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Intravenous Anesthetics: Muscle Relaxants (page 4)

Muscle Relaxants
Muscle relaxants play an important role in anesthesia even though they don't provide any pain relief or produce unconsciousness. Total muscle relaxation is often necessary to help the surgeon achieve the best possible operating conditions. Muscle relaxation also adds to the safety of the patient during certain delicate or critical operations.

This class of drugs has its effect at the neuromuscular junction by preventing the effects of acetylcholine. Normally, when a nerve stimulus acts to contract a muscle, it releases acetylcholine. The binding of this acetylcholine to receptors causes the muscle to contract.

P>Two categories of muscle relaxants are in common usage today - depolarizing agents and non-depolarizing agents.

Depolarizing Muscle Relaxants
These agents act by actually binding to the acetycholine receptors and blockiing access to the receptors. In the process of binding, the receptor is actually activated - causing a process known as depolarization. Succinycholine is the only drug in this class that is used by anesthesia providers today.

The duration of action of succinylcholine is very short, because it is metabolized in the body very quickly by an enzyme called plasma cholinesterase. This short duration of action makes succinylcholine a useful drug in situations where muslce relaxation is needed for only a short time.

Side effects of succinycholine include fasciculations (small muslce movements caused when the drug binds to receptors), myalgias (muscle soreness which may be the result of the fasciculations), cardiac rhythm disturbances, increases in ocular and gastric pressure, hyperkalemia in at risk patients and triggering of a syndrome known as malignant hyperthermia.

Some of these side effects can be eliminated by pre-treating the patient with a small amouint of non-depolarizing muscle relaxant prior to administration of succinylcholine.

Non-depolarizing muscle relaxants
These drugs also bind to acetycholine receptors in a reversible manner and act by blocking access to the receptors. However, they do not "activate" the receptor in the process, no depolarization takes place and fasciculations do not occur.

These agents are classified and chosen by their duration of action. Common short-acting agents include mivacurium and rapacuronium. Common intermediate-duration drugs include rocuronium, vecuronium, atracurium and others. Common long-acting agents include pancuronuim and d-tubocurarine.

Each of these agents has its own profile of side effects which are partly a result of their activation of similar receptors in areas of the body outside of the muscle.

Acetycholinesterase is an enzyme that is responsible for breaking down acetylcholine. Drugs that block this enzyme, known as acetylcholinesterase inhibitors, will result in a larger amount of acetycholine being available. This will reverse the effect of the non-depolarizing muscle relaxants in a competitive manner.

Back to First Page ---> Intravenous Anesthetics Page 1, 2, 3, 4

 

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