Potassium-Channel Blockers (Class III Antiarrhythmics)
Effects on action potentials
The primary role of potassium channels in cardiac action potentials is cell repolarization. In non-nodal tissue (see figure), action potentials are initiated when a cell is depolarized to a threshold potential by an adjacent cell. This leads rapid opening of fast sodium channels and a slower opening of L-type calcium channels that permit calcium to enter the cell (phase 0 and 2, respectively). As these channels become inactivated, potassium channels open permitting potassium ions to leave the cell (Figure: K+ out), which causes repolarization of the membrane potential (phase 3). Potassium channels remain open until the next action potential is triggered. There are also different potassium channels that are responsible for the initial repolarization (phase 1) that occurs as the fast sodium channels become inactivated. Potassium channels are also responsible for repolarizing slow-response action potentials in the sinoatrial and atrioventricular nodes.
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. The electrophysiological changes prolong the period of time that the cell is unexcitable (refractory) and therefore make the cell less excitable.
By increasing the ERP, these drugs are very useful in suppressing tachyarrhythmias caused by reentry mechanisms. Reentry occurs when an action potential reemerges into normal tissue when that tissue is no longer refractory. When this happens, a new action potential is generated prematurely (before normal activation) and a circular, repeating pattern of early activation can develop, which leads to a tachycardia. If the ERP of the normal tissue is lengthened, then the reemerging action potential may find the normal tissue refractory and premature activation will not occur.
Specific Drugs and Therapeutic Indications
The following table summarizes Class III compounds in terms of their therapeutic use and some special or distinguishing characteristics. More detailed information on specific drugs can be found at www.rxlist.com.
|amiodarone||ventricular tachycardia, includuing ventricular fibrillation; atrial fibrillation and flutter (off-label use)||very long half-life (25-60 days); Class I, II, III & IV actions and therefore decreases phase 4 slope and conduction velocity; potentially serious side effects (e.g., pulmonary fibrosis; hypothyroidism)|
|dronedarone||atrial fibrillation (non-permanent) and flutter||structurally related to amiodarone, but has a much smaller volume of distribution and shorter elimination half-life (13-19 hr); Class I, II, III & IV actions; containdicated in severe or recently decompensated, symptomatic heart failure; based on results from the PALLAS trial in 2011, the FDA has concluded there are concerns regarding increased risk for severe liver injury and serious cardiovascular adverse events in patients with permanent atrial fibrillation, and therefore, this drug should be used only in patients in sinus rhythm with a history of non-permanent atrial fibrillation|
|bretylium||life-threatening ventricular tachycardia and fibrillation||IV only; initial sympathomimetic effect (norepinephrine release) followed by inhibition, which can lead to hypotension|
|sotalol||ventricular tachycardia; atrial flutter and fibrillation||also has Class II activity|
|ibutilide||atrial flutter and fibrillation||slow inward Na+ activator, which delays repolarization; –inhibits Na+-channel inactivation, which increases ERP; IV only; can cause life-threatening ventricular arrhythmias|
|dofetilide||atrial flutter and fibrillation||very selective K+-channel blocker; can cause life-threatening ventricular arrhythmias|
Abbreviations: IV, intravenous.
Side Effects and Contraindications
All of these compounds, like Class I compounds, are proarrhythmic as well as being antiarrhythmic. For example, the increase in action potential duration can produce torsades de pointes (a type of ventricular tachycardia), especially in patients with long-QT syndrome. Amiodarone, because of its Class IV effects, can cause bradycardia and atrioventricular block, and therefore is contraindicated in patients with heart block, or sinoatrial node dysfunction.