Why we use them
Flecainide and propafenone are the workhorse class IC agents for rhythm control in patients with structurally normal hearts — paroxysmal AF, frequent symptomatic SVT, and as a “pill-in-the-pocket” to convert an AF episode on demand. They are effective and generally well tolerated as long as the structural-heart-disease line is respected.
Mechanism
Class IC drugs block the fast sodium channel with slow on-off kinetics, producing marked use-dependent conduction slowing (the effect grows at faster heart rates). They widen the QRS but have minimal effect on repolarization — so, unlike class III agents, they are not QT-prolonging torsades risks. The trade-off is proarrhythmia in diseased myocardium.
Dosing
- Flecainide: 50–150 mg BID maintenance; pill-in-the-pocket 200–300 mg as a single dose.
- Propafenone: 150–300 mg TID (immediate) or 225–425 mg BID (sustained-release); PIP 450–600 mg once.
- Pair every PIP dose with an AV-nodal blocker taken ≥30 min prior.
The structural-heart-disease rule
This is the non-negotiable safety point. CAST (flecainide/encainide post-MI) showed increased mortality from proarrhythmia in patients with prior infarction. Therefore class IC agents are contraindicated in:
- Coronary/ischemic heart disease or prior MI
- Significant LV dysfunction or heart failure
- Significant structural disease (marked LVH, valvular disease)
Confirm a structurally normal heart (echo ± ischemic evaluation) before starting.
1:1 atrial flutter — the classic trap
IC drugs slow the atrial rate of AF/flutter. A flutter that was conducting 2:1 at 150 bpm can slow to ~200 bpm atrial and then conduct 1:1 to a dangerous ventricular rate, sometimes with aberrancy mimicking VT. Always co-prescribe a beta-blocker or non-dihydropyridine calcium-channel blocker.
Monitoring
- Baseline and on-therapy ECG — watch QRS duration; a >25% widening suggests excess effect.
- Exercise/treadmill testing — IC conduction slowing is use-dependent, so proarrhythmia can surface only at higher rates; an exercise study helps confirm safety.
- Periodic reassessment of LV function and ischemic status.
Interactions
- Propafenone raises digoxin and warfarin levels and interacts with CYP2D6 substrates.
- Additive AV/sinus node depression with beta-blockers, CCBs, and other antiarrhythmics.
Common pitfalls
- Starting an IC agent without excluding structural/ischemic disease.
- Forgetting the AV-nodal blocker and seeing 1:1 flutter.
- Reading IC-induced wide-complex 1:1 flutter as VT.
- Giving propafenone to a patient with reactive airway disease (its beta-blockade can provoke bronchospasm).