2021A Question 9

Explain the onset and offset of neuromuscular block at the diaphragm, larynx and adductor pollicis after an intubating dose of vecuronium. What are the clinical implications of these differences?

Examiner Report

29.5% of candidates achieved a pass in this question.

The major domains assessed in this question were:

  • The definition of an intubating dose of vecuronium
  • How the specified muscles differ in terms of fibre types and the sensitivity of these types to non-depolarizing muscle relaxants
  • The relative onset and offset of blockade at the different muscle groups and the factors that accounted for these differences
  • The clinical implications of these differences for intubating and extubating

Accurate graphs demonstrating the relative speed of onset and offset of block at peripheral vs central muscles, supplemented with an explanation were given credit.

Common problems included:

  • Ranking muscle groups by speed of onset or offset without further explanation
  • Listing factors affecting speed of onset or offset without explanation
  • Addressing only the onset of blockade; both onset and offset needed to be addressed to pass
  • Only addressing the clinical implications of differences in onset (timing of intubation) or offset (timing of extubation) but not both

Detailed explanation of the mechanism of neuromuscular blockade for all muscle groups did not explain the differences in speed of onset between different muscle groups and therefore did not attract marks.

Model Answer


  • Introduction
  • Table
  • Graph
  • Implications


Term Detail

- ED95: Dose that produces 95% depression of single twitch height in adductor pollicis

- 2 x ED95: Intubating dose


Adductor pollicis vs diaphragm/larynx:

- Slower onset

- Greater peak effect

- Slower offset


Effect Detail

Diaphragm and Larynx (D&L)

Corrugator supercilii (CS)

Adductor pollicis (AP)
Blood flow
ACh Release
AChR Density
AChE Activity
Factor Detail
Clinical Effect

Diaphragm and Larynx (D&L)

Corrugator supercilii (CS)

Adductor pollicis (AP)
Onset speed ↑ (25% faster)
Peak effect
Offset speed ↑ (33-50% faster)


Factor CS AP
Where to look Medial eyebrow (not orbicularis oculi) Thumb & forefinger (not hypothenar eminence)
Induction Intubate at TOF count = 0 Intubate at TOF count = 1 (?)
Maintenance (if immobility essential) Titrate against TOF count = 0 or by infusion Titrate against PTC or by infusion
Emergence Ahead of D&L. TOF count by inspection.

→ Risk of residual curarisation

Lag behind D&L. TOF ratio by accelerometry.

→ Reversal at TOF count 4

→ Extubate at TOF ratio >0.9

Last updated 2021-08-23

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