?
- Excludes signals below a set frequency from passing through
- Allows higher frequency signals to pass through (high pass)
Reducing a low frequency filter allows more low frequecy signals to pass through. What does this do to the waveforms?
It increases the duration of the waveforms because the duration is primarily a lower frequency response
As a high frequency filter is lowered more high frequency signals are excluded. What does this do to the waveform?
It decreases the amplitude of the waveform because amplitude is primarily a higher frequency response
Does depolarization take place beneath the cathode or the anode?
Cathode
Waveform measurement should be made between the _______ and __
Cathode and G1
What physiologic effects do cold temperatures have on Na channels and what does this do to depolarization?
It delays inactivation of Na channels and prolongs the time of depolarization
The time of depolarization increases with decreasing temperature. What does prolonged depolarization do to conduction velocity?
It slows conduction velocity down. The colder the temperature, the slower the conduction velocity.
For every 10 C drop in temperature, what happens to motor and sensory conduction velocity and distal latency?
- Conduction velocity decreases by 1.5 to 2.5 m/s
- Distal latency prolongs by 0.2 ms
Why do CMAPs and SNAPs increase in amplitude and duration with cooler temperatures?
Cooler temperatures result in longer channel opening which results in a larger influx of Na which lengthens the depolarization
What effect does cooler temperature have on EMG MUAPs?
- Increased amplitude
- Increased duration
- Number of phases may also increase
What is the ideal distal limb temperature for EMG/NCS?
32º to 34º C
Why does conduction velocity increase from birth to adult?
Myelination increases
What happens to conduction velocity with increasing age after adulthood? At what age is this most prominent?
- It decreases because of demyelination
- Most prominent after age 60
At what age is peripheral myelination complete?
Ages 3 to 5
After age 60 how much does conduction velocity decrease per decade?
0.5 to 4.0 m/s/decade
What affect does increasing age have on SNAP amplitudes?
It decreases amplitudes up to 50% by age 70
What effect does increasing age have on EMG MUAP?
It increases MUAP duration due to the slow dropout of motor units with some subsequent reinnervation
What affect does height have on nerve conduction velocity and why?
- Increasing height = slower conduction velocity
- The taller the individual, the longer the limb and the more tapered the distal nerve is
- Conduction velocity is directly proportional to nerve diameter (larger = faster)
- Limbs are cooler distally than proximally and since legs are longer than arms, conduction velocity in the legs is slower than in the arms
What is common mode rejection?
This occurs when the same electrical noise is present at both the active and reference electrodes, they cancel each other out and only the signal of interest is amplified
In recording motor conduction studies, what is the belly-tendon method?
Active electrode (G1) is placed on the muscle belly and the reference electrode (G2) is placed on the muscle’s distal tendon
Antidromic stimulation vs orthodromic stimulation
Antidromic stimulation
- Stimulate proximal nerve and record distal nerve
- Example: stimulating median nerve at wrist and recording at digit 2
Orthodromic stimulation
- Stimulate distal nerve and record at proximal nerve
- Example: stimulating at digit 2 and recording median nerve at wrist
Why does antidromic stimulation of a sensory nerve result in a higher amplitude
- SNAP amplitude is directly proportional to the distance between the recording electrodes and the nerve
- For most antidromic potentials, the active recording electrodes are closer to the nerve
- Example: stimulating median sensory nerve at the wrist and recording on digit 2
Disadvantage of antidromic sensory study
- Motor and sensory fibers are stimulated
- Usually the SNAP shows up before the volume conducted motor potential
- If the SNAP is absent the motor potential can be confused for the SNAP
What is the effect on onset latency, peak latency and amplitude of a sensory nerve if the recording electrode is lateral or medial to the nerve instead of directly on top?
- Onset latency is shorter (seems like it would be the opposite)
- Peak latency is the same (again, seems weird)
- Amplitude is lower (this makes sense)
All of the above occurs because of volume conduction through tissue
What effects do sweep speed and sensitivity have on the recorded latency of sensory and motor potentials?
- As the sweep speed is decreased (2 ms to 1 ms for example), onset latency measurement increases
- As sensitivity is increased (1 mV to 100 µV), onset latency measurement decreases
What is the transmembrane potential of skeletal muscle?
-90 mV
What does the endoneurium do?
Surrounds individual axons
What does the perineurium do?
It binds each fascicle and may act as a blood-nerve barrier
What does the epineurium do?
It binds fascicles together and merges with the dura mater of the spinal roots
Do changes in sweep speed and sensitivity have an effect on onset and peak latency?
They affect onset latency but not peak latency
What are the typical low and high frequency filter settings for motor nerve conduction studies?
10 Hz and 10 kHz
What are the typical low and high frequency filter settings for sensory nerve conduction studies?
20 Hz and 2 kHz
Sensory high frequency filter is set lower than for motor studies to reduce high frequency noise since SNAPs contain more high-frequency components compared to CMAPs
