Class IC antiarrhythmics

Class IC antiarrhythmics (flecainide and propafenone) are potent sodium channel blockers (Stage I of action potential) with strong effects on conduction velocity and minimal effect on action potential duration or refractoriness. They have the longest binding duration to sodium channels compared to other Class I agents. ECG effects include marked PR prolongation and QRS widening (> 150 ms possible). Mechanism: blocked sodium channels slow impulse propagation, increasing conduction times; in reentrant circuits with dual AV node pathways (AVNRT, AVRNT), this can terminate re-entry. Class IC drugs are reserved for: (1) Symptomatic AVNRT/AVRNT; (2) Atrial fibrillation with SVT features or structural heart disease; (3) Other refractory SVTs; they are NOT used for ventricular arrhythmias. Major limitation: high proarrhythmic potential, especially post-MI (CAST trial showed increased mortality with flecainide/encainide in post-MI patients with frequent ventricular ectopy despite suppressing PVCs). Contraindications: structural heart disease (coronary disease, cardiomyopathy, prior MI, LVH), baseline conduction abnormalities (prolonged QRS), bradycardia, electrolyte abnormalities (hypokalemia). Adverse effects: negative inotropic effects (heart failure risk), proarrhythmia (torsades, accelerated VT), and bradycardia. Flecainide undergoes hepatic metabolism (CYP2D6 inhibitor—drug interactions). Propafenone also blocks beta-adrenergic receptors at higher doses. On boards: link Class IC to SVT in structurally normal hearts, avoid in heart disease, and remember CAST trial. Beta-blockers or calcium channel blockers are preferred first-line for AF rate control.