L3i: Describe the monitoring of neuromuscular blockers

Defintion

Monitoring equipment which produces a direct current of specific AMPLITUDE, DUATION & FREQUENCY to cause depolarisation of nerves

Use

  • Intubation – ensure optimum intubating conditions
  • Extubation – known when to give reversal to assist with extubation

Residual NMB is common → 45% PACU patients have PORC after single intubation dose of intermediate acting MR

→ No ↑mortality, but ↑resp morbidity

  • Assessing NMB → Clinical/PNS

Clinically

Awake

Sustained head lift 5 sec

Hand grip

VT > 15mL/kg

Asleep

Bag-mask ventilation

Compliance/VT

Muscle tone (surgeon)

→ CRUDE assessment → influenced by many factors

→ Unreliable

Peripheral Nerve Stimulator

Generate a supramaximal stimulus (up to 80mA) to a motor nerve, close to skin, visible to contraction/accessible to evoked response monitoring

↓

All fibres need to be stimulated to produce maximal m. contraction (hence supramaximal stimulus)

Ideal Nerve Stimulator

  • Constant current
  • current up to 80mA
  • Polarity clearly marked
  • Twitch frequency 0.1 & 7.0Hz
  • Capable of variety of stimuli patterns
  • Light & portable
  • Battery operated with visible battery check

How does it work?

  • Negative electrode sits over most superficial part of nerve
  • Positive electrode sits where it cannot affect muscle
  • Energy delivered in form of SUPRAMAXIMAL STIMULUS
  • Causes an ↑membrane potential above threshold
  • Depolarisation + AP propagation of motor nerve (all fibres need to be stimulated to produce maximal m. contraction)
  • Contracting muscle must be clearly visible
    • Ulnar n. → adductor pollicis longus
    • Facial n. -> Oubicularis muscle
  • Different muscle groups have different sensitivities
    • DIAPH → least sensitive (good BF ∴ quick onset/offset)
    • HAND → most sensitive to MR (last to offset)
    • LARYNX/PHARYNX → @onset like diaphragm but v. sensitive to drugs ∴offset like hand/face muscles

@ Induction

  • Larynx, jaw, diaphragm paralysed
  • Orbicularis m. best correlates
  • Single twitch allows identification of SUPRAMAXIMAL STIMULUS
  • When To4 diappears → optimal intubating conditions

@ Maintenance

  • PTC & TO4 best for profound block monitoring

@ Extubation

  • Before giving AChE inhibitor TO4 ≥ 3
  • Peripheral m. (adductor pollicis) best to assess

Advantage

Inexpensive

Portable

Simple to use

Disadvantage

Interpretation of muscle twitches is subjective

  • Different patterns of monitoring
  • Skin has high resistance → ∴currents of 40 – 60mA are required

Single Twitch

Method

  • 1 supramaximal stimulus of 1Hz (2 twitch/sec) for 0.1 – 0.2msec
  • Prior to any MR being given
  • Then MR is given
  • @75% mAChR occupancy to muscle twitch amplitude diminishes
  • As you can identify @ what level SUPRAMAXIMAL STIMULUS is obtained

Use

    • @ onset of NMB to know when supramaximal stimulus is obtained

Disadvantages

  • Not useful to assess recovery
  • Need to give a control twitch prior to gaining MR

PTC

Method

    • 5 second tetanic stimulus @50Hz (pain!)
    • Pause 3 sec
    • Then 20 x single twitches @ 1Hz
    • The no. of muscle twitches in response to 20 pulses are counted
    • Predicts how long NMB will last
    • In profound block → no muscle twitch
    • But early in recovery stages you will get a muscle twitch before TO4 appears
    • Because after tetanus → ACh is synthesised + mobilised still for a few seconds
    • PTC = 12 – 15 → return of TO4 imminent
      • PTC = 9 = TO4 1
      • PTC = 0 = Retinal Surgery ready

Disadvantage

  • Cannot repeat for 6 mins (degree of block will be underestimated because you’ve mobilised ACh)

TO4

Method

    • 4 supramaximal stimuli 30 seconds apart
    • Response shows receptor occupancy

Twitch

0

1

2

3

4

% Receptors Occupied

100%

95%

90%

80%

<75%

  • TO4 RATIO = T1/T4
    • TOFR = 0.7 = diaphragm recovered
    • TOFR = 0.9 = pharyngeal m. recovered

Advantage

Commonly used → ∴ more experience

TOF ≥ 3 = give reversal

TOFR 0.9 → safe to extubate

Disadvantage

Larger inter-observer error assessing TOFR

Not good to monitor deep block

DBS

Method

  • 2 supramaximal bursts of tetany
  • 750ms apart
  • The fade that occurs between the 2 bursts is easier to see than with a single tetanic stimulus

Advantage

Better accuracy for observer cf. TO4

Easier to appreciate small degrees of block

Disadvantage

More painful → good at assessing PDRC

 Too awake