Class III antidysrhythmic medications

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.

Regarding interactions, potassium channel blockers should not be combined with other medications that prolong the QT interval, such as certain macrolide antibiotics or fluoroquinolones, as well as some antipsychotics, like risperidone.

If diuretics are combined with potassium channel blockers, there’s an increased risk of severe dysrhythmias due to electrolyte imbalances, like decreased potassium.

Excessive bradycardia can also result if these medications are combined with a beta blocker, and increased bleeding can result if they’re combined with warfarin.

Also, both amiodarone and dronedarone are major substrates of the enzyme CYP3A4, so their levels can be affected by medications and herbal supplements that either induce or inhibit this enzyme. Additionally, amiodarone can increase the serum level of digoxin, leading to toxic effects.

Lastly, all antidysrhythmics are considered high alert medications, meaning they have an increased risk of causing significant harm if used in error.

Now, when caring for a patient taking a class III antidysrhythmic medication, begin by performing a baseline assessment focused on their vital signs and cardiovascular status.

Next, review your patient’s medical history as well as nonpharmacologic and pharmacologic therapies to check for contraindications or possible drug interactions. Then, review diagnostic tests like ECGs and recent laboratory results including hepatic, renal, and thyroid function tests.

Also be sure to place your patient on continuous cardiac monitoring and keep emergency equipment nearby. Following administration, assess your patient for side effects, and evaluate the effectiveness of antidysrhythmic therapy.

Now, if your patient is discharged on a class III antidysrhythmic medication, focus your teaching on safe self-administration. Teach them about the importance of taking their medications exactly as prescribed and to never stop taking their medication abruptly, since this can cause reemergence of their dysrhythmia.

Also, explain that they should avoid grapefruit juice since it can affect the metabolism of their medication and explain that they shouldn’t begin new over-the-counter medications or herbal supplements without first discussing it with their health care provider.

Finally, review some of the most common side effects, how to recognize them, and when to contact their health care provider. For patients taking amiodarone, remind them of the importance of regular ophthalmic examinations, and instruct them to avoid sun exposure and to wear sunscreen and protective clothing when they're outdoors.

Alright, as a quick recap.... Class III antidysrhythmic medications, also known as potassium channel blockers, mostly work by prolonging repolarization and the refractory period of the heart. Side effects include increasing the risk of torsades de pointes by prolongation of the QT interval as well as toxicity to organs like the lungs and liver.

Nursing considerations for class III antidysrhythmic medications include establishing a baseline assessment, monitoring for side effects, evaluating the effectiveness of therapy, and providing teaching for safe self-administration.