3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.7. Neuromuscular blocking drugs
 3.1.3.7.4. Neuromuscular monitoring 

Neuromuscular monitoring

[RDM6:p1552-p1555; CEACCP 2004 Vol 4(1) "Pharmacology of neuromuscular blocking drugs"; SH4:p216-219]

 

[To be added later...

What are the common sites for monitoring

 

Ulnar

Red (positive) lead is placed on the proximal point.

Principles of peripheral nerve stimulation

  • Response of a single muscle fibre
    = All-or-none pattern
  • Response of a muscle
    = Variable depending on the stimulation and the number of fibres activated
  • Reduction in the muscle response to a supramaximal stimulus is proportional to the degree of blockade
    --> Supramaximal stimulus is usually at least 20-25% above what is neccessary for a maximal response

Fade

  • Normally ACh also act on prejunctional nAChRs
    --> ACh mobilised within the prejunctional terminal for immediate release
    --> Increase its own release during high frequency stimulation (>2Hz) (i.e. positive-feedback)
  • Non-depolarising NMBDs block the prejunctional nAChRs
    --> Impairs mobilisation
  • Fade response to continuous stimulation (50-100Hz) or train-of-four stimulation (TOF) reflect antagonism at the presynaptic receptors

Patterns of nerve stimulation

  • Single-twitch stimulation
  • Train-of-four stimulation (TOF)
  • Tetanic stimulation
  • Post-tetanic count stimulation
  • Double-burst stimulation (DBS)

Single-twitch stimulation

  • Supramaximal stimuli applied at frequencies ranging from 0.1 - 1Hz
    * 0.1Hz is usually used
  • Response to single-twitch stimulation depends on frequency
    * At frequencies above 0.15Hz, the response will gradually decrease and settle at a lower level
    * Results obtained using different frequencies are not comparable
  • The strength of response to single-twitch stimulation is the same for both depolarising and non-depolarising blockade (partial blockade)

Train-of-four stimulation

  • Four supramaximal stimuli are given every 0.5 seconds (2Hz)
    * If given repeatedly, usually each set (of TOF) are repeated every 10 - 20 seconds
  • TOF ratio (fade)
    = Amplitude of the fourth response divided by amplitude of the first response
  • During a partial non-depolarising block, the ratio is inversely proportional to the degree of the blockade
    * i.e. low ratio = higher degree of blockade
  • During a partial depolarising block, there is no fade
    --> TOF ratio is approximately 1.0
  • No response if the blockade was complete (depolarising or non-depolarising)

Advantage of TOF

  • Degree of block can be read without a preoperative value
  • Less painful
  • Does not affect the degree of NMJ blockade

Disadvantage of TOF

  • Can be hard to read manually

Tetanic stimulation

  • Very rapid stimulation
    * Usually 50Hz stimulation given for 5 seconds
  • During normal NMJ transmission or partial depolarising NMJ blockade
    --> A sustained response (no fade)
  • During a partial non-depolarising NMJ block (or a phase II block)
    --> Fade occurs

Disadvantage

  • Very painful
  • May produce a lasting antagonism of NMJ blockade
    --> The response of the tested muscle is no longer representative of other muscle groups

Post-tetanic facilitation

  • Post-tetanic facilitation of NMJ transmission refers to the increase in twitch tension after tetanic stimulation during a partial nondepolarising blockade
  • Caused by increase in mobilisation and synthesis of ACh after tetanic stimulation
  • Disappears after about 60 seconds of tetanic stimulation

Post-tetanic twitch potentiation

  • Different from post-tetanic facilitation
  • A muscular phenomenon
    * Not accompanied by an increase in the compound muscle action potential
  • Occurs before NMBDs were given

Post-tetanic count stimulation

  • Tetanic stimulation (50Hz for 5 seconds)
    --> Followed by single-twitch stimulation given at 1Hz starting 3 seconds afterwards
  • During very intense blockade, there will be no response to any stimulation
  • When the very intense blockade wears off, this method may elicit response before the first response to TOF occurs
  • Used when the NMJ blockade is still very strong and there is no response to TOF or single twitch stimulation
  • Due to possible antagonism of NMJ blockade, this should not be done more often than every 6 minutes

Double-burst stimulation (DBS)

  • DBS consists of:
    * Two bursts of tetanic stimulation, separated by 750 msec
    * Each burst consist of 3 impulses (duration 0.2 msec) at 20 msec intervals (50Hz)
  • In normal NMJ transmission, the two responses are of equal strength
  • In partial blockade, the second response is weaker than first
  • DBS ratio correlates closely to TOF ratio
    * But may be easier to evaluate manually

Sites of nerve stimulation

  • Most commonly to ulnar nerve --> contraction of the adductor pollicis muscle
  • Facial nerve stimulation --> contraction of orbicularis oculi muscle

Indicators of NMJ recovery

[??? reference]

  • TOF ratio of >0.9 --> Less risk of aspiration
  • Other indicators include:
    * Grip strength
    * Ability to sustain head lift
    * Vital capacity measurement
    * Generation of negative inspiratory pressure