1.3 Atrial Fibrillation, Atrial Flutter & Stroke Prevention

Cardiac arrhythmias and conduction disturbances span benign incidental findings to life-threatening emergencies. The ABIM exam emphasizes clinical recognition on 12-lead ECG, acute hemodynamic stabilization, stroke prevention in atrial fibrillation, antiarrhythmic drug selection tailored to underlying structural heart disease, and indications for pacing and defibrillation.

1. Atrial Fibrillation & Atrial Flutter

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, characterized by disorganized, rapid atrial electrical activation (350-600 bpm) with an irregularly irregular ventricular response and loss of atrial mechanical contraction.

Acute Management: Hemodynamic Instability vs. Stability

• Hemodynamically Unstable AF/Flutter: Immediate synchronized direct-current cardioversion (DCCV) (initial 120-200 J biphasic for AF; 50-100 J for flutter). Indications include hypotension/shock, acute pulmonary edema, ongoing myocardial ischemia, or acute altered mental status.
• Hemodynamically Stable AF: Triage into rate control versus rhythm control and assess thromboembolic risk.

Rate Control vs. Rhythm Control

• Evidence: The landmark AFFIRM and RACE trials demonstrated equivalent all-cause mortality and stroke rates between rate and rhythm control strategies. However, the EAST-AFNET 4 trial (2020) showed that early rhythm control (antiarrhythmic drugs or catheter ablation initiated within 1 year of AF diagnosis) significantly reduces cardiovascular death, stroke, and HF hospitalizations in patients with cardiovascular comorbidities.
• Rate Control Targets: Resting heart rate <80 bpm for symptomatic patients (strict control) or <110 bpm for asymptomatic patients with preserved LV systolic function (lenient control via RACE II trial).
• First-Line Rate Control Agents:
  • Beta-Blockers: Metoprolol Tartrate (2.5-5 mg IV q5min or 25-100 mg PO BID), Esmolol IV infusion, or Carvedilol (preferred in CAD/HFrEF).
  • Non-Dihydropyridine CCBs: Diltiazem (0.25 mg/kg IV bolus over 2 min, then 5-15 mg/h infusion, or 120-360 mg PO daily) or Verapamil. Strictly contraindicated in HFrEF (LVEF <=40%) due to potent negative inotropy precipitating pulmonary edema and cardiogenic shock.
  • Digoxin: Second-line add-on agent; particularly useful in sedentary patients with HFrEF. Ineffective for rate control during high sympathetic states (exercise, sepsis).
  • IV Amiodarone: Used for acute rate control in critically ill patients with severe hypotension, sepsis, or refractory HFrEF.

Antiarrhythmic Drug Selection for Rhythm Control

Antiarrhythmic selection depends strictly on the presence or absence of underlying structural heart disease (CAD, LVH, HFrEF):

Pill-in-the-Pocket Approach: Outpatient self-administration of a single oral dose of Flecainide (200-300 mg) or Propafenone (450-600 mg) combined with a beta-blocker or diltiazem (given 30 min prior to prevent 1:1 atrial flutter conduction) for termination of paroxysmal AF in patients with structurally normal hearts once safety is confirmed in a monitored setting.

Amiodarone Extracardiac Toxicities & Monitoring

• Pulmonary: Interstitial pneumonitis / pulmonary fibrosis (1-5%). Baseline chest radiograph and Pulmonary Function Tests (PFTs with DLCO); repeat for new respiratory symptoms or cough.
• Thyroid: Amiodarone-induced hypothyroidism (AIH; Wolff-Chaikoff effect, treat with levothyroxine without stopping amiodarone) or Amiodarone-induced thyrotoxicosis (AIT Type 1: Jod-Basedow iodine excess; AIT Type 2: destructive thyroiditis). Check baseline TSH/free T4, then q6 months.
• Hepatic: Elevated transaminases, cirrhosis. Monitor baseline LFTs, then q6 months.
• Ophthalmic: Corneal microdeposits (benign, reversible) and optic neuropathy/neuritis (requires discontinuation).
• Dermatologic: Photosensitivity and blue-gray skin discoloration (ceruloderma).
• Electrophysiologic: Sinus bradycardia, QT prolongation (low torsades risk compared to other Class III agents), heart block.

2. Stroke Prevention & Anticoagulation in Atrial Fibrillation

The CHA2DS2-VASc Risk Score

Anticoagulation Decision Rules

• Score >= 2 in Men or >= 3 in Women: Class I recommendation for long-term oral anticoagulation.
• Score = 1 in Men or = 2 in Women: Class IIb recommendation; oral anticoagulation may be considered.
• Score = 0 in Men or = 1 in Women: No antithrombotic therapy recommended (aspirin is ineffective for stroke prevention in AF and increases bleeding).

DOACs vs. Warfarin

• Direct Oral Anticoagulants (DOACs) are preferred over Warfarin for non-valvular AF due to superior efficacy/safety, lower rates of intracranial hemorrhage, and no need for INR monitoring:
  • Apixaban (Eliquis): 5 mg PO BID. Reduce dose to 2.5 mg PO BID if patient has >=2 of the following: Age >=80 years, Body weight <=60 kg, or Serum Creatinine >=1.5 mg/dL.
  • Rivaroxaban (Xarelto): 20 mg PO daily with the evening meal (15 mg daily if CrCl 15-50 mL/min).
  • Dabigatran (Pradaxa): Direct thrombin inhibitor, 150 mg PO BID (75 mg BID if CrCl 15-30 mL/min).
  • Edoxaban (Savaysa): 60 mg PO daily (30 mg daily if CrCl 15-50 mL/min or weight <=60 kg; do not use if CrCl >95 mL/min due to increased renal clearance and reduced stroke efficacy).
• Mandatory Warfarin (Target INR 2.0-3.0): DOACs are contraindicated in: (1) Moderate-to-severe rheumatic mitral stenosis; and (2) Mechanical prosthetic heart valves (target INR 2.5-3.5 for mechanical mitral valves). Warfarin must be used in these settings.
• HAS-BLED Score: Assesses 1-year major bleeding risk (Hypertension, Abnormal renal/liver function, Stroke, Bleeding history, Labile INR, Elderly >65y, Drugs/alcohol). A score >=3 identifies patients at high bleeding risk who need closer follow-up and modifiable risk factor correction—it is NOT a reason to withhold indicated anticoagulation.
• Left Atrial Appendage Occlusion (LAAO / Watchman Device): Percutaneous option for patients with high stroke risk (CHA2DS2-VASc >=2) who have a formal contraindication to long-term oral anticoagulation (e.g., recurrent life-threatening unprovoked bleeding).