Which of the following best describes the action of procainamide?

Procainamide is classified as a sodium channel blocker. It primarily exerts its effects by inhibiting the influx of sodium ions through voltage-gated sodium channels in cardiac myocyte membranes. This action helps to stabilize the cardiac cell membrane, which reduces excitability and conduction of impulses in the heart.

As a result, procainamide is frequently used to treat certain types of arrhythmias, particularly ventricular tachycardia and atrial fibrillation, because the sodium channel blocking mechanism slows conduction velocity and increases the refractory period in cardiac tissue. This makes it particularly effective for controlling abnormal heart rhythms.

While other classes of drugs mentioned have different primary mechanisms of action—like calcium channel blockers, which affect calcium ions to influence heart contraction and vascular tone, or potassium channel blockers which prolong repolarization—the unique action of procainamide as a sodium channel blocker is what defines its role in cardiac pharmacology.