Depolarising NMJ blockers

Suxamethonium (succinylcholine USAN) is the only useful drug in this class. It is an acetylcholine analogue which binds to receptors and causes depolarisation (like acetylcholine). Suxamethonium dissociates from the receptors slowly leaving them in an inactivated state and unable to respond to acetylcholine. Its action ends when it diffuses out of the synapse. The end result is that the muscle fibre twitches once then relaxes.

Pharmacokinetics

acts in one circulation time; duration of action in most species except dog is about 2 - 3mins (dog 20 mins) but broken down by plasma (butyryl)cholinesterase not acetylcholinesterase. These are both inhibited by organophosphate insecticides (for fleas) in which case its duration may be >24 hrs.
does not cross the placenta

Indications

used in cats and dogs, (rarely pigs, horses and rabbits) as part of a crash induction technique for anaesthesia (relaxes larynx to allow intubation)
cats: useful for intubation to overcome laryngeal spasm
dogs: not used much - duration of action 20 mins (but remember OPs) - sometimes used for caesarian section
horses: not used much any more - some horses have an excitable induction with thiopentone - low dose suxamethonium given with thiopentone to block this; used to be used in Australia for restraint
pigs and rabbits are difficult to intubate without muscle relaxants
widely used in man

Contra - indications

•if no means of artificially ventilating animal is available
•if there is any doubt that the animal is unconscious
•organophosphate administration in last month

Precautions

The initial depolarisation causes muscle fasiculation which can damage muscle fibres. This may cause an increase in plasma K+ and CPK, and post operative muscle pain.
transient bradycardia may occur.
some breeds of rabbits are very sensitive to its effects
attempted reversal with anticholinesterases will prolong the block
may trigger malignant hyperthermia in susceptible pigs

Clinical use

The animal is given an induction dose of anaesthetic. When it loses consciousness, the suxamethonium is injected rapidly iv. The muscles fasiculate as the fibres are depolarised and then relax, allowing rapid intubation. If the animal is not intubated rapidly, it must be ventilated by mask. After intubation, the animal is ventilated with low doses of inhalation anaesthetic agent until the block wears off. If the block does not wear off, ventilation must be continued.

back

copyright Massey University