29. Neuromuscular Blockade Monitoring Flashcards
Whys it important
It is important to know what degree of neuromuscular blockade is present in our patients so that we can manage the various stages of anaesthesia from tracheal intubation, muscle relaxation to facilitate safe surgery in the best possible conditions, the return of adequate spontaneous ventilation through to extubation.
Monitoring neuromuscular blockade involves two steps:
Monitoring neuromuscular blockade involves two steps:
1 Stimulation of a motor nerve
2 Assessment of the muscular response
Stimulation of a motor nerve is done
clinically in two ways:
(i) Needle electrodes,
e. g. Stimuplex needle,
inserted into the tissue near the nerve.
This method can be used to identify
nerves to block during regional anaesthesia,
although it is less commonly used now
because ultrasound-guided
regional anaesthesia has become popular.
Here, the current applied to the nerve is
low amplitude
(1–3 mA to locate the general area
of the nerve, reducing to 0.2–0.5 mA
as the needle is brought nearer to the nerve prior to injection of local anaesthetic.)
(ii) Skin electrodes placed over a
peripheral nerve, which deliver a
supramaximal current (50 mA)
to ensure recruitment of all muscle units.
After nerve stimulation,
assessment of the resulting
motor response is made.
This assessment can be:
1 Visual/tactile:
1 Visual/tactile:
the anaesthetist observes
‘twitches’ and feels for muscle movement,
e.g. holding the twitching thumb
during ulnar nerve stimulation.
- Advantages – simple, convenient, cheap
- Disadvantages – prone to interpretation error, inaccurate, crude
2 Mecahnomyography
3 Aceleromyogaphy
4 Electromyography
2 Mechanomyography:
2 Mechanomyography:
a small weight is hung from the muscle
to maintain isometric contraction.
A strain gauge measures tension generated
in the muscle following stimulation
and converts it into an electrical signal.
The tension generated is proportional to
the force of contraction and
so inversely proportional to the
degree of neuromuscular blockade.
• Advantages – more accurate than visual monitoring
• Disadvantages – hand must be splinted and stable, fiddly and inconvenient.
Used mainly in research
3 Acceleromyography:
3 Acceleromyography:
a transducer using piezoelectric crystals
is secured to the end of a digit.
The digit moves following stimulation
and its acceleration is
proportional to the force of muscle contraction
and so inversely proportional to the
degree of neuromuscular blockade.
- Advantages – more accurate than visual monitoring
- Disadvantages – hand must be splinted and stable, fiddly and inconvenient
4 Electromyography
a skin or needle electrode is
placed over the adductor pollicis
(this is the most commonly used muscle).
The ulnar nerve is
stimulated and the electrodes
record the magnitude of the compound
muscle action potentials generated as a consequence.
• Advantages –
more accurate than visual monitoring.
Avoids the problems of position
and calibrating transducers attached to joints
• Disadvantages –
even small movements of the hand
can alter the
response of the electrode
Tell me about the stimulation patterns used
in neuromuscular monitoring.
It is easy to become confused
by the seeming array of numbers and
patterns that have to be learnt.
In fact, this question is very simple, because all twitches that we deliver in theatre with the nerve stimulators are uniform and so you only have to learn a couple of facts.
• All twitches are delivered at 50 mA (i.e. supramaximal stimulus)
• All twitches last 0.2 ms
Once you have these facts,
the rest becomes easier to remember.
In theatre, we mainly use visual
and tactile assessment of the degree of
neuromuscular blockade because
of the relative convenience of this method.
Several different patterns of
stimulation have been developed to try to
improve the sensitivity of monitoring.
These are:
- Single twitch
- Train of four (TOF)
- Tetanic stimulation
- Post-tetanic count
- Double burst stimulation (DBS)
Single twitch
Single twitch
• Twitch current 50 mA
- Duration of twitch 0.2 ms
- Frequency of twitches 1 Hz (i.e. 1 every second)
• Number of twitches:
as many as operator chooses to give
This is the simplest stimulation pattern.
At a frequency of 1 twitch per
second, there is time for complete
recovery of the muscle units between
each stimulation.
This means that we will not see
‘fade’ during single-twitch stimulation.
Single twitch should be used before and after the administration of neuromuscular blocking drugs (NMBDs) to assess the degree of receptor occupancy by comparing the height of the twitch before drug administration and after.
When 75% of the ACh receptors are occupied, the twice height starts to reduce; when 100% are occupied, there are no twitches at all.
Single twitch can be used to assess block in depolarising NMBD, i.e.
suxamethonium, where fade and post-tetanic facilitation do not occur (see
below).
Train of four (TOF)
Train of four (TOF)
- Twitch current 50 mA
- Duration of twitch 0.2 ms
• Frequency of twitches 2 Hz
(i.e. 2 every second)
• Number of twitches:
4
In the TOF, the ratio of twitch height
T4:T1 indicates the degree of receptor
occupancy by the NMBD.
Twitches height may be reduced or absent, and the disappearance of T4 = 75% occupancy, T3 = 80%, T2 + 90%, T1 = 100%.
This phenomenon is called ‘fade’ (see below)
Fig. 78.4 Train of four – smaller amount of NMBD at receptor
Smaller amount NMBD present
at receptors
Fade in TOF
Fig. 78.5 Train of four – larger amount of NMBD at receptor Larger amount NMBD present at receptors Fade and loss of 4th twitch
Accepted values for TOF:
• 1 twitch for tracheal intubation
• 1–2 twitches during surgery
• 3–4 twitches before attempting reversal with anticholinesterases
Tetanic stimulation
- Twitch current 50 mA
- Duration of twitch 0.2 ms
• Frequency of twitches 50 Hz
(i.e. 50 every second)
• Number of twitches:
stimulation lasts 5 seconds =
5 × 50 = 250 twitches
If neuromuscular block is present,
tetanic stimulation will
demonstrate fade inversely proportional
to the percentage receptor occupation.
Tetanic stimulation is applied before the tetanic count (see below).
It is used to assess when there is profound block such that there are no twitches on TOF. Tetanic stimulation is extremely painful in an awake patient and may leave an unpleasant sensation in those who were anaesthetised.
Post-tetanic count (PTC)
Post-tetanic count (PTC)
• Twitch current 50 mA • Duration of twitch 0.2 ms • Frequency of twitches 1 Hz (i.e. 1 every second) • Number of twitches: as many as operator chooses to give
The stimulation pattern here is
identical to the single twitch.
Following tetanic stimulation,
post-tetanic facilitation mobilises presynaptic ACh, making it available to produce contractions in response to the current applied in the post-tetanic count (PTC).
The number of twitches is
possible to produce is inversely
proportional to the degree of receptor blockade.
- PTC < 5 = profound block
- PTC >15 = equivalent to two twitches on TOF
Double burst stimulation (DBS)
Double burst stimulation (DBS)
- Twitch current 50 mA
- Duration of twitch 0.2 ms
• Frequency of twitches 50 Hz
(i.e. 50 every second)
• Number of twitches:
3 twitches – break of 750 ms –
3 more twitches
Double burst stimulation was
developed to try to improve our
ability to detect fade clinically,
as the stimulation yields only
two muscle contractions.
It is easier for us to compare the heights of
T1:T2 than detecting fade over four
twitches.
What is fade? ‘
Fade’ describes the phenomenon of
decreasing twitch height when
competitive neuromuscular blocking drugs
are present at the neuromuscular junction (NMJ).
ACh is stored in vesicles in the terminal button at the NMJ. Each vesicle contains over
10 000 ACh molecules.
Eighty per cent of the vesicles are readily releasable,
while 20% form a stationary store.
At the normal NMJ, the arrival of an action potential of sufficient magnitude will cause >100 vesicles to fuse with the pre-synaptic membrane and discharge their ACh into the synaptic cleft.
The ACh diffuses across the cleft,
binds to post-synaptic ACh receptors
and triggers the muscle AP, which causes contraction.
In addition to the post-synaptic ACh receptors,
there are pre-junctional receptors found
on the terminal button.
These too are stimulated by the release of ACh and,
in the face of repeated APs,
they have a positive feedback role by stimulating
an increase in ACh production by second messenger systems.
This helps to prevent the muscle fatigue
with prolonged stimulation.
When molecules of NMBD are
bound to the post-synaptic ACh receptors,
it leaves fewer for the ACh to bind to.
There is a large safely margin at
the NMJ and there will be no
discernible weakness until 75% of receptor
sites are occupied.
Fade occurs when there is sufficient NMBD
present to compete significantly with
binding of ACh at both the
pre and post-synaptic receptors.
Binding of the drug to the
pre-synaptic receptors prevents the
positive feedback mechanism,
which results in increased production of ACh.
Consequently, a decreasing amount of
ACh is released with each stimulation
and this is reflected in a decreasing
twitch height: so called ‘fade’.
What is post-tetanic potentiation?
What is post-tetanic potentiation?
Tetanic stimulation is a
supra-maximal stimulation,
applied to the NMJ for
a prolonged period of time.
It is sufficient to produce a
substantial increase in ACh release,
enough to overcome competition from
NMBD in all but the most profound of blocks.
The positive feedback mechanism described
above is activated and this increases the amount of ACh available for release.
This is called post-tetanic potentiation.
What are phase I and phase II blocks?
These terms refer to the blocks seen
following the administration of suxamethonium.
A phase I block describes the block
seen following the administration of a
single dose of suxamethonium.
Suxamethonium binds to the ACh receptor,
which causes opening of the sodium channel
and membrane depolarisation.
This results in disorganised muscle contraction, seen as fasciculation followed by flaccid paralysis because suxamethonium causes prolonged depolarisation of the motor end plate
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