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Neuromuscular-blocking drugs

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Neuromuscular-blocking drugs block neuromuscular transmission at the neuromuscular junction, causing paralysis of the affected skeletal muscles. This is accomplished either by acting presynaptically via the inhibition of acetylcholine (ACh) synthesis or release, or by acting postsynaptically at the acetylcholine receptor. While there are drugs that act presynaptically (such as botulin toxin and tetrodotoxin), the clinically-relevant drugs work postsynaptically.

Clinically, neuromuscular block is used as an adjunct to anesthesia to induce paralysis, so that surgery can be carried out with less complications. Because neuromuscular block may paralyze muscles required for breathing, mechanical ventilation should be available to maintain adequate respiration. These drugs fall into two groups:

  • Non-depolarizing blocking agents: These agents constitute the majority of the clinically-relevant neuromuscular blockers. They act by blocking the binding of ACh to its receptors, and in some cases, they also directly block the ionotropic activity of the ACh receptors (PMID 8866353).
  • Depolarizing blocking agents: These agents act by depolarizing the plasma membrane of the skeletal muscle fiber. This persistent depolarization makes the muscle fiber resistant to further stimulation by ACh.

Patients are still aware of pain even after full conduction block has occurred; hence, general anesthetics and/or analgesics must be given to prevent anesthesia awareness.

Non-depolarizing blocking agents

All of these agents act as competitive antagonists against ACh at the site of postsynaptic ACh receptors.

Tubocurarine, found in curare of the South American plant genus Strychnos, has this effect. Tubocurarine has a slow onset (>5min) and a long duration of action (1-2 hours). Side effects include hypotension. This hypotension is partially explained by its effect of increasing histamine release, which is a vasodilator (PMID 2429800), as well as its effect of blocking autonomic ganglia (PMID 2682131). Route of excretion in the urine.

This drug needs to block about 70-80% of the Ach receptors for neuromuscular conduction to fail, and hence, for effective blockade to occur. At this stage, EPPs (end-plate potentials) can still be detected, but are too small the reach the threshold potential needed for activation of muscle fiber contraction.

Examples:

   * Curare (d-tubocurarine in clinical practice)
       -Prototype non-depolarizing agent
       -Not currently used in clinically

Ultra short-acting:

   * Rapacuronium

Short-acting:

   * Mivacurium (Mivacron)
        -Onset: 90 seconds, Duration: 12-18 minutes
        -Benzyl-Isoquinolinium agent: needs to be refrigerated, and causes release of histamine
        -No longer manufactured secondary to marketing, manufacturing, financial concerns

Intermediate-acting:

   * Atracurium (Tracrium)
        -Onset: 90 seconds, Duration: 60-80 minutes
        -Benzyl-Isoquinolinium agent: needs to be refrigerated, and causes release of histamine
        -Racemic mixture
        -Toxic metabolite called laudanosine, greater accumulation in individuals with renal failure
        -Laudanosine decreases seizure threshold
   * Cisatracurium (Nimbex)
        -Onset: 90 seconds, Duration: 60-80 minutes
        -Benzyl-Isoquinolinium agent: needs to be refrigerated, and causes release of histamine
        -Stereospecific enantiomer
        -Non-organ elimination via Hoffmann elimination (pH & temperature specific)
   * Vecuronium (Norcuron)
        -Onset: 60 seconds, Duration: 70-120 minutes
        -Aminosteroid: non-refrigerated, and may promote muscarinic block
   * Rocuronium (Zemuron)
        -Onset: 75 seconds, Duration: 45-70 minutes
        -Aminosteroid: non-refrigerated, and may promote muscarinic block

Long-acting:

   * Pancuronium (Pavulon)
        -Onset: 90 seconds, Duration: >180 minutes
        -Aminosteroid: non-refrigerated, and may promote muscarinic block

Depolarizing blocking agents

Depolarizing blocking agents work by depolarizing the plasma membrane of the muscle fiber, similar to acetylcholine. However, these agents are resistant to degradation by acetylcholinesterase, and can persistently depolarize the muscle fibers as opposed to the transient depolarization by ACh which is rapidly degraded. Initially, they cause muscular fasciculations (muscle twitches) while they are depolarizing the muscle fibers. Eventually, after sufficient depolarization has occurred, the muscle is no longer responsive to ACh released by the motoneurons. Hence, full neuromuscular block has been achieved.

Inhibition of acetylcholinesterase, the enzyme responsible for degrading acetylcholine, will cause ACh to have the same effect as these agents.

Examples:

    * Decamethonium 
        -Not used clinically
    * suxamethonium (US: succinylcholine)
        -Onset: 30 seconds, Duration: 5 minutes 
        -The only depolarizing blocking agent in general clinical use.
        -Composed of two ACh molecules joined with a methyl group
        -Has a short action time, and elevated action effects at the end-plate of muscles.

Comparison of drugs

The main difference is in the reversal of these two types of neuromuscular-blocking drugs. Non-depolarizing blockers are reversed by anticholinesterase inhibitor drugs. Since they are competitive antagonists at the ACh receptor so can be reversed by increases in ACh. The depolarizing blockers already have ACh-like actions, so these agents will have prolonged effect under the influence of anticholinesterase inhibitors.

The administration of depolarizing blockers will initially exhibit fasciculations (a sudden twitch just before paralysis occurs). This is due to the depolarization of the muscle. Also, post-operative pain is associated with depolarizing blockers.

The tetanic fade is the failure of muscles to maintain a fused tetany at sufficiently-high frequencies of electrical stimulation. Non-depolarizing blockers will have this effect on patients, while depolarizing blockers will not.

Adverse effects

Since these drugs may cause paralysis of the diaphragm, mechanical ventilation should be at hand to provide respiration.

Additionally, these drugs may exhibit cardiovascular effects, since they are not fully selective for the nicotinic receptor and hence may have effects on muscarinic receptors (PMID 2682131). If muscarinic receptors of the autonomic ganglia or adrenal medulla are blocked, these drugs may cause hypotension and tachycardia. Additionally, neuromuscular blockers may facilitate histamine release, which causes hypotension, flushing, and tachycardia.

In depolarizing the musculature, suxamethonium may trigger a transient release of large amounts of potassium from muscle fibers. This puts the patient at risk for life-threatening complications, such as hyperkalemia and cardiac arrhythmias.

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