Class III: Potassium Channel Blockers
- Treat tachyarrhythmias, including re-entry arrhythmias.
Potassium channel blockers delay repolarization, prolong the action potential, and lengthen the effective refractory period.*
The drugs we'll focus on the most, amiodarone and its derivative, slow the heart rate and atrioventricular node conduction; these additional actions are due to their abilities to block beta adrenoreceptors, sodium, potassium, and calcium channels.*
- Class III drugs prolong the QT interval on the ECG.
- Examples
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Amiodarone, which, due to the various channels and receptors it blocks, is a mix of classes I-IV. Its wide spectrum of action makes it a popular drug of choice in a variety of arrhythmias.
Amiodarone is lipophilic, and is widely distributed through the body tissues.
Widespread potential side effects:
Liver toxicity and cirrhosis
Interstitial pneumonitis, which can lead to
pulmonary fibrosis
Corneal microdeposits, which are caused by amiodarone-induced lipidosis in the eye
(we show their "whorl-like" pattern(
Photosensitivity/phototoxicity can produce a rash or even give the skin a
gray-blue tint
And, because amiodarone is iodine-rich, it can cause
hypo- or hyperthyroidism.
Amiodarone can also cause
sinus bradycardia
–
Dronedarone is a synthetic, non-iondinated derivative of amiodarone (it doesn't have iodine).
It is used to treat
atrial fibrillation, and is generally considered less toxic, but less powerful, than amiodarone.
It is, however, associated with
liver toxicity.
–
Sotalol, which also has class I beta blocking actions, is associated with
torsades de pointes.
As a warning, note that most class III drugs are significantly pro-arrhythmic; amiodarone and dronedarone are important exceptions, as they less likely to precipitate arrhythmias.*
For References, please see our full tutorial on Class III Anti-arrhythmic drugs.
