Monitoring the Neuromuscular Junction Flashcards
What is the NMJ (Neuromuscular Junction)?
A synapse at which an electrical impulse traveling down a motor nerve, releases chemical transmitter which causes the muscle to contract
Steps to contraction:
- 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
Criteria for Nerve Selection:
-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
Criteria of nerve stimulator:
-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)
Pulse Width:
The duration of the individual impulse delivered by the nerve stimulator
Time to monitor:
-Time to intubation -Degree of relaxation -Time to reverse -Time for extubation -Residual curarization
Indications of Monitoring:
-Long interventions -Changed pharmacokinetics/dynamics -No moving/straining allowed -No reversal preferred -Disturbed electrolyte balance -Expected drug interactions
Techniques:
-Peripheral nerve stimulation (PNS) -Mechanomyograph (MMG) -Electromyograph (EMG) -Acceleromyography (AMG)
Peripheral Nerve Stimullus
-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
Mechno-myograph
-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
Electromyography
-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
Acceleromyography
-Newton’s second law (F = M X a) -Transducer is easily placed but must move freely for reliable measurement
Stimulating patterns: Single Twitch (ST)
-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
Stimulating patterns: Train of Four (TOF)
-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
Non-Depolarizing Block Response:
Depolarizing Block Response
Train of 4 Symptoms: 0.7 - 0.75
- Diplopia and visual disturbances
- Decreased handgrip strength
- Inability to maintain apposition of incisor teerth
- “Tongue depressor test” negative
- Inability to sit up w/o assistance
- Severe facial weakness
- Speaking a major effort
- Overall weakness and tiredness
Train of 4: 0.85-0.9
- Diplopia and visual disturbances
- Generalized fatigue
Stimulating Patterns: Tetanus
- Normally 50 Hz for 5 sec
- Fade w/ non-depolarizing block
- Post-tetanic facilitation
- Painful
- May produce lasting antagonism
Tetanus:
Stimulating Patterns: Post-tetanic count (PTC)
- 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)
Blocks
Stimulating Patterns: Double Burst Stimulation
- 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
Double Burst Stimulation
Muscle Choices for Monitoring:
- 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)
Which nerve to stimulate When: Induction
- Oricularis oculi
- similar to central nerves, such as laryngeal muscle
Which nerve to stimulate When: Maintenance
- Oribularis oculi
- Central muscle more reflective of diaphragm
Which nerve to stimulate When: Reversal and recovery
- Adductor pollicis
- Larger margin of safety if peripheral nerve is assessed
Supra-maximal stimulus:
20 - 25% above that necessary for maximal response to evoke response in all muscle fibers
Clinical Applications:
- 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%
TOF Count
Phases
TOF
