Why should class IC antiarrhythmics be avoided in patients with structural heart disease?

• A. Because they are always well tolerated.
• B. Because they are recommended as first-line agents.
• C. Because they reliably suppress ventricular function.
• D. Because they carry a higher risk of proarrhythmia in structural heart disease.

Answer: (D)

Class IC antiarrhythmics are potent sodium channel blockers that slow conduction significantly. In hearts with structural disease—such as prior myocardial infarction, scar, or dilated cardiomyopathy—the myocardium has heterogeneous areas of scar and viable tissue. Slowing conduction across these areas can paradoxically promote reentrant circuits and trigger dangerous ventricular arrhythmias. This proarrhythmic risk is why these drugs are avoided in structural heart disease, a finding reinforced by historical trials showing increased mortality with flecainide/other IC agents in such patients. The best reason to avoid them is that they carry a higher risk of proarrhythmia when structural heart disease is present.