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.
    • VT > 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 (), 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 prejunctional 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.
Last updated 2017-09-21

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