What are the Class 1c anti‑arrhythmic drugs, their dosing regimens, indications, contraindications, and monitoring recommendations?

Class 1c Antiarrhythmic Drugs

Overview

Class 1c antiarrhythmic drugs—flecainide and propafenone—are first-line agents for rhythm control in patients with atrial fibrillation or supraventricular tachycardia who have structurally normal hearts. These sodium channel blockers are highly effective but carry significant contraindications that must be strictly observed 1, 2.

The Two Drugs

Flecainide

• Initial dose: 50 mg every 12 hours 1
• Maximum dose: 300 mg daily (150 mg every 12 hours) 1
• For cardioversion: 200-300 mg oral or 1.5-3.0 mg/kg IV over 10-20 minutes 3

Propafenone

• Immediate release: 150 mg every 8 hours initially, up to 300 mg every 8 hours 1
• Extended release: 225 mg every 12 hours initially, up to 425 mg every 12 hours 1
• For cardioversion: 600 mg oral or 1.5-2.0 mg/kg IV over 10-20 minutes 3

Primary Indications

Class 1c drugs are recommended for:

• Rhythm control in paroxysmal or persistent atrial fibrillation in patients without structural heart disease 2, 4
• Pharmacological cardioversion of recent-onset AF 5
• Prevention of AVNRT recurrence when AV nodal blockers fail 6, 7
• Maintenance of sinus rhythm after successful cardioversion 4

The 2020 Canadian guidelines specifically state these agents should be used "in the absence of HF or CAD" 2, while the 2024 ESC guidelines recommend them "excluding those with impaired left ventricular systolic function, severe left ventricular hypertrophy, or coronary artery disease" 5.

Absolute Contraindications

You must exclude class 1c drugs in patients with:

1. Structural heart disease 1, 2, 4:

   • LVEF <40% 4
   • Significant LV hypertrophy 1, 4
   • Ischemic heart disease or history of MI 4
   • Active ischemia 4

2. Conduction system disease 1, 4:

   • Second- or third-degree AV block (without pacemaker) 1, 4
   • Sinus node dysfunction (without pacemaker) 1
   • Left bundle branch block 4
   • Right bundle branch block with left hemiblock 4

3. Other cardiac conditions:

   • Cardiogenic shock 1
   • Brugada syndrome 1
   • Atrial flutter without concomitant AV nodal blockade 1, 4

4. Organ dysfunction:

   • Severe hepatic impairment 1, 4
   • Severe renal impairment (CrCl <35 mL/min for flecainide) 4

5. Reactive airway disease (propafenone specifically) 1

Critical Safety Requirement

Always combine class 1c drugs with AV nodal blocking agents (beta-blockers or non-dihydropyridine calcium channel blockers) 4. This combination is mandatory because class 1c drugs can organize atrial fibrillation into atrial flutter with 1:1 AV conduction, causing dangerously rapid ventricular rates 1, 4.

Pre-Treatment Screening

Before initiating class 1c drugs in patients >50 years or with atherosclerotic risk factors, perform formal ischemia assessment (stress test) 4. This stems from the CAST trial findings showing increased mortality when flecainide was used post-MI 4.

Monitoring Requirements

At Baseline:

• 12-lead ECG to document PR interval, QRS duration, and QTc 4
• Assess renal function (creatinine clearance) 4
• Assess hepatic function 1
• Evaluate for structural heart disease 4

After Initiation:

• Repeat ECG to monitor for PR and QRS prolongation 4
• QRS widening >25% from baseline increases proarrhythmia risk and warrants dose reduction or discontinuation 4

Long-Term:

• Annual assessment for CAD symptoms in patients on chronic therapy 4
• Formal stress testing if symptoms develop 4
• Monitor for atrial flutter development 1

Adverse Effects to Monitor

Common adverse effects include:

• Atrial flutter with 1:1 AV conduction (most dangerous) 1
• QT prolongation 1
• Torsades de pointes 1
• Bradycardia 1
• Worsening heart failure 1
• Hypotension 1, 3
• Bronchospasm (propafenone) 1
• CNS effects (dizziness, visual disturbances) 6, 7

In the 2022 pill-in-the-pocket study, significant adverse events occurred in 3% of patients, all with higher doses (flecainide 300 mg or propafenone 600 mg), including syncope, symptomatic bradycardia/hypotension, and 1:1 atrial flutter 8.

Drug Interactions

Important interactions to manage:

• Amiodarone: Monitor levels closely 1
• Digoxin: Monitor digoxin concentrations 1
• Ritonavir, saquinavir, tipranavir: Avoid combination 1
• Verapamil: Monitor flecainide levels 1
• QT-prolonging drugs: Use with extreme caution 1

Emerging Evidence on Structural Heart Disease

Recent data suggest class 1c drugs may be safer in select patients with structural heart disease than previously thought 9, 10, 11. A 2026 study showed improved survival with class 1c use in real-world AF patients 10, and a 2024 study demonstrated safety in nonischemic cardiomyopathy patients with ICDs 11. However, current guidelines still restrict use in structural heart disease 2, 4, 5, and this remains the standard of care until guidelines are updated.

Pill-in-the-Pocket Approach

For intermittent AF, the pill-in-the-pocket strategy uses single loading doses taken at symptom onset. The first dose must be administered in a monitored setting due to 3% risk of significant adverse events requiring intervention 8. After safe initial dosing, patients may self-administer at home 8.

When to Choose Alternatives

Use amiodarone instead when:

• Structural heart disease is present 2, 4, 5
• LVEF <40% 4
• Significant LV hypertrophy exists 4
• Coronary artery disease is present 2, 4
• Class 1c drugs have failed or are contraindicated 4

Use dronedarone for:

• Patients with HFpEF, HFmrEF, ischemic heart disease, or valvular disease 2, 5

Key Clinical Pitfall

The most dangerous error is using class 1c drugs without concomitant AV nodal blockade, risking 1:1 atrial flutter conduction 1, 4. The second most common error is prescribing these drugs to patients with unrecognized structural heart disease—always screen appropriately before initiation 4.