Depolarizing blocking drugs

There are two main classes of natural and synthetic drugs used as neuromuscular blockers—nondepolarizing and depolarizing.

Depolarizing blocking drugs

Succinylcholine is the only therapeutic depolarizing blockingdrug. Although it’s similar to the nondepolarizing blockers in its therapeutic effect, its mechanism of action differs. Succinyl-choline acts like acetylcholine, but it isn’t inactivated by cholinesterase. It’s the drug of choice when short-term muscle re-laxation is needed.

Pharmacokinetics

Because succinylcholine is absorbed poorly from the GI tract, the preferred administration route is I.V.; the I.M. route can be used, if necessary.

Metabolism and excretion

Succinylcholine is hydrolyzed in the liver and plasma by the en-zyme pseudocholinesterase, producing a metabolite with a nonde-polarizing blocking action. Succinylcholine is excreted by the kid-neys, with a small amount excreted unchanged.

Pharmacodynamics

After administration, succinylcholine is rapidly metabolized, but at a slower rate than acetylcholine. As a result, succinylcholine re-mains attached to receptor sites on the skeletal muscle membrane for a longer period of time. This prevents repolarization of the mo-tor end plate and results in muscle paralysis.

Pharmacotherapeutics

Succinylcholine is the drug of choice for short-term muscle relax-ation, such as during intubation and ECT.

Drug interactions

The action of succinylcholine is potentiated by a number of anes-thetics and antibiotics. In contrast to their interaction with nonde-polarizing blockers, anticholinesterases increase succinylcholine blockade. (See Adverse reactions to succinylcholine.)