Class I Antidysrhythmic Therapy (Sodium Channel Blockers)

Antidysrhythmic medications help correct irregular heart rhythms. Class I antidysrhythmic medications, also known as sodium channel blockers, are divided into three groups which differ by how they impact electrical conduction and repolarization.

Class IA antidysrhythmics like quinidine, are used for atrial and ventricular dysrhythmias, as well as supraventricular dysrhythmias; whereas class IB antidysrhythmics, such as lidocaine, are used for acute ventricular dysrhythmias; and class IC antidysrhythmics, like flecainide, are used to treat the most serious, life-threatening ventricular dysrhythmias that have not responded to other antidysrhythmic medications.

Alright, so class I antidysrhythmics work by blocking sodium channels and reducing the influx of sodium into cardiac cells. This slows the electrical impulses through the heart’s conduction system, ultimately resulting in a slower, more regular heart rhythm.

The three medication subgroups have minor differences in how they affect the electrical impulses. First, class IA antidysrhythmics slow conduction and prolong repolarization; class IB antidysrhythmics slow conduction and shorten repolarization; and class IC antidysrhythmics prolong conduction with little or no effect on repolarization.

Now, even though antidysrhythmics are indicated to treat dysrhythmias, they tend to have pro-dysrhythmic properties, meaning they can make the dysrhythmia worse, or even cause new dysrhythmias. Other side effects of antidysrhythmic medications will vary depending on the subgroup.

So, quinidine can cause headaches, dizziness, bradycardia, and diarrhea. Some of the more severe effects include cardiotoxicity, including ventricular tachycardia, heart block, asystole, as well as arterial embolism. It can also cause cinchonism, which is an adverse effect of medications made from plant alkaloids from the cinchona bark, that causes symptoms like nausea, tinnitus, and visual disturbances.

On the other hand, lidocaine can cause drowsiness, anxiety, restlessness, and blurred or double vision; and more severe effects include hypotension, bradycardia, cardiac or respiratory arrest, and seizures.

Then, flecainide has a high potential to worsen existing cardiac dysrhythmias; and because it also decreases cardiac contractility, it can worsen or even cause heart failure.