29. Neuromuscular Blockade Monitoring Flashcards

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1
Q

Whys it important

A
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.
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2
Q

Monitoring neuromuscular blockade involves two steps:

A

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.

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3
Q

After nerve stimulation,
assessment of the resulting
motor response is made.
This assessment can be:

1 Visual/tactile:

A

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

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4
Q

2 Mechanomyography:

A

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

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5
Q

3 Acceleromyography:

A

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
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6
Q

4 Electromyography

A

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

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7
Q

Tell me about the stimulation patterns used

in neuromuscular monitoring.

A

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)
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8
Q

Single twitch

A

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).

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9
Q

Train of four (TOF)

A

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

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10
Q

Tetanic stimulation

A
  • 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.
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11
Q

Post-tetanic count (PTC)

A

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
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12
Q

Double burst stimulation (DBS)

A

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.

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13
Q

What is fade? ‘

A

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’.

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14
Q

What is post-tetanic potentiation?

A

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.

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15
Q

What are phase I and phase II blocks?

A

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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16
Q

The characteristics of a phase I block:

A

The characteristics of a phase I block:

• Reduced twitch height,
but sustained response to tetanic stimulation

• No post-tetanic facilitation

• TOF ratio >70%
(height of fourth twitch to that of first).

This is a measure
of the pre-synaptic effect of suxamethonium.

The block is potentiated by the effect of anticholinesterases because these
will further decrease the rate of suxamethonium breakdown.

17
Q

A phase II block describes the block seen

A

A phase II block describes the block seen

following the repeated administration/infusion

of suxamethonium and can develop
with doses in excess of 2.5 mg/kg.

It occurs because in the continued presence
of suxamethonium,

the receptors eventually close
and the membrane repolarises,
at least partially.

However, it is now desensitised to ACh
and so cannot open again to propagate
an action potentials.

In this way, a phase II block is similar
to a non-depolarising block.

Phase II blocks are also called
‘desensitisation blocks’.

18
Q

Characteristics of a phase II block:

A
  • Exhibits fade on tetanic stimulation
  • Exhibits post-tetanic facilitation
  • TOF ratio < 0.3 (fourth to first twitch height)
  • Antagonised by anticholinesterases
  • Tachyphylaxis is seen with the need to increase suxamethonium infusion rate or bolus dose.
19
Q

Which nerves can we monitor clinically?

A

Commonly monitored nerves in theatre are:

• Facial nerve –
twitch of the eyebrow with
orbicularis oculi contraction

• Ulnar nerve –
twitch of the thumb with
adductor pollicis contraction

• Posterior tibial nerve –
twitch of the big toe with
flexor hallucis brevis contraction.

20
Q

Which muscles are affected first

by NMBDs?

A

NMBDs cause paralysis of all voluntary muscles
in the body,
but some are more sensitive than others.
In order of decreasing sensitivity:

  • Eyes (affected first)
  • Facial muscles
  • Neck
  • Extremities
  • Limbs
  • Abdominal muscles
  • Glottis
  • Intercostal muscles (affected last).

Muscle function returns in the reverse order.
This is why it is traditional to
wait until a patient can lift their head of the pillow before extubation.