2018B Question 04

Compare and contrast the pharmacology of suxamethonium and rocuronium.

Examiner Report

86% of candidates achieved a pass in this question.

This question is asking for the comparative pharmacology of two commonly encountered drugs in anaesthesia. A table format would be an ideal way to answer this question. A pass would require information on pharmaceutics (including classification and uses), pharmacokinetics and pharmacodynamics (including mechanism of action). The majority of candidates passed this question, but those that did well included discussion on the clinical relevance and implications of these differences.

Some areas of confusion and/or omission included: incorrect ED95s and dosing of rocuronium and suxamethonium; metabolism of both drugs into active or inactive metabolites; how they should be stored; omitting a discussion pharmaceutics altogether; and alternative routes of administration and why IM or intralingual administration might be useful for suxamethonium.

Model Answer


  • PC
  • PK
  • PD


Suxamethonium Rocuronium
Container Plastic ampoule Glass vial or ampoule
Contents: Drug and additives

Aqueous solution pH 3.5

(Precipitate with thiopentone pH 11)

suxamethonium chloride 50mg/mL

Aqueous solution pH 4

Precipitation rare

Rocuronium bromide 10mg/mL

Needs refrigeration Y Y

Life at room temp

(accept 10% loss of potency)

2/52 2/12
Needs reconstitution No No


Administration Suxamethonium Rocuronium
ED95 mg.kg-1 0.3 0.3
Intubating dose mg.kg-1 1-1.5 (4x) 0.6-1.2 (2-4x)



(Useful if larygospasm during paediatric gas induction without IVC)

Intralingual Y (as for IM) N
Phase 2 blockade mg.kg-1 3-5 N/A
Time course
Onset IV 2xED95 dose (mins)


(limited by cardiac output)

1-1.5 (≤1 if 4x ED95)
Duration IV (mins)

5 (normal)

(ideal for very short case e.g. ECT)


(Longer if 4x ED95)

Distribution Suxamethonium Rocuronium
VDSS 0.2L/kg 0.2L/kg
%PPB Unknown 10%
Crosses placenta Minimal Minimal
Metabolic Suxamethonium Rocuronium

PChE near 100%

Two stage

Products: Succinic acid, choline

Hepatic ≤5%
Active metabolites Nil 17-desacetyl-rocuronium (much less active, hepatic elimination)
t1/2β (mins) 1-2 70
Excretion Suxamethonium Rocuronium
Overall (minimal) (95%)
% In bile -


(liver failure → Prolonged)

% In urine

10% parent

100% metabolites


(renal failure → Prolonged)


Therapeutic Suxamethonium Rocuronium
Action at junctional nAChR

Depolarising blockade

Non-classical receptor effect

?Non-competitive partial agonist

Non-depolarising blockade

Competitive antagonist

Use Suxamethonium Rocuronium
Rapid sequence induction Y Y (↑ dose → ↑ Speed of onset but ↑ duration)
Surgical access e.g. Laparotomy Y (but usually too brief) Y
Infusion N (develop phase 2 block) Y (but offset variable)
Monitoring Suxamethonium Rocuronium
Single twitch
TOF ratio

Phase 1 >0.7

Phase 2 ≤0.7

Response to tetany

Phase 1 sustained

Phase 2 fade

Post-tetanic potentiation

Phase 1 yes

Phase 2 no

Side Effects Suxamethonium Rocuronium
Myalgia Y N

Sinus bradycardia

AV block

↑ K+


(contraindicated if ↑↑ extra-junctional receptors e.g. Burns, prolonged immobility, spinal cord injury)

Histamine release Y N
Anaphylaxis (arguments!!) 1 in 2000 1 in 2500
↑ Intragastric pressure +10cmH2O (but also ↑ lower oesophageal sphincter tone)
↑ Intra-ocular pressure +10cmH2O? (avoid if open globe injury)
Polymorphism → Prolonged paralysis


(avoid and test if FHx suxamethonium apnoea)

Malignant hyperthermia Y N
Reversal Suxamethonium Rocuronium
Spontaneous Y and fast (5min after 4xED95) Y but slow (30-40mins after 2xED95)
AChEi reversal

Phase 1 augmented

Phase 2 antagonised

Chelation reversal No g-cyclodextrin

Last updated 2021-08-23

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