Suxamethonium

A tracheal Intubation

  Suxamethonium chloride (otherwise known as succinyl chloride or “sux”) is a drug used to create short-term muscle paralysis. The drug can effectively stop the action of all skeletal muscles in the body in 30-60 seconds enabling doctors to perform a Tracheal Intubation (placing a flexible plastic tube into the trachea to maintain an open airway as shown in the diagram) without the patient struggling. The effect typically lasts 5-10 minutes meaning that the patient does not need to have artificial breathing for too long.

 

   Suxamethonium works by works by depolarising the cell membrane of the muscle fibres for much longer than normal (I’ll explain what that means soon). In normal muscle contraction the signal is sent by the movement of sodium and potassium ions through channels in the membrane. First a few sodium ions move into the cell through. This causes the charge inside the cell to increase from its normal slightly negative state. If the inside of the cell is positive enough then channels open and sodium ions move into the cell making the inside of the membrane even more positive and the outside more negative i.e. increasing the difference in charge. One sodium channel opening causes a chain reaction until all are open. When all of the sodium channels are open a different process starts. The sodium channels close and the potassium channels open and potassium ions leave the cell; this makes the inside of the cell more negative until the charge is about normal (there is a slight overshoot then readjustment to prevent the electrical signal going backwards).

   The increase in the charge of the cell from negative to positive (hence lowering the difference in charges between inside and out i.e. depolarisation) involves a molecule called acetylcholine bonding to the sodium channel causing it to open. The acetylcholine is then broken down in milliseconds by an acetylcholinesterase enzyme.

Suxamethonium works because it is made of two acetylcholine groups joined together. This means the acetylcholine ends can open the sodium channel but the molecule is two big to be broken down by the acetylcholinesterase enzyme. This means the channel is kept open and can only close when the suxamethonium molecule is eventually broken down by cholinesterase enzymes that are only present in small amounts and can therefore only act very slowly. This mean the impulse is stopped half way through and the signal cannot cause a contraction in the muscle.  

A suxamethonium chloride molecule

   Suxamethonium is not unique in this property. Various different molecules consisting of two acetylcholine groups were synthesised and have a similar effect. Suxamethonium was chosen because it started more quickly and stopped more slowly than other variations. The discovery and development of suxamethonium led to a Nobel Prize in medicine for Daniel Bovet in 1957.

   Suxamethonium is sometimes used as a student entertainment by participants injecting a small amount of it into each thigh and then trying to run as far as possible before the paralysis kicks in.