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I&M for Anesthesia - Fall 2013 > Monitoring the NMJ > Flashcards

Monitoring the NMJ Flashcards

0
Q

Steps of muscle contraction

A
  1. Impulse arrives at motor end plate and ach is released from vesicles in the axon terminus
  2. Ach diffuses across the synapse to bing to receptors on the sarcolemma
  3. Binding cause Na to enter the cell, causing depolarization
  4. If threshold depolarization occurs, a new impulse (action potential) moves along the sarcolemma
  5. Sarcolemma repolarizes, K leaves cell, Na/K pump restores polarity
  6. Ach is broken down by acetylcholinesterase
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1
Q

The neuromuscular junction is

A

A synapse at which an electrical impulse traveling down a motor nerve releases a chemical transmitter, causing the muscle to contract

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2
Q

The sarcolemma is polarized by

A

The sodium-potassium pump

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3
Q

The intermediate in causing exocytosis by ach vesicles

A

Calcium

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4
Q

Criteria for nerve selection

A

Must have motor element
Must be close to skin
Contraction must be visible

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5
Q

What is a nerve stimulator?

A

Battery powered device that delivers depolarizing current via electrodes

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6
Q

Pulse width

A

The duration of the individual impulse delivered by the nerve stimulator

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7
Q

How long should each impulse be

A

Less than .5 msec and .1 sec to elicit nerve firing at a readily attainable current

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8
Q

Pulse width beyond .5 msec

A

Extends beyond refractory period of the nerve, resulting I repetitive firing

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9
Q

What does the nerve stimulator monitor

A 
Time of extubation
Degree of relaxation
Time to reverse
Time for extubation
Residual curarization
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10
Q

Indication for monitoring

A 
Long interventions
Changed pharmacokinetics/dynamics
No moving allowed
No reversal preferred
Disturbed electrolyte balance
Expected drug interaction
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11
Q

Features of monitoring

A

Increased safety
Cost effective
Easy documention

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12
Q

Techniques

A

Peripheral nerve stimulation (PNS)
Mechanomyograph (MMG)
Electromyograph (EMG)
Acceleromyography (AMG)

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13
Q

Peripheral nerve stimulus

A

Visual or tactile
Muscle should be in sight
Lacks accuracy or reliability
Acceptable TOF ratio of >70% for extubation
Double burst stimulus: only 40% of anesthesiologist able to recognize fade

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14
Q

Mechanomyograph

A 
Isometric measurement of force of contraction with a force displacement transducer
Simple, accurate, and reliable
Sensitive to external influences
Limb must be fixed in one position
Used for scientific studies
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15
Q

Electromyography

A

Measures evoked compound muscle potential
Correct positions of electrodes very important
Extensive and sensitive equipment
Diathermy interference
Seems to underestimate block during recovery
Scientific use but not popular for routine clinical use

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16
Q

Accerleromyography

A

Newton’s second law

Transducer is easily placed but must move freely for reliable measurement

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17
Q

Single twitch

A

Reflects events at post junction all membrane
Single supra maximal electrical stimuli applied to peripheral motor nerve
Used for monitoring onset of block
Same response to both groups of NMBA
Response influenced by position of muscle, muscle temp
Calibration required before relaxation

18
Q

Frequency for single twitch

A

Every second (1 Hz) or every 10 seconds (0.1 Hz)

19
Q

Train of four

A

Reflects events at pre synaptic membrane
Used successfully for onset, maintenance, and recovery of block
Four supra maximal stimuli q 0.5 seconds (2 Hz) may be repeated q 12-15 seconds

20
Q

What is the advantage of train of four?

A

Relative ratio of fourth to first response remains the same despite changes in absolute response

21
Q

Train of four ratio

A

Fourth to first twitch

22
Q

.70-.75 TOF ratio

A 
Diplopia, visual disturbances
Decreased handgrip strength
Inability to maintain apposition of incisors
Negative tongue depressor test
Inability to sit up without assistance
Severe facial weakness
Speaking is a major effort
Overall weakness and tiredness
23
Q

.85-.90 TOF ratio

A

Diplopia and visual disturbances

Generalized fatigue

24
Q

Tetanus

A 
Normally 50 Hz for 5 sec
Fade with non depolarizing block
Post tetanic fasciculation
Painful
May produce long lasting antagonism
25
Q

Post tetanic count (PTC)

A

If no response with ST or TOF: block can’t be assessed. PTC assesses the intensity of deep block due to fasciculation
50 Hz tetanus for 5 sec… 3 sec later, single twitch at 1 Hz and count the number of responses
Should not be repeated for 6 sec (possible antagonism)
TOF is zero at PTC of 5 (T1 appears in 5 min if PTC > 15 for pancuronium)

26
Q

Double burst stimulation

A

Two short (.2 msec) bursts of 50 Hz tetanic stimuli separated by 750 msec
DBS with 3 impulses in each bursts (3,3) commonly used
Ratio of second to first equivalent to TOF ratio
Easily seen or felt by the anesthesiologist

27
Q

Choice of muscle

A

Diaphragm (most resistant) > other resp, upper airway and facial muscles > peripheral and abdominal (least resistant)
Adductor pollicis (hand) and Flexor hallucis brevis (leg): sensitive (may be unreliable for intubation), less chance of overdosing
Orbicularis oculi: onset, duration, and sensitivity same as resp muscles
Other: laryngeal, masseter, other facial muscles (research purposes only)

28
Q

Which nerve to stimulate during induction?

A

Orbicularis oculi

Similar to central nerves, such as laryngeal muscle

29
Q

Which nerve to stimulate for maintenance?

A

Orbicularis oculi

Central muscle more reflective of diaphragm

30
Q

Which nerve to stimulate for reversal and recovery?

A

Adductor pollicis

Larger margin of safety if peripheral nerve is accessed

31
Q

Facial nerve placement of electrodes

A

Above eyebrow and near ear lobe

32
Q

Stimulation current

A

Single muscle fiber = all or none pattern
Whole muscle response depends on number of fibers activated
If sufficient current… All muscle fiber will react with max response
Supra maximal stimulus (20-25% above that necessary for maximal response) to evoke response in all fibers

33
Q

Conical applications - onset

A

Orbicularis oculi: ST or TOF

34
Q

Clinical applications - surgical relaxation

A

1 or 2 responses to TOF = sufficient block.

When intense block required = PTC

35
Q

Clinical applications - recovery

A

TOF ratio, DBS

36
Q

Clinical applications - reversal

A

When 2 or more TOF responses

37
Q

Clinical applications - extubation

A

When TOF reaches 70-90%

38
Q

TOF responses and percent of receptors blocked

A

1 - 95%
2 - 90%
3 - 85%
4 - 75%

40
Q

How many receptors can still be occupied when TOF ratio is 1?

A

40-50%

41
Q

T or F. Established cut off values for adequate recovery guarantee adequate ventilatory function and airway protection.

A

False

42
Q

T or F. Increased skin impedance from hypothermia limits the appropriate interpretation of evoked responses

A

False

59
Q

Limitations of monitoring

A

Neuromuscular responses may appear normal despite persistence of receptor occupancy
Because of individual variability in evoked responses, some patients may exhibit weakness at TOF ratio as high ad .8 to .9

I&M for Anesthesia - Fall 2013 (13 decks)

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