Class III anti-arrhythmic drugs prolong the action potential plateau and refractory period, with therapeutic benefits and pro-arrhythmic risks.
Class actions and risks
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Class III drugs prolong the plateau phase of the action potential, lengthening the refractory period, and are effective at preventing atrial and ventricular tachyarrhythmias.
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They prolong the QT interval and can predispose to torsades de pointes and ventricular tachycardia, especially when other risk factors are present.
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Amiodarone is the main drug in this class. Disopyramide and sotalol also have some class III activity.
Amiodarone
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Amiodarone primarily has class III activity but also has class I, II and IV activity. The source describes it as probably the most effective available drug for controlling paroxysmal atrial fibrillation.
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It prevents recurrent ventricular tachycardia, particularly in patients with poor left ventricular function or implantable defibrillators, where it helps prevent unnecessary DC shocks.
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Its tissue half-life is 25–110 days. Intravenous or oral loading is often used to achieve therapeutic tissue concentrations, and effects may persist for weeks or months after treatment stops.
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Adverse effects occur in up to one-third of patients and are numerous and potentially serious. Drug interactions are also common.
Dronedarone
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Dronedarone is related to amiodarone but has a short tissue half-life and fewer adverse effects. It is effective at preventing episodes of atrial flutter and atrial fibrillation.
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It is contraindicated in permanent atrial fibrillation, heart failure or left ventricular impairment because it increases mortality.
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Regular liver and renal function monitoring is required.