Muscle Relaxants

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Muscle Relaxants Nerve Monitoring
2
Overview of Nervous System
3
Motor Neuron
4
Nerve Conduction

• More Na outside membrane than inside at rest
  (15x).Na gt K
• More K inside membrane than outside at rest (30x)

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Synaptic Cleft

• Acetylcholine stored in vesicles at motor end
  plate
• Nerve impulse ends with Ach released into cleft
  from vesicles

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Acetylcholine

• Synthesized from choline and acetate
• Stored in vesicles at motor end plate
• Quanta contain 5-10 Thousand ACh
• 2-4 hundred Quanta(1-4 million Ach) with each
  action potential
• Some quanta are released in absence of nerve
  stimulation
• Ach metabolized by Cholinesterase within
  thousandths of a second

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Nicotinic Acetylcholine Receptor

• Consists of 5 glycoprotein subunits to form an
  ion channel.
• 2 alpha, 1 beta,1 delta,1 gamma(epsilon)
• Ach binds to the 2 alpha subunits opening the
  channel
• 1-10 million receptors across cell membrane
• Only 1 alpha subunit needs NDMR attached to block
• Both alpha subunits need SCh attached to block

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MUSCLE RELAXANTS

• Depolarizers Succinylcholine Decamethoniu
  m
• Non-Depolarizers Curare Pancuronium Ve
  curonium Rocuronium Atracurium
  Cis-Atracurium Mivacurium

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Depolarizers
!
Succinylcholine is 2 Ach molecules acting like
Ach at post-synaptic receptor. Not fully
understood exactly how it exerts action

• CHARACTERISTICS
• Phase I block
• Decrease single twitch
• No FADE (decrease but Sustained response) to
  continual stimulus
• TOF ratio greater than 0.7
• Potentiation by Anticholinesterase drugs
• Cause fasiculations
• Phase II block
• Resembles non-depolarizer blockade characteristics

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Non-depolarizers
!
...act on at least one alpha subunit to block
conduction. Competitive antagonist

• CHARACTERISTICS
• Decrease twitch response to a single stimulus
• Produce FADE (unsustained response) to continual
  stimulus
• TOF ratio less than 0.7
• Post tetanic facilitation
• Potentiation by other non-depolarizers
• Antagonism by anticholinesterase drugs

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Why relax or paralyze Muscles?

• To obtain adequate intubating conditions
• To facilitate surgical exposure or manipulation
• To improve mechanical ventilation

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Nerve Stimulators

• Many styles and sizes

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Nerve Monitoring

• Single twitch 0.1-0.15 Hz
• Train of Four 2Hz (0.5secs apart)
• Tetanus 50Hz
  100Hz both 5 secs apart
• Double Burst Stimulation 50Hz (0.2 ms,750ms
  apart) 2 sets of 3 bursts appear as two Twitches
• Post-tetanic count stim. 50Hz, 5 secs
• Impulse durations are 0.2 sec.
• PURPOSE To evaluate degree of muscle paralysis
  or recovery from paralysis

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Nerve Monitoring Train of Four
Single Twitch
Train of


Four
FADE on Train of Four
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Non-Depolarizers

• Exhibit Fade on Train of four.
• Fade may progress to no twitches if enough NDMR
  is given

or
or
or (none)
Increasing NDMR dose
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Nerve MonitoringTrain of Four
!
R4 decreases at 75 receptors blocked R3
decreases at 85 receptors blocked R2
decreases at 90 receptors blocked
R1 disappears at 95 receptors blocked
What is your degree ( receptor blockade) of
paralysis shown to the right?
R1 R2 R3 R4 RResponse
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Clinical Application
!

• Surgical relaxation appreciated at gt90
• Intubation facilitated at 95
• Total Flaccidity at 99

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Nerve Monitoring Double Burst
Fade on Double Burst
Equal Double Burst
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Phase 1 and 2 Block
Seen with depolarizing blockade

• Phase 1 block
• Phase 2 block

typical non-depolarizing TOF
Baseline TOF
high or repeated doses of depolarizer- 0/4 TOF
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REVIEW

• NDMR have FADE
• Depolarizers have Phase 1 block seen as 4/4
  TOF but with LESS amplitude or Phase 2
  block seen as 0/4 TOF That is NO TWITCHES

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Placement of Nerve Monitor Leads
THERE ARE MANY NERVES YOU CAN MONITOR Most
frequently used ULNAR-adductor pollicis
FACIAL-orbicularis oculi POSTERIOR
TIBIAL-flexor hallucis
We stimulate a nerve to observe a muscle response
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Ulnar nerve monitoring
Note proper lead placement
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Proper Twitch evaluation

• Adduction of thumb by Ulnar nerve stimulation
  causing ADDUCTOR POLLICIS muscle contraction

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Improper Twitch evaluation

• Median nerve and / or direct muscle
  stimulation(note finger contraction, not thumb
  contraction)

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Direct muscle and Ulnar nerve (Adductor Pollicis)
stimulation
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Facial Nerve Monitoring
Lead Placement
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Facial Nerve Twitch
Twitch of ORBICULARIS OCULI
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Posterior Tibial Nerve monitoring
Placement- Behind the external malleolus.
Stimulation causes flexion of the big toe by
contraction of the flexor hallucis
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Which nerve is better to Monitor?
!

• Good Question!
• Orbicularis Oculi-relatively resistant to
  blockade. Similar to Diaphragms resistance
• Go to sleep closing your eyes, wake up thumbs up.
  Monitor Facial nerve for intubating conditions
  and Adductor Pollicis ( Ulnar nerve) for
  extubation adequacy

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Which nerve is better to Monitor? continued

• Adductor Pollicis-easily blocked
• Ulnar nerve monitoring beneficial for two main
  reasons Ease of placement and access. And
  contralateral innervation of the monitored
  muscle. This allows discrimination between direct
  muscle stimulation and direct nerve stimulation.
• Note-Ulnar nerve also innervates hypothenar
  muscles of 5th digit though these muscles are
  relatively resistant to blockade.

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Which nerve is better to Monitor? continued

• Posterior Tibial nerve- Allows monitoring on
  patients who we have limited access to.
• Sensitivity of flexor hallucis undetermined

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Residual Muscle Paralysis Evaluation
!

• TEST RECEPTORS OCCUPIED
• Normal tidal vol. Up to 80 may be blocked
• Holds tetanus 50Hz Up to 75-80 may be blocked
• Equal TOF, DBS Up to 75-80 may be blocked
• Holds tetanus 100Hz Up to 50 may be blocked
• Head lift x 5secs Up to 33 may be blocked
• Remember 0 UP TO the above number
  ie. Normal tidal vol.represents 0 or anywhere up
  to 80 of ACh receptors competitively blocked
  with muscle relaxant.

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What do each of these modes mean?

• Single twitch- allows continual evaluation of
  Depolarizing block (must know baseline)
• Train of Four (TOF) - Allows estimation of DEGREE
  of non- depolarizing block
• Double Burst Stimulation (DBS)- Easier visual
  evaluation of fade. Gives same info. As TOF

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What do each of these modes mean? continued

• Tetanic stimulation- More detailed assessment of
  degree of non-depolarizing block
• Post Tetanic Stimulation Count 1)Allows
  quantification of block if NO TOF.
  2) Represents what chemical reversal drugs will
  achieve for you
• Post Tetanic facilitation- release of large
  amount of ACh floods the NMJ and accentuates
  subsequent twitch responses

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Reversal of muscle relaxants

• Anti Cholinesterases ie.Neostigmine
• Cholinergic/(muscarinic) side effects
  Bradycardia
• Give with Anti-Cholinergic / (anti-muscarinic)

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Additional Readings

• Appliguide Neuromuscular monitoring, Datex-Ohmeda
  pp.1-21, 31-35. Available in Anesthesia Office.

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REVIEW

• How are impulses conducted along the axon?
• How are impulses conducted to the motor cells?
• What is the resting phase of the motor neuron
• Where is most potassium in the body?
• In what part of the peripheral nervous system
  designations do we find the majority of motor
  neurons?
• What compartment do muscle relaxants work in?
• Explain the anatomy and function of an
  acetylcholine receptor
• How do muscle relaxants effect ACh receptors
• What is the duration of ACh?
• How is ACh metabolized
• Why do we need to paralyze muscles during
  surgery?
• What is a nerve stimulator/monitor?
• How does a nerve stimulator work?
• What is Ohms law and how does it apply to nerve
  monitoring?
• What is a Hertz?
• What is Fade?
• Which muscle relaxants cause Fade? Which do not?
• What is a Twitch,Train of Four, Double Burst
  Stimulation,Tetanus and what do these represent?
• What degree of paralysis is needed before we see
  any appreciable fade on train of four?