Suxamethonium — Clinical Use

Current · V5 (2025) → I3.i Historical · V4 (2023) → L2.i 2 exam appearances

2018A Q10

Exam question

Outline the advantages (15% of marks) and disadvantages (85% of marks) of the clinical use of suxamethonium.

CICMWrecks answer

Master answer

Suxamethonium is the dicholine ester of succinic acid which acts as an ultrashort acting depolarising muscle relaxant.
It is used in rapid sequence induction and to modify seizures caused by ECT.
Mechanism of Action:
- mimics the actions of Ach by binding to the nicotinic Ach receptor and causing membrane depolarization.
- However, because its hydrolyzing enzyme is not present at the NMJ, the effect lasts longer than for Ach.
- The persistent depolarization renders the voltage sensitive Na channels inactive within 1-2 mins.
- This prevents the transmission of further APs. Initially causes fasciculations, then muscle relaxation.

Partial agonists are drugs that bind to and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist
A partial agonist is unable to achieve a maximal biological response following active site binding, despite increasing doses

A non-competitive antagonist binds to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor
Since it induces a lower biological response on active site binding than acetylcholine, it is a non-competitive antagonist of acetylcholine at the nicotinic receptor
No matter how high levels of acetylcholine are at the neuromuscular junction, maximal response will not be reached

Effect	Advantage	Disadvantage
Pharmaceutics	Inexpensive	Needs to be stored at 4 degrees -Shelf life only 7 days when kept in resuscitation trolley
	Can be given peripherally
		Excretion of 10% of drug unchanged in urine means active metabolites may accumulate and prolong the effects.
		0.03% of population are abnormal metabolisers resulting in prolonged apnoea
Causes fasiculations	Offset of fasiculations indicates paralysis	Muscle pains
		Causes malignant hyperthermia
Prolonged suxemethonium apnoea		If intubation fails, prolonged apnoea
Some agonism at muscarinic receptor		Second dose of suxemethonium commonly causes bradycardia
Widespread neural depolarisation		Hyperkalaemia Raised intraocular pressure Raised GIT sphincter pressures
Rapid onset (30-60 seconds)	Short period of time before intubating conditions achieved – Less risk of desaturation / apnoea when intubating
Rapid offset (5-10 minuts)	If intubation fails, apnoea will not be prolonged	Muscle paralysis may disappear before airway secured or patient made safe
Metabolised by plasma cholinesterase	Metabolism not dependant on organ function	Plasma cholinesterase may be lower in some patients – myriad reasons

Mooney / JC 2020

Answer by JC, Mooney

Past papers

2 appearances

Exam	Exact exam wording	Candidate success
2018A Q10	Outline the advantages (15% of marks) and disadvantages (85% of marks) of the clinical use of suxamethonium.	46%
2012B Q02	Suxamethonium is a non-competitive partial agonist. Explain what is meant by this statement using definitions of the underlined terms (50% of marks). List the advantages and disadvantages of suxamethonium within Intensive Care practice (50% of marks).	59.1%