Monitoring the NMJ Flashcards by Helen Wang

Steps of muscle contraction

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

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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

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The sarcolemma is polarized by

The sodium-potassium pump

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The intermediate in causing exocytosis by ach vesicles

Calcium

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Criteria for nerve selection

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

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What is a nerve stimulator?

Battery powered device that delivers depolarizing current via electrodes

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Pulse width

The duration of the individual impulse delivered by the nerve stimulator

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How long should each impulse be

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

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Pulse width beyond .5 msec

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

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What does the nerve stimulator monitor

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

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Indication for monitoring

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

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Features of monitoring

Increased safety
Cost effective
Easy documention

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Techniques

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

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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

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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

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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

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Accerleromyography

Newton’s second law

Transducer is easily placed but must move freely for reliable measurement

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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

Frequency for single twitch

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

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

What is the advantage of train of four?

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

Train of four ratio

Fourth to first twitch

.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

.85-.90 TOF ratio

Diplopia and visual disturbances

Generalized fatigue

Tetanus

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

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)

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

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)

Which nerve to stimulate during induction?

Orbicularis oculi Similar to central nerves, such as laryngeal muscle

Which nerve to stimulate for maintenance?

Orbicularis oculi Central muscle more reflective of diaphragm

Which nerve to stimulate for reversal and recovery?

Adductor pollicis Larger margin of safety if peripheral nerve is accessed

Facial nerve placement of electrodes

Above eyebrow and near ear lobe

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

Conical applications - onset

Orbicularis oculi: ST or TOF

Clinical applications - surgical relaxation

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

Clinical applications - recovery

TOF ratio, DBS

Clinical applications - reversal

When 2 or more TOF responses

Clinical applications - extubation

When TOF reaches 70-90%

TOF responses and percent of receptors blocked

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

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

40-50%

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

False

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

False

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