Class III Antidysrhythmic Therapy (Potassium Channel Blockers)

Class III antidysrhythmic medications, also called potassium channel blockers, are a class of medications used to treat life-threatening dysrhythmias like ventricular tachycardia or ventricular fibrillation. They can also be used to treat certain atrial dysrhythmias when other medications have not been effective. Examples of potassium channel blockers include amiodarone and dronedarone.

Okay, amiodarone and dronedarone mostly work by blocking potassium channels, which prolongs repolarization and the refractory period of the heart.

They also block sodium channels, which slows conduction through the heart’s conduction system.

Finally, they also block calcium channels and beta-adrenergic receptors, which decreases sinoatrial, or SA, node automaticity, and delays the conduction through the atrioventricular, or AV, node.

Now, even though antidysrhythmics are indicated to treat dysrhythmias, antidysrhythmics do tend to have prodysrhythmic properties, meaning they can make the dysrhythmia worse, or even cause new dysrhythmias.

Potassium channel blockers also prolong the refractory period in the heart, which prolongs the QT interval, which is the length of time from the beginning of the QRS complex to the end of the T wave on an ECG. This represents the time it takes for the ventricles to depolarize, or contract; and repolarize, or relax.

Widening of the QT interval increases the risk of torsades de pointes, a dysrhythmia that can degenerate into ventricular fibrillation, which is a life-threatening dysrhythmia.

There's also a high risk for organ toxicities, especially in patients taking the medication long-term or prescribed larger doses.

Both medications can cause pulmonary toxicity and hepatotoxicity. Amiodarone can cause cardiotoxicity, and thyroid toxicity.

Amiodarone can also cause other serious complications such as ophthalmic effects like optic neuropathy and neuritis. Amiodarone may also cause photosensitivity, and the skin can turn a bluish-gray color with prolonged exposure to the sun.

As far as contraindications go, both medications are contraindicated in patients with second- or third-degree heart block, sick sinus syndrome, or severe bradycardia in patients without a pacemaker.

And they should not be used in patients with cardiogenic shock, due to their effects on beta-adrenergic receptors.

Because amiodarone is a source of iodine, it should also be avoided in patients with iodine hypersensitivity. Lastly, due to its potential for toxicity, amiodarone has a Black Box warning stating that it should only be used in patients with life-threatening dysrhythmias.

On the other hand, dronedarone is contraindicated for patients who have had previous liver or lung toxicities from amiodarone, atrial fibrillation, or heart failure. In fact, dronedarone has a Black Box warning regarding its contraindication in patients with heart failure.