Neuromuscular Monitoring

Describe the concept of depth of neuromuscular blockade and explain the use of neuromuscular monitoring

Describe the clinical features and management of inadequate reversal of neuromuscular blockade

The degree of neuromuscular blockade can be assessed:

• Clinically
  Crude compared to electrical assessment. Tests include:
  • Sustained head lift > 5 seconds
    Suggests < 30% blockade.
  • V_T > 10ml.kg^-1
  • Tongue protrusion
• Electrically
  Using a nerve stimulator. Can be:
  • Visual/tactile
    Monitoring of twitch height by anaesthetist.
  • Electrical
    Monitoring of twitch height by a device:
    • Accelerometer
      Acceleration is proportional to force for any given mass ($F = ma$), therefore an accelerometer taped to the thumb can be used to assess force of contraction.
    • Mechanical force transducers
      Muscle tension is measured using a strain gauge. Requires control prior to administration.
    • Electromyography
      EMG response is measured using electrodes over the muscle. The AUC of the response curve can be used to calculate degree of blockade.

Nerve Stimulator

A nerve stimulator:

• Consists of two electrodes, a power supply, and some buttons for control
• Produces a monophasic, square wave at constant current, lasting no more than 0.3ms
• Generates a supra-maximal stimulus
  Ensures every nerve fibre is depolarised, which means a consistently reproducible response will be generated. A supra-maximal stimulus is 25% greater than the maximum required to depolarise all nerve fibres.
• Allows assessment of different muscle groups
  Not all muscle groups are affected equally by neuromuscular blockade.
  • Typically smaller muscle groups are more sensitive
  • The positive (red) lead is placed proximal
  • Ulnar nerve
    Electrodes are placed along the ulnar border of the wrist at the flexor crease, and thumb adduction is assessed.
  • Facial nerve
    The positive electrode is placed at the outer canthus, and the negative electrode is placed anterior to the tragus. Eyebrow twitching is assessed.
  • Posterior tibial nerve
    Electrodes are placed posterior to the medial malleolus, and plantar flexion is assessed.

Stimulation Patterns

There are five common stimulation patterns:

• Train of Four
  Four single twitches (0.1ms) delivered at 2Hz (i.e. 1.5s for all 4).
  • Number of observed twitches gives an indication of receptor occupancy
    With increasing blockade, the amplitude and number of observed twitches decreases.
    • Fade is the reduction of twitch height with repeated stimuli during a partial neuromuscular block
      Occurs due to the effect of non-depolarising agents on the presynaptic membrane, reducing ACh production.
    • Number of observed twitches depends on the degree of blockade:
      • No twitches ≈ 100% blockade
      • One twitch ≈ 90% blockade
      • Two twitches ≈ 80% blockade
      • Three twitches ≈ 75% blockade
        Reversal agents should not be given with a ToF count < 3.
      • Four twitches ≈ < 75% blockade
  • The ratio of the amplitude of T₁ to T₄ (ToF ratio) can also be used as a measure of blockade:
    • ToF ratio > 90% is adequate for extubation
    • ToF ratio > 70% suggests adequate respiratory function
  • Should not be repeated faster than every 10s

• Tetanic stimulation
  High frequency (50-200Hz) supramaximal stimulus for 5 seconds.
  • Normal muscle will exhibit tetanic contraction
  • Partially paralysed muscle exhibits fade
    Degree of fade is proportional to degree of blockade, and is very sensitive.

• Post-tetanic count (PTC)
  Used in deep blockade when there is no response to ToF. A tetanic stimulus is given, followed 3s later by single twitches at 1Hz.
  • No response may be seen in very deep blockade
  • However, twitches may be seen prior to the return of a ToF response.
    This is called post-tetanic facilitation, and occurs due to the tetanic stimulus mobilising ACh vesicles into the pre-junctional area.
    • Typically, a ToF of 1 will occur when the PTC ≈ 9
  • Should not be repeated faster than every 6 minutes
    Due to residual post-tetanic potentiation.

• Double burst
  Two 0.2ms 50Hz (tetanic) stimuli are applied 750ms apart.
  • Two identical contractions occur in normal muscle
  • Amplitude of the second burst is reduced in partially paralysed muscle
    DB ratio is similar to the ToF ratio, but is easier to assess clinically.
  • A ratio > 0.9 is required for adequate reversal

• Single twitch
  A single stimulus lasting ~0.2ms is applied.
  • > 75% blockade causes a depressed response
  • A twitch must be assessed prior to blockade so a baseline can be established

References

1. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.
2. Saenz, AD. Peripheral Nerve Stimulator - Train of Four Monitoring. 2015. Medscape.
3. McGrath CD, Hunter JM. Monitoring of neuromuscular block. Continuing Education in Anaesthesia Critical Care & Pain, Volume 6, Issue 1, 1 February 2006, Pages 7–12.