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Patient Monitoring/Instrumentation & SIM > Nerve Stimulation > Flashcards

Nerve Stimulation Flashcards

1
Q

Length of a nerve stimulus

A

0.2 msec (200 usec)

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2
Q

How does a nerve stimulator work?

A
  1. Stimulator is connected to electrodes that are placed on the ulnar or facial nerve
  2. The nerve is electrically stimulated for 0.2 msec
  3. Ach is released from the presynaptic nerve
  4. Ach binds to nictonic Ach receptors
  5. After Ach is released, it is eventually degraded
  6. Prejunctional receptors allow acetic acid and choline to enter the nerve
  7. Once inside the nerve, acetic acid and choline form acetylcholine
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3
Q

True/false: if we could directly stimulate a muscle, it would underestimate how paralyzed a patient is

A

True

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4
Q

Factors that could cause direct muscle stimulation

A
  1. Placing electrodes directly over a muscle instead of over a nerve
  2. Longer pulse durations (>500usec) when stimulating a nerve
  3. Using a higher current when stimulating a nerve (over 80mA)
  4. Using needle electrodes
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5
Q

Describe a “weaker” muscle contraction

A

Some receptors are blocked, so the muscle can still contract, just not as forcefully

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6
Q

Describe no muscle contraction

A

All receptors are blocked, so no muscle contraction can occur

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7
Q

Types of nerve stimulation patterins

A
  1. Single twitch (1Hz)
  2. Train of Four (2Hz)
  3. Tetanus (50Hz)
  4. Post tetanic count (1Hz after Tetanus)
  5. Double burst stimulation (20ms)
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8
Q

2 types of single twitch stimulation

A

1 Hz (1 stimulation per second) and 0.1 Hz (1 stimulation per 10 seconds)

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9
Q

Duration of Train of Four (TOF) stimulation

A

4 stimuli (0.2 msec each) over a 2 second (2Hz) period with 500 msec between each stimulus

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10
Q

Duration/frequency of Tetanus

A
  • continuous nerve stimulation at 50-100 Hz, mainly 50

- Very painful, so limit to under 5 seconds

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11
Q

Duration of Post Tetanic Count

A

50 Hz of tetanus applied for 5 seconds, followed by a 3 second pause, then a single twitch stimulation at 1 Hz

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12
Q

Duration of Double Burst Stimulation

A

2 short tetanic stimulations followed by a 750 msec pause
-the first tetanic stimulation is 3 impulses at 50Hz
-750 msec pause
-the second can be: 1. Two impulses at 50 Hz (3,2)
or 2. Three impulses at 50 Hz (3,3)

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13
Q

Lowest stimulation frequency, longest duration

A

Single twitch

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14
Q

Faster stimulation frequency, shorter duration

A

Train of Four and Tetanus

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15
Q

Occurs when a nerve is stimulated multiple times in a row and a patient has a partial neuromuscular block
(Twitch gets weaker)

A

Fade

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16
Q

What does fade mean?

A

Something is causing less Ach to be released from the presynaptic nerve during each subsequent twitch

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17
Q

What can cause fade?

A

Partial nondepolarizing blocks

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18
Q

What does it mean when twitches are all at the same strengh?

A

The same amount of Ach is released from the presynaptic nerve each time

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19
Q

Blocks presynpatic and postsynaptic receptors

A

Nondepolarizing muscle relaxants

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20
Q

Description of partial nondepolarizing block

A

When presynaptic receptors are blocked, acetic acid and choline have a harder time entering the nerve
-With repeated, back to back stimuli, stockpiles of Ach will diminish and less Ach will be available

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21
Q

Mechanism of depolarizing muscle relaxants

A

Only block postsynaptic Ach receptors

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22
Q

Factors necessary for fade to occur

A
  1. There must be a partial NONDEPOLARIZING block

2. The nerve must be stimulated at a fairly high frequency

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23
Q

Gold standard in assessment of recovery of neuromuscular blockade

A

fade

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24
Q

What does it mean if fade is present?

A

The patient is still paralyzed

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25
Q

When does fade occur clinically?

A

During recovery of a partial nondepolarizing neuromuscular block
With TOF, Tetanus, PTC and DBS

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26
Q

True/false: Fade is observed with single twitch stimulation and succinylcholine

A

False; fade is not seen with either one

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27
Q

Why is single twitch not used to observe fade?

A

You cannot differentiate between depolarizing and nondepolarizing blocks

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28
Q

What is the significance of quantitative nerve stimulators?

A

Quantitative nerve stimulators can tell you how strong the twitches are as well as how many

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29
Q

Negatively charged electrode

A

Black

Depolarizing membrane

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30
Q

Positively charged electrode

A

Red

Hyperpolarizes membrane

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31
Q

When does the maximal twitch occur?

A

When the negative electrode is placed in closest proximity to the nerve

32
Q

Where do the electrodes get placed on the ulnar nerve?

A

Red should be placed proximally (3-6cm away from black)

Black should be placed distally

33
Q

Where do electrodes get placed on the face?

A

The black electrode should be placed as close to the temporal branch of the facial nerve as possible, probably on the zygomatic arch
The red electrode can be placed on the forehead

34
Q

What is subthreshold stimulus?

A

No motor units respond

35
Q

What is threshold stimulus?

A

One motor unit responds

36
Q

Submaximal stimuli

A

Increasing number of units responding

37
Q

Maximal stimulus

A

All motor units respond

Pre-relaxant control response

38
Q

Supramaximal stimuli

A
  • All motor units respond
  • Stimulating the nerve at a current ABOVE that of which is required for maximum twitch height
  • Should always use this one
39
Q

How do you determine supramaximal stimulus?

A
  1. Put pt to sleep and attach nerve stimulator
  2. Using single twitch summation, start at lower current and gradually increase until maximal twitch height is observed (b/t 40-80mA)
40
Q

Indications for single twitch nerve stimulation

A
  1. Muscle relaxant onset/satisfactory conditions for intubation
  2. Supramaximal stimulus
  3. Post Tetanic Count
41
Q

How to use single twitch to determine muscle relaxant onset

A
  1. Put pt to sleep and stimulate nerve once per second
  2. Administer muscle relaxant and continue stimulating nerve
  3. Twitch strength will fade gradually, as the twitch begins to decrease once 75% of receptors are blocked
42
Q

How to use single twitch to determine recovery

A
  1. Obtain max twitch height prior to dosing muscle relaxant
  2. Compare supramaximal twitch height after reversal to the one prior to dosing the muscle relaxants

*not common bc 75% of receptors could still be blocked even with max height

43
Q

Disadvantages to single twitch stimulation

A
  1. Fade during recovery is not likely

2. Cannot distinguish between depolarizing and nondepolarizing blocks

44
Q

Clinical use of train of four

A
  1. Indicates how “paralyzed” a patient is/how profound the neuromuscular block is
  2. Patients are considered reversible with Neostigmine if they display 1/4 twitches (but it is recommended they display 2-3 twitches)
45
Q

How does 0/4 twitches correlate with paralysis?

A

> 90% receptor block

46
Q

How does 1/4 twitches correlate with paralysis?

A

90% receptor block

47
Q

How does 2/4 twitches correlate with paralysis?

A

80% receptor block

48
Q

How does 3/4 twitches correlate with paralysis?

A

75% receptor block

49
Q

How does 4/4 twitches correlate with paralysis?

A

<75% receptor block

50
Q

What is the train of four ratio?

A

The strength of the 4th twitch compared to the strength of the 1st twitch
T4:T1 =60% means the 4th twitch is 60% as strong as the 1st twitch

51
Q

What TOF ratio is considered to have residual neuromuscular blockade?

52
Q

What does a higher TOF ratio mean?

A
  1. The stronger the 4th twitch will be to the 1st twitch
  2. The lower the fade will be
  3. The stronger the muscle function
53
Q

Disadvantages to TOF?

A
  1. It is less useful in assessing partial depolarizing blocks (no fade)
  2. Not as good at measuring deep levels of blockade
  3. Not as useful at determining muscle relaxant onset
54
Q

Clinical use of tetanus

A
  1. Can assess deeper levels of blockade than TOF
  2. Can assess if patient’s paralysis has been adequately reversed w/sustained tetanus >5 seconds
  3. Can distinguish between nondepolarizing and depolarizing
55
Q

Clinical use of post tetanic count

A
  1. Can assess even deeper levels of blockade than tetanus
  2. The higher the number of post tetanic twitches, the less time to wait before the return of a single twitch
  3. Reversal with neostigmine can happen with more than 10 PTC twitches
56
Q

Clinical use of double burst stimulus

A
  1. Better indicator of fade than TOF or tetanus

2. Less painful than tetanus

57
Q

Why must you wait a certain period of time before repeating a nerve stimulation pattern?

A

The NMJ will be flood with Ach, causing a stronger twitch than what is really there, causing you to underestimate the paralysis

58
Q

How long do you wait to repeat DBS?

A

12-15 seconds

59
Q

How often can TOF be repeated?

A

10-30 seconds

60
Q

How often can tetanus be repeated?

61
Q

How often can PTC be repeated?

62
Q

What is the order of muscle recovery from shortest time to longest time?

A
  1. Diaphragm is soonest to recover
  2. Rectus abdominus
  3. Laryngeal adductors
  4. Orbicularis oculi
  5. Adductor pollicus (ulnar)
63
Q

What is the most useful stimulator site for determining onset time for intubation?

A

Orbicularis oculi (facial nerve)

64
Q

True/false: Direct muscle stimulation is more likely with the facial nerve than the ulnar nerve

65
Q

Stimulation at this point reduces PONV

A

Median nerve

66
Q

Indicators of adequate reversal

A
  1. Sustained head lift
  2. Sustained tetanus (>5 seconds without fade)
  3. Tidal volumes
  4. Strong hand grip
  5. Negative Inspiratory Force (greatest negative pressure a patient can generate during inspiration: -50 to -100 cm/H2O)
  6. TOF ratio
67
Q

Factors that prolong the duration of muscle relaxants (14)

A
  1. Hepatic and renal disease
  2. Hypothermia
  3. Old age
  4. Premature neonates
  5. Metabolic acidosis>respiratory acidosis
  6. Myasthenia Gravis
  7. Ca++ abnormalities
  8. Mg abnormalities
  9. Hypokalemia
  10. Hypernatremia
  11. Antibiotics (aminoglycosides)
  12. Antiarrhythmic agents
  13. Inhalational agents
  14. Administration of succs prior to nondepolarizing can prolong nondepolarizing effects
68
Q

Advantages to awake extubation

A
  1. Pt is less likely to have airway obstruction or laryngospasm
  2. Airway is protected
69
Q

Disadvantages to awake extubation

A
  1. Coughing and bronchospasm are more likely

2. Turnover times are prolonged if pt is slow to awaken

70
Q

Awake extubation criteria

A
  1. Pt must be breathing spontaneously
  2. Must show adequate reversal
  3. Must be able to follow commands/protect airway and prevent laryngospasm
71
Q

Advantages of deep extubation

A
  1. Shorter turnover times

2. Less likely to cough or have bronchospasm

72
Q

Disadvantages of deep extubation

A
  1. Airway is not protected
  2. Airway will obstruct
  3. Laryngospasm is possible until pt wakes up
73
Q

Deep extubation criteria

A
  1. Breathing spontaneously with adequate tidal volumes
  2. Must truly be deep
  3. Must be thoroughly suctioned
74
Q

Deep extubation contraindications

A 
Absolute:
1. Full stomach/GERD/hiatal hernia
2. Difficult airway
3. Airway edema
Relative
1. Morbidly obese
2. OSA
75
Q

Positive pressure is higher during inspiration than expiration

Patient Monitoring/Instrumentation & SIM (12 decks)

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