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I & M Pt. 2 > Monitoring the Neuromuscular Junction > Flashcards

Monitoring the Neuromuscular Junction Flashcards

1
Q

What is the NMJ (Neuromuscular Junction)?

A

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

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2
Q

Steps to contraction:

A
  1. Impulse arrives at the motor end plate causing release of Ach from vesicles in axon terminus (Ca++ causes Ach to leave) 2. Ach diffuses across synapse to bind to receptors on the sarcolemma 3. Binding of Ach to receptors causes Na+ to enter cell and cause depolarization 4. If threshold depolarization occurs a new impulse moves along sarcolemma 5.Sarcolemma repolarizes (K+ leaves cell, Na/K pump restores polarity) 6. Ach broken down by Ach-E
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3
Q

Criteria for Nerve Selection:

A

-It must have a motor element -It must be close to skin -Contraction in the muscle or muscle group which the nerve supplies must be visible or accessible to evoked response monitoring

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4
Q

Criteria of nerve stimulator:

A

-Battery powered device that delivers depolarizing current via electrodes -Impulse < 0.5 msec and 0.1 sec in duration to elicit nerve firing at a readily attainable current (Pulse width > 0.5 msec extends beyond the refractory period of the nerve resulting in repetitive firing)

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5
Q

Pulse Width:

A

The duration of the individual impulse delivered by the nerve stimulator

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6
Q

Time to monitor:

A

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

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7
Q

Indications of Monitoring:

A

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

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8
Q

Techniques:

A

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

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9
Q

Peripheral Nerve Stimullus

A

-Visual and/or tactile -Muscle monitored should be in sight -Lacks accuracy and reliability -Acceptable TOF ratio of >70% for extubation (only 10% correct observations) -Double burst stimulus (DBS):Only 40% of anesthesiologists are able to recognize a fade

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10
Q

Mechno-myograph

A

-Isometric measurement of force of contraction with a force displacements transducer -Simple, accurate and reliable -Sensitive to external physical influences and limb has to be fixed in one position -Used for studies

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11
Q

Electromyography

A

-Measures evoked compound muscle action potential -Correct positioning of electrodes very important -Extensive and sensitive equipment -Diathermy interference -Seems to underestimate block during recovery -For scientific use

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12
Q

Acceleromyography

A

-Newton’s second law (F = M X a) -Transducer is easily placed but must move freely for reliable measurement

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13
Q

Stimulating patterns: Single Twitch (ST)

A

-Reflects events at post junctional membrane -Single supra maximal electrical stimuli applied to peripheral motor nerve -Frequency every second (1 Hz) or every 10 seconds (0.1 Hz) -Used for monitoring onset of block -Same response to both groups of NMBs -Response influenced by position of muscle, muscle temp -Calibration required before relaxation -Not for day to day clinical practice

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14
Q

Stimulating patterns: Train of Four (TOF)

A

-Reflects events at pre-synaptic membrane -Used successfully for onset, maintenance and recovery of block -4 supra maximal stimuli q 0.5 seconds (2 Hz) May be repeated q 12-15 seconds -Advantage: relative ration of 4th to 1st response remains the same despite changes in absolute responses

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15
Q

Non-Depolarizing Block Response:

A
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16
Q

Depolarizing Block Response

A
17
Q

Train of 4 Symptoms: 0.7 - 0.75

A
  1. Diplopia and visual disturbances
  2. Decreased handgrip strength
  3. Inability to maintain apposition of incisor teerth
  4. “Tongue depressor test” negative
  5. Inability to sit up w/o assistance
  6. Severe facial weakness
  7. Speaking a major effort
  8. Overall weakness and tiredness
18
Q

Train of 4: 0.85-0.9

A
  1. Diplopia and visual disturbances
  2. Generalized fatigue
19
Q

Stimulating Patterns: Tetanus

A
  • Normally 50 Hz for 5 sec
  • Fade w/ non-depolarizing block
  • Post-tetanic facilitation
  • Painful
  • May produce lasting antagonism
20
Q

Tetanus:

A
21
Q

Stimulating Patterns: Post-tetanic count (PTC)

A
  • If no response w/ ST or TOF: block can’t be assessed. PTC assesses the intensity of a deep block (Due to facilitation)
  • 50 Hz tetanus for 5 sec -> 3 sec later, single twitch a 1 Hz and the no of responses
  • Should NOT be repeated for 6 min (possible antagonism of the muscle)
  • TOF is zero at PTC of 5 (TI appears in 5 min if PTC > 15 for Pancuronium)
22
Q

Blocks

A
23
Q

Stimulating Patterns: Double Burst Stimulation

A
  • Two short (0.2 milliseconds) bursts of 50 Hz tetanic stimuli separated by 150 mseconds
  • DBS w/ 3 impulses in each of bursts (3,3) most commonly used
  • Ratio of second response to the first is quicalent to TOF ratio
  • Easily seen or felt by the anesthesiologist
24
Q

Double Burst Stimulation

A
25
Q

Muscle Choices for Monitoring:

A
  1. Diaphragm: Most resistant >Other resp, upper airway and facial muscles > Peripheral and abdominal (least resistant)
  2. Adductor pollicis (hand) and Flexor hallucis brevis (leg): Sensitive (May be unreliable for intubation), less chance of overdosing
  3. Orbicularis oculi: Onset, duration and sensitivity same as resp muscles
  4. Other: Laryngeal, masseter, other facial muscles (research purposes only)
26
Q

Which nerve to stimulate When: Induction

A
  • Oricularis oculi
  • similar to central nerves, such as laryngeal muscle
27
Q

Which nerve to stimulate When: Maintenance

A
  • Oribularis oculi
  • Central muscle more reflective of diaphragm
28
Q

Which nerve to stimulate When: Reversal and recovery

A
  • Adductor pollicis
  • Larger margin of safety if peripheral nerve is assessed
29
Q

Supra-maximal stimulus:

A

20 - 25% above that necessary for maximal response to evoke response in all muscle fibers

30
Q

Clinical Applications:

A
  • Induction: obicularis oculi -> ST or TOF
  • Surgical relaxation: 1 or 2 responses to TOF= sufficient block. (When intense block required -> PTC)
  • Recovery: TOF ratio, DBS
  • Reversal: When 2 or more TOF responses
  • Extubation: When TOF reaches 70-90%
31
Q

TOF Count

A
32
Q

Phases

A
33
Q

TOF

A
34
Q
A

I & M Pt. 2 (4 decks)

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