Class 3 Antiarrhythmic Drugs (Potassium Channel Blockers)

Class 3 antiarrhythmics are drugs that block cardiac tissue K channels . The medications in this class include amiodarone , dronedarone, sotalol, ibutilide, dofetilide, and bretylium. The main mechanism of action includes blocking the cardiac K channels to prolong repolarization . However, some medications in this class also exert effects on Na channels , calcium channels , and adrenergic receptors .

Pharmacogenetics of potassium channel blockers

Some potassium channel blocking antiarrhythmics produce little or no other cardiac ion channel electrophysiologic effect and are termed “pure” potassium channel blockers. Sotalol, dofetilide, and possibly ibutilide fall into this category although sotalol’s effects are also mediated by beta-blockade and early reports suggested ibutilide may also enhance late sodium current...

Potassium channel blockade: A mechanism for suppressing ventricular fibrillation

The suppression of ventricular fibrillation by antidysrhythmic drugs is well correlated with their ability to block potassium channels in nerve and cardiac membranes. Blockade of potassium channels reduces electrical inhomogeneities in action potential and conduction parameters that lead to ventricular fibrillation. These actions tend to effectively decrease the electrical size of the heart, which suggests a mechanism for antifibrillatory drug action.

Potassium-Channel Blockers (Class III Antiarrhythmics)

Potassium-channel blockers comprise the Class III antiarrhythmic compounds according to the Vaughan-Williams classification scheme. These drugs bind to and block the potassium channels that are responsible for phase 3 repolarization. Therefore, blocking these channels slows (delays) repolarization, which leads to an increase in action potential duration and an increase in the effective refractory period (ERP). On the electrocardiogram, this increases the Q-T interval. This is the common effect of all Class III antiarrhythmic drugs.