Other Nondepolarizing Blockers of Importance

Atracurium besylate (Tracrium) is a benzylisoquinolin-ium compound like d-tubocurarine. Its actions are similar to those of d-tubocurarine, but its duration of action is shorter (45 minutes) because of spontaneous degradation of the molecule (Hofmann elimination). Because of this, atracurium is useful in patients with low or atypical plasma cholinesterase and in patients with renal or hepatic impairment.

Mivacurium chloride (Mivacron) is a newer agent that is chemically related to atracurium. The primary mechanism of inactivation is hydrolysis by plasma cholinesterase. Although it is useful for patients with renal or hepatic disease, some caution is warranted, since these individuals may have reduced plasma cholinesterase as a result of the disease. Mivacurium has an onset of action (1.8 minutes) and duration of effect (20 minutes) only twice that of succinylcholine, and in this respect, it is the most similar to succinylcholine of all of the nondepolarizing agents.

Pancuronium bromide (Pavulon) is a synthetic bis-quaternary agent containing a steroid nucleus (amino steroid), as denoted by the -curonium suffix. It is five times as potent as d-tubocurarine. Unlike d-tubocu-rarine, it does not release histamine or block ganglionic transmission. Like d-tubocurarine, it has a moderately long onset (2.9 minutes) and duration of action (110 minutes). Pancuronium and its metabolite are eliminated in the urine.

Vecuronium bromide (Norcuron) is chemically identical to pancuronium except for a tertiary amine in place of a quaternary nitrogen. However, some of the drug will exist as the bisquaternary compound, depending on body pH. Vecuronium has a moderate onset of action (2.4 minutes) and a duration of effect of about 50 minutes. Like pancuronium, it does not block ganglia or vagal neuroeffector junctions, does not release hista-mine, and is eliminated by urinary excretion.

Rocuronium bromide (Zemuron) is a recently approved amino steroid neuromuscular blocking agent. It has a rapid onset of action (1 minute), but its duration of action is intermediate (55 minutes), about that of ve-curonium. On rare occasions, it may release histamine and cause cardiac irregularities. Rapacuronium bromide (Raplon) is the most recent neuromuscular blocking agent approved by the United States Food and Drug Administration (FDA). It is an analogue of vecuronium and is thus categorized as an amino steroid. It has a rapid onset of action (1.5 minutes) and a short to intermediate duration of action (20 minutes). This makes it a suitable alternative to mivacurium or succinylcholine for short procedures. It is eliminated mainly by the liver. Adverse effects are dose dependent; they include tachycardia, hypotension, and bronchospasm. These effects may be related to the ability of the drug to release a small amount of histamine.

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