Class Iii Antiarrhythmics

Amiodarone. Amiodarone, 2-butyl-3-benzofuranyl-4-[2-(diethylamino)ethoxy]-3,5-diiodophenyl ketone (Cordarone), was introduced as an antianginal agent. It has very pronounced class III action and is especially effective in maintaining sinus rhythm in patients who have been treated by direct current shock for atrial fibrillation.41 Like class III antiarrhythmic drugs, amiodarone lengthens the effective refractory period by prolonging the action potential duration in all myocardial tissues. Amiodarone is eliminated very slowly from the body, with a half-life of about 25 to 30 days after oral doses.42 Although the drug has a broad spectrum of antiarrhythmic activity, its main limitation is a slow onset of action. Drug action may not be initiated for several days, and the peak effect may not be obtained for several weeks.

Amiodarone has adverse effects involving many different organ systems. It also inhibits metabolism of drugs cleared by oxidative microsomal enzymes. It contains iodine in its molecular structure and, as a result, has an effect on thyroid hormones. Hypothyroidism occurs in up to 11% of patients receiving amiodarone.43 The principal effect is the inhibition of peripheral conversion of T4 to T3. Serum reverse T3 (rT3) is increased as a function of the dose as well as the length of amiodarone therapy. As a result, rT3 levels have been used as a guide for judging adequacy of amiodarone therapy and predicting toxicity.44

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