Potassium channel blockers
One might say that some drugs prolong the QTc as an accidental side effect, but Class III agents do it intentionally - Deranged Physiology
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Classification of antiarrhythmic agents
It is hard to discuss the Class III effects because the poster child for this class is amiodarone, and it acts promiscuously on all Class I-IV molecular targets. In fact, all Class III agents have some kind of extra weirdness (sotalol is a beta-blocker, ibutilide acts on slow inward depolarising sodium currents, etc). A "pure" potassium channel blocker effect is therefore difficult to describe using an example. Strictly speaking, they should only prolong repolarisation.
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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.
Classification of antiarrhythmic agents
It is hard to discuss the Class III effects because the poster child for this class is amiodarone, and it acts promiscuously on all Class I-IV molecular targets.
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