Monitoring the NMJ Flashcards
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
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
The sarcolemma is polarized by
The sodium-potassium pump
The intermediate in causing exocytosis by ach vesicles
Calcium
Criteria for nerve selection
Must have motor element
Must be close to skin
Contraction must be visible
What is a nerve stimulator?
Battery powered device that delivers depolarizing current via electrodes
Pulse width
The duration of the individual impulse delivered by the nerve stimulator
How long should each impulse be
Less than .5 msec and .1 sec to elicit nerve firing at a readily attainable current
Pulse width beyond .5 msec
Extends beyond refractory period of the nerve, resulting I repetitive firing
What does the nerve stimulator monitor
Time of extubation Degree of relaxation Time to reverse Time for extubation Residual curarization
Indication for monitoring
Long interventions Changed pharmacokinetics/dynamics No moving allowed No reversal preferred Disturbed electrolyte balance Expected drug interaction
Features of monitoring
Increased safety
Cost effective
Easy documention
Techniques
Peripheral nerve stimulation (PNS)
Mechanomyograph (MMG)
Electromyograph (EMG)
Acceleromyography (AMG)
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
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
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
Accerleromyography
Newton’s second law
Transducer is easily placed but must move freely for reliable measurement
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
