Summary
Muscle relaxants are commonly used as an adjunct to general anaesthesia and to facilitate ventilator care in the intensive care unit. The muscle relaxants are unique in that the degree of neuromuscular blockade can be directly measured. Thus, for some of the muscle relaxants it is possible to correlate the degree of neuromuscular blockade with the plasma concentration of drug. This quantitative pharmacokinetic approach has been applied primarily to d-tubocurarine and to a lesser extent to suxamethonium (succinylcholine), gallamine and pancuronium. The pharmacokinetic information for the other relaxants is mostly descriptive and incomplete.
The variation in drug concentration over time is influenced by the distribution, metabolism and excretion of drug. Metabolism by plasma Cholinesterase plays a major role in the termination of action of suxamethonium. Although pancuronium is partly metabolised its major metabolites have moderate pharmacological activity. The other relaxants are excreted through the kidney. For gallamine and diinethyl-tubocurarine, renal excretion appears to be the only means of elimination. However, biliary excretion probably provides an alternative route of elimination for d-tubocurarine and pancuronium. In patients with impaired renal function the duration of neuromuscular blockade may be markedly prolonged following standard doses of gallamine or diinethyl-tubocurarine, may be slightly prolonged following standard doses of pancuronium, and is near normal following standard doses of d-tubocurarine. Following large or repeated doses of pancuronium or d-tubocurarine, the duration of neuromuscular blockade may be markedly prolonged.
Because of their relatively large extracellular fluid volume, infants require more suxamethonium on a weight basis than do adults to produce equal neuromuscular blockade. At a single equipotent dose of suxamethonium, the time to recover full neuromuscular transmission is the same in infants, children and probably adults. Neonates appear to be sensitive to non-depolarising muscle relaxants; dosage criteria are unpredictable.
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Wingard, L.B., Cook, D.R. Clinical Pharmacokinetics of Muscle Relaxants. Clin Pharmacokinet 2, 330–343 (1977). https://doi.org/10.2165/00003088-197702050-00002
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DOI: https://doi.org/10.2165/00003088-197702050-00002