From the chemical point of view, amiodarone is completely different from other antiar-rhythmics. It has two iodide atoms and a diethylaminoethanol group as substituents in the benzoyl part, and overall it is very similar to the structure of thyroxin-like molecules.
Amiodarone's antiarrhythmic action is connected to its ability to block K+, Na+, and Ca2+ channels while noncompetitively blocking a- and fi-adrenergic receptors of the heart, thus prolonging the action potential and effective refractive period of atrial cells, atrioven-tricular junctions, and ventricles of the heart, which is accompanied by decreased automatism of sinus node and slowing of atrioventricular conductivity.
Clinical use of amiodarone is limited because of its high toxicity, which consists of cardiac block, bradycardia, cardiac insufficiency, damaged thyroid gland function, neu-ropathology, and increased sensitivity to light, all of which significantly limit use of amiodarona, and it is only used in therapy for extremely serious tachyarrhythmias such as reoccurring ventricular fibrillation and hemodynamic unstable ventricular tachycardia, and only under supervision of a physician in a clinical situation. Synonyms of amiodarone are cordarone, rythmarone, and others.
Bretylium: Bretylium, N-(o-bromobenzyl)-W-ethyl-W,N-dimethylammonium tosylate (18.1.22), is synthesized by reacting o-bromobenzyltosylate with ethyldimethylamine .
Bretylium is poorly absorbed when taken orally, and it is used only in the form of intravenous or intramuscular injections. However, like many other quaternary ammonium salts, it initiates a response of neuronal catechoamines, which can cause tachycardia, elevate blood pressure, and so on.
Bretylium possesses sympatholytic action, which is associated with blockage of norep-inephrine (noradrenaline) from presynaptic nerve endings. It also has a direct effect on ischemic myocytes. Bretylium is an urgent treatment that is used in situations of ventricular tachycardia and ventricular fibrillation, primarily in the severe phase of a myocardial infarction, during which use of other medications or procedures have proven unsuccessful. It requires great caution and should be used only in urgent situations. Synonyms of this drug are vretilol, ornid, and others.
Drugs of this group are calcium channel blockers that inhibit slow transmembrane calcium ion flow in the cell of the conductive system of the heart during depolarization, which causes a slowing of atrioventricular conductivity and increased effective refractive period of atrioventricular ganglia, which eventually leads to the relaxation of smooth muscle of heart musculature and restores normal sinus rhythm during supraventricular tachycardias.
Today, this group is represented by a single calcium channel-blocking drug, verapamil, which is primarily used as an antianginal drug as well as for controlling hypertension.
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