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

Flashcards in Monitoring the NMJ Deck (43):
0

The neuromuscular junction is

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

1

Steps of muscle contraction

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

2

The sarcolemma is polarized by

The sodium-potassium pump

3

The intermediate in causing exocytosis by ach vesicles

Calcium

4

Criteria for nerve selection

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

5

What is a nerve stimulator?

Battery powered device that delivers depolarizing current via electrodes

6

Pulse width

The duration of the individual impulse delivered by the nerve stimulator

7

How long should each impulse be

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

8

Pulse width beyond .5 msec

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

9

What does the nerve stimulator monitor

Time of extubation
Degree of relaxation
Time to reverse
Time for extubation
Residual curarization

10

Indication for monitoring

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

11

Features of monitoring

Increased safety
Cost effective
Easy documention

12

Techniques

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

13

Peripheral nerve stimulus

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

14

Mechanomyograph

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

15

Electromyography

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

16

Accerleromyography

Newton's second law

Transducer is easily placed but must move freely for reliable measurement

17

Single twitch

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

Frequency for single twitch

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

19

Train of four

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

What is the advantage of train of four?

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

21

Train of four ratio

Fourth to first twitch

22

.70-.75 TOF ratio

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

.85-.90 TOF ratio

Diplopia and visual disturbances
Generalized fatigue

24

Tetanus

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

25

Post tetanic count (PTC)

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

Double burst stimulation

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

Choice of muscle

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

Which nerve to stimulate during induction?

Orbicularis oculi

Similar to central nerves, such as laryngeal muscle

29

Which nerve to stimulate for maintenance?

Orbicularis oculi

Central muscle more reflective of diaphragm

30

Which nerve to stimulate for reversal and recovery?

Adductor pollicis

Larger margin of safety if peripheral nerve is accessed

31

Facial nerve placement of electrodes

Above eyebrow and near ear lobe

32

Stimulation current

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

Conical applications - onset

Orbicularis oculi: ST or TOF

34

Clinical applications - surgical relaxation

1 or 2 responses to TOF = sufficient block.
When intense block required = PTC

35

Clinical applications - recovery

TOF ratio, DBS

36

Clinical applications - reversal

When 2 or more TOF responses

37

Clinical applications - extubation

When TOF reaches 70-90%

38

TOF responses and percent of receptors blocked

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

40

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

40-50%

41

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

False

42

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

False

59

Limitations of monitoring

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