8: Artifacts and Technical Factors

Question 1

What does cooler temperatures mean with regards to nerve physiology?

Answer 1

Delayed inactivation of Na channels, and prolonged depolarization time

Question 2

With motor and sensory conduction velocities, how many m/s is the CV slowed for every 1 degree C drop in temp? How long does distal latency prolong?

Answer 2

1.5-2.5 m/s;

.2 ms

Question 3

Logic behind enlarged sensory nerve action potentials with colder temps?

Answer 3

The SNAP’s prolong, with less phase cancellation

Question 4

What can happen with MUAPs with cooler temps?

Answer 4

Prolonged amplitude, duration, and phases

Question 5

CV for newborn? At 1 year of age? When equal to adults?

Answer 5

50% of adult; 75% of adult; 3-5

Question 6

Effect of age on CV? SNAP amplitude?

Answer 6

Past age 60, can see CV decrease about .5-4 m/s per decade;

SNAP amplitude can drop up to 50% by age 70

Question 7

For folks who are taller, what is an adjustment in CV?

Answer 7

2-4 m/s typically at most below normal limits

Question 8

Which segments of nerve are faster, proximal or distal? Why?

Answer 8

Proximal (more tapering with distal segments, and distal segments usually cooler)

Question 9

Most common cause of electrical noise?

Answer 9

60-Hz interference generated by other electrical devices (lights, fans, heaters, computers)

Question 10

Best ways to eliminate 60-Hz interference?

Answer 10

1. Same type of active and reference recording electrodes
2. Ensure all contacts are intact w/o frayed or broken connections
3. Clean dirt and oil from skin with alcohol or acetone
4. Apply conducting electrode jelly b/w skin and electrodes
5. Secure electrodes firmly to the skin
6. Place ground between stimulator and recording electrodes
7. Use coaxial recording cables

Question 11

What are the typical low- and high-freq filters for motor and sensory conduction studies?

Answer 11

10 and 10 kHz;

20 and 2 kHz

Question 12

Methods to reduce stimulus artifact

Answer 12

1. Place ground b/w stimulator and recording electrodes
2. Reduce electrode impedance mismatch between the recording electrodes
3. Use coaxial recording cables
4. Ensure stimulator position right over the nerve
5. Lower stimulus intensity
6. Rotate anode of the stimulator
7. Increase distance between the stimulator and recording electrodes
8. Make sure stimulator and recording electrode cables do not overlap

Question 13

With stimulation at Erb’s point or at the C8-T1 nerve roots, how can co-stimulation of the ulnar and median nerves potentially be avoided?

Answer 13

Collision studies

Question 14

How to avoid Co-stimulation of adjacent nerves?

Answer 14

1. Ensure stimulator position is optimized right over the nerve
2. Look for abrupt waveform morphology change
3. Look for change in muscle twitch
4. Avoid excessive stimulation currents
5. Co-record muscles maybe from adjacent nerves
6. Consider possible distal co-stimulation when visualizing conduction block

Question 15

What happens if recording electrodes are placed lateral or medial to the nerve with regards to latencies?

Answer 15

Onset latency shortens while peak latency remains unchanged (due to effects of volume conduction through tissue)

Question 16

What happens to onset latency as sensitivity is increased?

Answer 16

The onset latency measurement tends to decrease