A 52-year-old woman with paroxysmal atrial fibrillation (CHA2DS2-VASc score 3) is being started on flecainide as 'pill-in-the-pocket' therapy. She has no structural heart disease and normal LV function. Which condition MUST be ruled out before initiating flecainide therapy?

• A Long QT syndrome (flecainide prolongs QTc via potassium channel blockade)
• B Wolff-Parkinson-White syndrome (flecainide accelerates accessory pathway conduction)
• C Hypertrophic cardiomyopathy (flecainide increases outflow tract gradient)
• D Brugada syndrome (sodium channel mutation — flecainide can unmask or worsen) ✓

Explanation

Flecainide is a Class IC antiarrhythmic (pure sodium channel blocker) contraindicated in structural heart disease and, crucially, in Brugada syndrome. Flecainide is a standard provocation test for unmasking latent Brugada pattern (sodium channel blockade exacerbates the J-point elevation and ST elevation in V1–V3 characteristic of Brugada) and can precipitate life-threatening ventricular arrhythmias in affected individuals. All patients being considered for flecainide should have a 12-lead ECG reviewed for Brugada pattern. Flecainide primarily blocks fast sodium channels and does not significantly prolong QTc (that is a concern with Class IA/III agents). It does not accelerate accessory pathway conduction (adenosine's limitation) but is actually used cautiously in WPW-related AF (though amiodarone or procainamide preferred acutely). HCM is not a contraindication to flecainide specifically.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.