Atracurium is unique in that it is altered spontaneously in the body to an inactive form (t]/2 30 min) by a passive chemical process (Hofmann elimination). The duration of action (15-35 min) is thus uninfluenced by the state of the circulation, the liver or the kidneys, a significant advantage in patients with hepatic or renal disease and in the aged. It has very little direct effect on the cardiovascular system but at doses of greater than 0.5-0.6 mg/kg histamine release may cause hypotension and bronchospasm.
Cisatracurium is a stereoisomer of atracurium; it is less prone to cause histamine release.
Vecuronium is a synthetic steroid derivative that produces full neuromuscular blockade about 3 minutes after a dose of 0.1 mg/kg. After this dose, its duration of action is 20-30 minutes. It has no cardiovascular side-effects and does not cause histamine release.
Rocuronium is another steroid derivative that has the advantage of a rapid onset of action. After a dose of 0.6 mg/kg tracheal intubation can be achieved after 60 seconds. It has negligible cardiovascular effects and has a similar duration of action to vecuronium.
Mivacurium belongs to the same chemical family as atracurium. It is the only nondepolarisng neuromuscular blocker that is metabolised by plasma cholinesterase. It is comparatively short acting (10-15 minutes), depending on the initial dose. Mivacurium can cause some hypotension because of histamine release.
Pancuronium was the first steroid-derived neuromuscular blocker in clinical use. It is longer acting than vecuronium and causes a slight tachycardia.
Tubocurarine is obsolete and is no longer available in the UK. It is a potent antagonist at autonomic ganglia and causes significant hypotension.
Antagonism of competitive neuromuscular block: neostigmine
The action of competitive acetylcholine blockers is antagonised by anticholinesterase drugs, which allow accumulation of acetylcholine. Neostigmine (p. 437) is given intravenously, mixed with gly-copyrronium to prevent bradycardia caused by the parasympathetic autonomic effects of the neostigmine. It acts in 4 minutes and lasts for about 30 minutes. Too much neostigmine can cause neuromuscular block by depolarisation, which will cause confusion unless there have been some signs of recovery before neostigmine is given. Progress can be monitored with a nerve stimulator.
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