Suxamethonium apnoea

Also known as:

What is it?
Apnoea means inability to breathe. Suxamethonium (succinylcholine) apnoea is rare and occurs when a patient has been given this muscle relaxant prior to surgery but is incapable of metabolising the drug sufficiently rapidly. They remain paralysed and unable to breathe after the surgery is complete because they cannot regain their muscle function quickly enough.

Uses
Suxamethonium is a fast acting muscle relaxant and its main use in anaesthesia is to allow the rapid placement of a breathing tube (intubation) in the trachea. It is also used to modify fits after ECT (electroconvulsive therapy).

Suxamethonium is useful in these situations because it acts and subsides quickly for the vast majority of patients lasting just long enough to meet the needs of the surgical procedure.

How it works
Suxamethonium relaxes skeletal muscle and is the fastest acting of all the available muscle relaxants and lasts for between 2 and 10 minutes. Further doses can be given as required. Part of the structure is similar to the neurotransmitter acetylcholine and as such it is able to prevent acetylcholine from binding at the neuromuscular junction. It binds tightly at the synaptic junction which means that it cannot be reversed by other drugs and as a result it wears off very slowly until the drug is eliminated by other routes.

Causes of suxamethonium apnoea
The majority of suxamethonium is metabolised by an enzyme called cholinesterase and about 10% is excreted by the kidneys. Suxamethonium apnoea is rare and can be either inherited or acquired. In the case of inherited suxamethonium apnoea the degree of sensitivity to this drug varies with the genetic makeup of the patient (see Tests section on cholinesterase).

Each genetic makeup (called a phenotype or genotype) expresses a particular form of the enzyme cholinesterase. Those most susceptible to apnoea are the AA, AS, AF, FF, FS, SS and to a variable degree, UA phenotypes.

The action of suxamethonium can be increased from 30 minutes to over two hours in these cases which means that the patient will not start breathing spontaneously again until the drug has worn off.

In the acquired form situations, such as pregnancy, liver and renal disease, hypothyroidism and if taking drugs such as methotrexate or monoamine oxidase inhibitors, the action of suxamethonium can be increased by minutes as opposed to hours.