Muscle relaxants in canine anaesthesia 1: History and the drugs

The history of the introduction of muscle relaxants into medical and veterinary anaesthesia is discussed. It is interesting to note that the compounds were used in animals for therapeutic purposes some 100 years before their introduction into human anaesthetic practice. The concept of the triad approach to anaesthesia by separate consideration of the three components of narcosis, reflex depression and muscle relaxation is considered. The properties of the depolarising and non-depolarising muscle relaxant drugs are described. The only depolarising relaxant which is in common use is suxamethonium which produces an initial depolarisation of the muscle end-plate which is accompanied by fasciculation of muscle and is non-reversible. There are a number of non-depolarising or competitive muscle relaxants which are available for use in the dog. They are reversible by anticholinesterase drugs and are potentiated by volatile anaesthetic agents. The properties of the more recently introduced compounds atracurium besylate and vecuronium bromide are considered in the light of their differences when compared with the older compounds.