Physiology and Monitoring Neuromuscular Blockade Flashcards
prevents muscle contraction by interfering with the transmission of an action potential from the nerve ending to the muscle
action of neuromuscular blockers
- facilitate intubation
- decrease muscle tone to provide appropriate operating conditions
- alleviate muscle activity with ECT
- allow balance anesthesia w/o pt movement
- assist in controlled vent pts
uses of neuromuscular blockers
- impulse arrives at the motor nerve terminal
- Ca influx release ACh into cleft (synapse)
- ACh diffuses across cleft to nicotinic receptor
- ACh binds with alpha sites on postsynaptic receptor causing ion channel to open
- Na and K ions move across channel causing depolarization
- action potential spreads over surface of muscle fibers causing contraction
- ACh diffuses away from end plate region or is metabolized by acetylcholinesterase (AChE)
normal neuromuscular function
release of acetylcholine is ___ dependent and triggered by increases in concentrations of free ___ ions in nerve terminals
Calcium
opens calcium ion channels
cyclic adenosine monophosphate (cAMP)
what is the primary neurotransmitter?
acetylcholine
the principle site of action of neuromuscular blocking agents
“site of effect”
neuromuscular junction
what inhibits release of ACh
magnesium
synthesized in the motor nerve ending by acetylation of choline which is controlled by choline acetylase enzyme
acetylcholine
what is ACh rapidly (<15 ms) hydrolyzed by turning it into choline and acetic acid where choline is taken back into nerve ending to be used to synthesize more ACh
acetylcholinesterase (AChE)
- on nerve ending
- affects neurotransmitter release
- ion channel allow flow of Na
- activation mobilizes additional ACh for release
ACh presynaptic (prejunctional) receptors
blockade of presynaptic (prejunctional) receptors causes a __ in the release of ACh
decrease
- similar to those found in fetus
- receptors found throughout muscle cell, as call matures, less are formed
- not involved in neuromuscular transmission
- sensitive to agonists AND channels remain open 4x longer (hyperkalemia)
- creating supressed by nerve ending activity
extrajunctional (perijunctional) receptors
occurs if the muscle is damaged, diseased or denervated (burns, paralysis, stroke, muscular dystrophies, immobilization)
proliferation of extrajunctional receptors
[means there are more places NMB can attach to but will not have same effect]
- in junctional folds of muscle membrane
- made of 5 linear protein subunits (2 alpha, beta, delta, and epsilon)
- form a channel for flow of Na, K, Ca
postsynaptic receptors
- ACh binds to extracellular sites on alpha subunits
- channel opens
- Ca and Na flow in, K out
- depolarization occurs
- muscle contraction
postsynaptic receptor events
attaches to both alpha subunit sites and mimics ACh and causes depolarization
depolarizing blockers
attaches to one alpha subunit to prevent ACh from binding and thus prevents depolarization
nondepolarizing blocks
can occur with antibiotics (mycins), quinidine, tricyclic antidepressants, and naloxone
physically blocks an open channel or a closed channel around the extracellular entrance
closed channel blockade
time from administration to max effect
onset time
time from admin to 25% recovery of twitch response (1/4 TOF)
clinical duration
time from admin to 90% recovery of twitch respone
total duration
time from 25% to 75% recovery of twitch responses
recovery index
the dose needed to produce 95% of suppression of single twitch response
ED 95
- titration of dosage of NMB to desired effect
- monitor for unusual resistance or sensitivity
- evaluate reversibility
- determine recovery from block
objectives of clinical monitoring
how do we know prejunctional receptor activity?
TOF monitoring
how long do you do the tetunus
5 seconds is standard up to 10
how do you monitor NMB?
by electrical stimulation of the peripheral motor nerve to observe the muscular contractions in response
Two most common locations to check TOF?
ulnar nerve and facial nerve
- located lateral to the flexor carpi ulnaris tendon and medial to the ulnar artery
- when stimulated causes adduction of thumb via adductor pollicis brevis muscle
ulnar nerve
located behind medial maleolus of the tibia and posteromedial to the pt artery
-causes plantar flexion of big toe when stimulated
posterior tibial nerve
- located behind the head of the fibula and around neck of fibula
- foot will dorsiflex
lateral popliteal or peroneal nerve
lies within the parotid gland after it emerges from the stylomastoid foramen
facial nerve
[place electrodes close to the tragus of the ear]
stimulation of facial nerve causes response in
orbicularis oculi
greater density of ACh receptors causes less dense block and more rapid recovery
orbicularis oculi
onset time
- small rapidly moving muscles (eyes, fingers)
- trunk, abdominal muscles, long muscles
- intercostal and diaphragm
recovery time
- diaphragm first to recover
- rapidly moving muscles
- long muscles
- inability to focus vision, keep eyelids open, double vision
- inability to swallow
- inability to phonate
- hearing acuity intensified
first signs of relaxation
the delivery of four stimuli at a frequency of 2 Hz (4 stimuli in 2 seconds)
train of four (TOF)
technique relies on the reduction of ACh release with rapid rates of stimulation
must wait 10 seconds before repeat
TOF
TOF Recovery of 2/4 means ___ receptors blocked
90%
TOF recovery 3/4 means ___ receptors blocked
80%
TOF recovery of 4/4 means __ receptors blocked
70-75% (still potentially clinically blocked)
clinical relaxation requires what % blocked
75-90%
difficult to detect TOF fade when ratio is __ or >
0.4
if no fade, only 50% chance that true TOF is >__
0.6
[pt could still have signs of residual block post extubation]
all 4 twitches are reduced, will not see a fade
depolarizing block
TOF ratio decreases or fades and is inversely proportionate to the degree of of block
[if TOF 4/4 less blocked then 1/4]
nondepolarizing block
when administering SCh, fade occurs
phase II block
two short bursts of 3 stimuli at frequency of 50 Hz
prob wont do clinically
easier to detect fade
double burst
- stimulation of 50 Hz to 100 Hz
- used to assess recovery
- wait 10 mins between assessments
tetanic stimulation, or tetanus
what is appropriate TOF to give reversal
1/4
consists of 3 sec 50 Hz tetanus, 3 sec pause, twitch stimuli at 1 Hz
can determine time it will take to get to 1/4 TOF
[would not do if you had 1/4> reading TOF]
posttetanic twitch
how many posttetanic twitches coincide with 1/4 TOF
10
requires baseline before drug admin, generally used as qualitative and not quantitative
single twitch
reflects blockage from 70-100%, useful during onset, maintenance, and emergence but really maintenance best
TOF
should be used sparingly for deep block assessment
tetanus
used only when TOF or double burst stim response is absent, count <8 indicated deep block and prolonged recovery
posttetanic
adequate reversal may take as long as 30 mins
1/4 TOF
recovery may take 4-12 mins
2/4 or 3/4 twitches
