Pharmacology for Anaesthesia and Intensive Care

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11 Muscle relaxants and anticholinesterases

Suxamethonium

Succinylmonocholine Choline

Plasma cholinesterase

Plasma cholinesterase

Succinic acid Choline

Figure 11.5.Metabolism of suxamethonium.

Other effects

Apart from its useful effects at the NMJ, suxamethonium has many other effects all

of which are detrimental:


Arrhythmias – sinus or nodal bradycardia, and ventricular arrhythmias can occur

following suxamethonium, via stimulation of the muscarinic receptors in the sinus

node. The bradycardia is often more severe after a second dose but may be pre-

vented by atropine. This phenomenon is often more pronounced in children.


Hyperkalaemia–asmall rise in serum K+is expected following suxamethonium

in the normal subject as depolarization involves K+efflux into extracellular fluid.

Patients with burns (of >10%) and neuromuscular disorders are susceptible to a

sudden release of K+,which may be large enough to provoke cardiac arrest. Burn

patients are at risk from about 24 hours after injury and for up to 18 months. Extra-

junctional ACh receptors (which contain a fetalγsubunit in place of an adultsub-

unit) proliferate over the surface of the muscle, and when activated release K+into

the circulation. Patients with paraplegia, progressive muscle disease or trauma-

induced immobility are at risk via a similar mechanism. The period of particular

risk in those with paraplegia is during the first 6 months but it continues in those

with progressive muscle disease, becoming more severe as more muscle is involved.

Those with renal failure are not at increased risk of a sudden hyperkalaemic

response to suxamethonium per se. However, serum K+may be grossly deranged

in acute renal failure leading to an increased risk of arrhythmias.


Myalgia – muscle pains are commonest in young females mobilizing rapidly in

the post-operative period. Pre-treatment with a small dose of non-depolarizing

muscle relaxant (e.g. gallamine), diazepam or dantrolene have all been used with

limited success in an attempt to reduce this unpleasant side effect.


Intra-occular pressure (IOP) – is raised by about 10 mmHg for a matter of minutes

following suxamethonium (normal range 10–15 mmHg) and is significant in the

presence of a globe perforation. However, concurrently administered thiopental

will offset this rise so that IOP remains static or may even fall. The mechanism by

which suxamethonium increases IOP has not been clearly defined, but it is known