A single depolarizing agent, succinylcholine, is in general clinical use; multiple competitive or nondepolarizing agents are available (Figure 9-2). Therapeutic selection should be based on achieving a pharmacokinetic profile consistent with the duration of the interventional procedure and minimizing cardiovascular compromise or other adverse effects (Table 9-1). Two general classifications are useful in distinguishing side effects and pharmacokinetic behavior. The first relates to the duration of drug action; these agents are categorized as long-, intermediate-, and short-acting. The persistent blockade and difficulty in complete reversal after surgery with D-tubocurarine, metocurine, pancuronium, and doxacurium led to the development of vecuronium and atracurium, agents of intermediate duration, followed by development of a short-acting agent, mivacurium. Often, the long-acting agents are the more potent, requiring the use of low concentrations. The necessity of administering potent agents in low concentrations delays their onset. Rocuronium is an agent of intermediate duration but of rapid onset and lower potency. Its rapid onset allows it to be used as an alternative to succinylcholine in rapid-induction anesthesia and in relaxing the laryngeal and jaw muscles to facilitate tracheal intubation.
The second useful designation is the chemical class of the agents (Table 9-1). The natural alkaloid D-tubocurarine and the semisynthetic alkaloid alcuronium seldom are used. Apart from a shorter duration of action, the newer agents exhibit greatly diminished frequency of side effects, chief of which are ganglionic blockade, block of vagal responses, and histamine release. The prototype ammonio steroid, pancuronium, induces virtually no histamine release; however, it blocks muscarinic receptors, and this antagonism is manifested primarily in vagal blockade and tachycardia. Tachycardia is eliminated in the newer ammonio steroids, vecuronium and rocuronium. The benzylisoquinolines lack vagolytic and ganglionic blocking actions but show a slight propensity for histamine release. Mivacurium is extremely sensitive to catalysis by cholinesterase or other plasma hydrolases, therein accounting for its short duration of action.
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