what is Suxamethonium chloride ?

Suxamethonium chloride is a nicotinic acetylcholine receptor antagonist, used to induce muscle relaxation and short-term paralysis, usually to facilitate tracheal intubation. Suxamethonium is sold under the trade names Anectine and Quelicin. It is used as a paralytic agent for euthanasia/immobilization of horses.
Its medical uses are limited to short-term muscle relaxation in anesthesia and intensive care, usually for facilitation of endotracheal intubation. Despite its adverse effects, including life threatening malignant hyperthermia, hyperkalaemia, and anaphylaxis, it is perennially popular in emergency medicine because it arguably has the fastest onset and shortest duration of action of all muscle relaxants. The former is a major point of consideration in the context of trauma care, where endotracheal intubation may need to be completed very quickly. The latter means that, should attempts at endotracheal intubation fail and the patient cannot be ventilated, there is a prospect for neuromuscular recovery and the onset of spontaneous breathing before hypoxaemia occurs.
Suxamethonium is also commonly used as the sole muscle relaxant during electroconvulsive therapy, favoured for its short duration of action.
Suxamethonium chloride is quickly degraded by plasma butyrylcholinesterase and the duration of effect is usually in the range of a few minutes. When plasma levels of butyrylcholinesterase are greatly diminished or an atypical form is present  paralysis may last much longer.