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Atrial Fibrillation (AF)
Definition and Overview
Atrial fibrillation is the most common sustained cardiac arrhythmia. It is characterized by disorganized, rapid, irregular atrial electrical activity (350-700 impulses/min) that results in loss of effective atrial contraction. The ventricles respond irregularly through the AV node, producing the hallmark "irregularly irregular" pulse.
ECG Findings
The two pathognomonic ECG features are:
- Absence of distinct P waves - replaced by irregular fibrillatory (f) waves, best seen in V1 and II
- Irregularly irregular QRS complexes - varying R-R intervals with usually narrow QRS (unless aberrant conduction or pre-excitation)
Classification
| Type | Definition |
|---|
| Paroxysmal | Terminates spontaneously or with intervention within 7 days |
| Persistent | Lasts >7 days or requires cardioversion |
| Long-standing persistent | Continuous AF >12 months |
| Permanent | Accepted by patient and clinician; no further rhythm control |
| Lone AF | No identifiable structural heart disease, hypertension, or other cause (historical term) |
Epidemiology
AF is extremely common, affecting approximately 12% of patients ≥75 years and 18% of those ≥85 years. The 2019 AHA/ACC/HRS guideline estimates that about one-third of all AF patients are ≥80 years. Global prevalence is rising due to aging populations and increasing rates of hypertension, obesity, and heart failure. - Braunwald's Heart Disease, p. 2808
Pathophysiology
AF requires both a trigger and a substrate:
Electrophysiological Mechanisms
- The classical model is Moe's multiple-wavelet hypothesis: multiple disorganized reentrant wavelets perpetuate the arrhythmia
- A competing theory involves focal discharge or rotor sources - single or small numbers of drivers that sustain AF. Evidence strongly supports the pulmonary veins (PVs) as the dominant source of both focal triggers and reentrant activity; PV isolation is thus the cornerstone of ablation therapy
- Underlying organization is reflected in dominant frequencies detectable by spectral analysis - Braunwald's Heart Disease, p. 721
Structural Remodeling
- Atrial fibrosis and conduction heterogeneity provide the substrate for reentry
- Age-related atrial changes, LA enlargement (e.g., from mitral stenosis or hypertension), and inflammation all promote remodeling
- AF begets AF ("electrical remodeling"): sustained AF shortens atrial effective refractory periods, stabilizing the arrhythmia
Ion Channel Abnormalities
- Familial (monogenic) AF: mutations in genes encoding K+ (IK) and Na+ (INa) channel subunits
- Gain-of-function IK mutations → shortened APD and atrial refractoriness → facilitates reentry
- Augmented inward INa → increased excitability and triggered activity
- Mutations in KCNJ2, KCNA5, and GJA5 (connexin 40) have been identified
- Polygenic forms: GWAS studies have identified multiple susceptibility loci - Braunwald's Heart Disease, pp. 733-762
Risk Factors / Causes
Cardiac:
- Hypertension (most common modifiable RF)
- Heart failure (HFpEF, HFrEF)
- Coronary artery disease
- Valvular disease (especially mitral stenosis - carries particularly high embolic risk)
- Cardiomyopathy, pericarditis, myocarditis
- Congenital heart disease
Non-cardiac:
- Obesity, obstructive sleep apnea
- Hyperthyroidism
- Diabetes mellitus
- Chronic kidney disease
- Excessive alcohol ("holiday heart")
- Pulmonary disease (COPD, PE)
- Post-surgery (especially cardiac surgery)
Clinical Features
Symptoms:
- Palpitations (may be less prominent in older adults)
- Dyspnea, reduced exercise tolerance
- Fatigue, light-headedness
- Chest discomfort
- Acute pulmonary edema (with rapid ventricular response, especially in HFpEF/stiff LV)
- Syncope, falls, or stroke as initial presentation (particularly in elderly)
- Many patients are asymptomatic - Braunwald's Heart Disease, p. 2814
Hemodynamic consequences:
- Loss of atrial kick reduces cardiac output by ~15-25% (more in diastolic dysfunction)
- Rapid ventricular rate further impairs diastolic filling
- Tachycardia-mediated cardiomyopathy if rate is uncontrolled chronically
Diagnosis
- ECG: Hallmark test - irregular rhythm without P waves
- Holter monitor / event recorder: For paroxysmal AF
- Echocardiogram: Assess structural disease, LA size, LV function, valvular disease, LAA thrombus
- TEE: Gold standard for excluding LAA thrombus before cardioversion
- Blood tests: TFTs (thyroid), CBC, electrolytes, renal function, BNP
- Wearable devices: Apple Watch and similar devices show utility for screening; ESC recommends AF screening at age ≥65 by pulse palpation or ECG rhythm strip (Class I)
Stroke Risk: CHA₂DS₂-VASc Score
Nonvalvular AF confers a fivefold increase in stroke risk. Stroke risk is quantified using the CHA₂DS₂-VASc score: - Tintinalli's Emergency Medicine, p. 1723
| Factor | Points |
|---|
| C - Congestive Heart Failure | 1 |
| H - Hypertension | 1 |
| A₂ - Age ≥75 years | 2 |
| D - Diabetes mellitus | 1 |
| S₂ - Prior Stroke/TIA/thromboembolism | 2 |
| V - Vascular disease (prior MI, PAD) | 1 |
| A - Age 65-74 years | 1 |
| Sc - Sex category (female) | 1 |
Anticoagulation thresholds:
- Score 0: anticoagulation not recommended
- Score 1: individualize (consider anticoagulation in men; generally treat in women)
- Score ≥2: oral anticoagulation strongly recommended
- All patients ≥75 years automatically score ≥2 and are candidates for anticoagulation - Braunwald's Heart Disease, p. 2821
Special case - Valvular AF (rheumatic mitral stenosis or mechanical heart valve): DOACs are contraindicated; warfarin (target INR 2-3) is mandated regardless of CHA₂DS₂-VASc score.
Bleeding Risk: HAS-BLED Score
Before initiating anticoagulation, assess bleeding risk:
| Factor | Points |
|---|
| H - Uncontrolled Hypertension (SBP >160) | 1 |
| A - Abnormal renal/liver function | 1-2 |
| S - Stroke history | 1 |
| B - Bleeding history or predisposition | 1 |
| L - Labile INR | 1 |
| E - Elderly (>65 years) | 1 |
| D - Drugs (antiplatelets, NSAIDs) or alcohol | 1-2 |
Score ≥3 = high bleeding risk. Importantly, a high HAS-BLED score should prompt modification of reversible bleeding risk factors, not automatic withholding of anticoagulation (in most patients, stroke risk still outweighs bleed risk). - Braunwald's Heart Disease, p. 2824
Anticoagulation Therapy
Direct Oral Anticoagulants (DOACs) - Preferred for Nonvalvular AF
| Drug | Mechanism | Dose |
|---|
| Apixaban (Eliquis) | Factor Xa inhibitor | 5 mg BID (2.5 mg BID if ≥2 of: age ≥80, wt ≤60 kg, Cr ≥1.5) |
| Rivaroxaban (Xarelto) | Factor Xa inhibitor | 20 mg OD with evening meal |
| Dabigatran (Pradaxa) | Direct thrombin inhibitor | 150 mg BID (110 mg BID if high bleed risk) |
| Edoxaban (Savaysa) | Factor Xa inhibitor | 60 mg OD (30 mg OD if CrCl 15-50 or wt ≤60 kg) |
DOACs offer similar or better stroke prevention versus warfarin, no dietary restrictions, no INR monitoring, and in elderly patients (≥75 years) demonstrate similar or less bleeding. - Braunwald's Heart Disease, p. 2826
Warfarin
- Target INR: 2.0-3.0 (2.0-2.5 in elderly); 2.5-3.5 for mechanical heart valves
- Required for valvular AF (rheumatic MS, mechanical valves) where DOACs are contraindicated
- Challenges: dietary interactions, multiple drug interactions, regular monitoring, increased osteoporosis risk
- Estimated maintenance dose in elderly: typically 2-5 mg/day - Braunwald's Heart Disease, p. 2826
Rate Control vs. Rhythm Control
Rate Control
Goal: Resting heart rate <80-100 bpm (lenient: <110 bpm acceptable in asymptomatic patients)
| Drug Class | Examples | Notes |
|---|
| Beta-blockers | Metoprolol, bisoprolol, carvedilol | First-line; preferred in CAD, HFrEF |
| Non-DHP CCBs | Diltiazem, verapamil | Avoid in HFrEF or accessory pathways |
| Digoxin | Digoxin | Useful in HF; less effective at exercise |
| Amiodarone | Amiodarone | Reserved for refractory cases |
Rhythm Control
Goal: Restore and maintain sinus rhythm
Indications:
- Persistent symptoms despite rate control
- Young patients with first-episode AF
- AF precipitating heart failure
- Patient preference
Cardioversion:
- Electrical cardioversion (DC cardioversion): 200 J biphasic; effective for acute restoration
- Pre-cardioversion anticoagulation ≥3 weeks if AF >48 hours (or TEE to exclude LAA thrombus)
- Post-cardioversion anticoagulation ≥4 weeks regardless of CHA₂DS₂-VASc score (due to "atrial stunning")
Antiarrhythmic Drugs (AADs):
| Drug | Use Case | Cautions |
|---|
| Flecainide / Propafenone | No structural disease | Contraindicated in CAD/HF ("pill-in-pocket" strategy) |
| Amiodarone | Any structural disease; HF | Pulmonary/thyroid/hepatic toxicity with long-term use |
| Dronedarone | Non-permanent AF without severe HF | Reduced mortality in ATHENA; avoid in HFrEF |
| Sotalol | Mild-to-moderate structural disease | QTc prolongation; requires renal monitoring |
| Dofetilide | HF, post-MI | Requires in-hospital initiation |
Ablation (Catheter Ablation / PVI)
- Pulmonary vein isolation (PVI) is the cornerstone procedure
- Superior to AADs for maintaining sinus rhythm in paroxysmal and persistent AF
- A 2024 meta-analysis (PMID 38040282) found ablation superior to AADs as first-line therapy in treatment-naive AF for maintaining sinus rhythm, with lower AF recurrence and comparable safety
- A separate 2024 meta-analysis (PMID 37937825) confirmed ablation improves LV function and quality of life vs. medical therapy
- Surgical maze procedure (Cox-Maze): used during concomitant cardiac surgery; high efficacy for permanent rhythm restoration
AF with Special Conditions
AF + Heart Failure with Preserved EF (HFpEF)
- AF frequently complicates HFpEF; loss of atrial kick and fast rate can precipitate acute decompensation
- Anticoagulation mandatory if coexisting AF
- Whether rhythm control reduces HF hospitalizations remains under investigation; rhythm control trials in HFrEF (e.g., CASTLE-AF) showed benefit - Braunwald's Heart Disease, p. 329
AF + Mitral Stenosis (Valvular AF)
- MS + AF carries embolic risk as high as mechanical prosthetic valves
- DOACs are contraindicated; warfarin is mandatory irrespective of CHA₂DS₂-VASc score
- Even transient episodes (<30 sec) detected on ambulatory monitoring significantly raise thromboembolic risk
- Rate control is difficult; ivabrading has been tried
- Restoring sinus rhythm improves QoL, especially with small LA and short AF duration - Braunwald's Heart Disease, p. 760
AF in the Elderly (≥75 years)
- All patients ≥75 automatically qualify for anticoagulation (CHA₂DS₂-VASc ≥2)
- DOACs preferred over warfarin (similar/better efficacy, similar/less bleeding)
- Symptoms are often atypical or minimal; syncope, falls, or stroke may be first presentation
- Screening: ESC recommends pulse palpation or ECG strip at age ≥65 (Class I); systematic ECG screening at ≥75 is Class IIb - Braunwald's Heart Disease, p. 2817
Emergency Management (Acute AF)
Hemodynamically unstable (hypotension, acute pulmonary edema, angina):
- Immediate synchronized DC cardioversion regardless of anticoagulation status
Hemodynamically stable:
- Rate control (IV metoprolol, diltiazem, digoxin)
- Consider rhythm control if AF onset <48 hours and patient appropriate
- Anticoagulation: heparin bridge if cardioversion planned; initiate long-term OAC based on CHA₂DS₂-VASc
- Discharge criteria per Tintinalli's: SpO2 >95%, BP at baseline, resting HR <100 bpm; 80%+ of ED patients can be safely discharged - Tintinalli's Emergency Medicine, p. 1718-1720
Complications
| Complication | Notes |
|---|
| Stroke / thromboembolism | 5× increased risk; main driver of anticoagulation decisions |
| Heart failure | Loss of atrial kick + uncontrolled rate |
| Tachycardia-induced cardiomyopathy | Reversible with rate/rhythm control |
| Cognitive decline / dementia | Independent association |
| Reduced quality of life and physical performance | Even with rate control |
| Increased mortality | HR ~1.5-2× vs. age-matched controls |
Recent Evidence (2024)
- Ablation vs. AADs as first-line therapy (PMID 38040282) - Systematic review/meta-analysis: catheter ablation superior to AADs in treatment-naive AF for maintaining sinus rhythm with comparable safety profile
- Ablation vs. medical therapy - updated meta-analysis (PMID 37937825) - Ablation improves LVEF and reduces recurrence vs. medical therapy
- Emerging therapies in AF management (PMID 37716839) - Reviews novel antiarrhythmics, left atrial appendage occlusion (LAAO), and pulsed-field ablation as emerging strategies
Key Take-Home Points
- AF = irregularly irregular rhythm with no P waves on ECG
- Anticoagulate all patients with CHA₂DS₂-VASc ≥2 (DOACs preferred over warfarin for nonvalvular AF)
- Valvular AF (rheumatic MS, mechanical valve) = warfarin only, no DOACs
- Rate control (HR <100-110) is adequate for asymptomatic patients; rhythm control for symptomatic patients
- PVI catheter ablation is the most effective rhythm control strategy, increasingly used as first-line
- Assess both stroke risk (CHA₂DS₂-VASc) AND bleeding risk (HAS-BLED) before anticoagulation
- All patients ≥75 years qualify for anticoagulation regardless of other risk factors
Sources: Braunwald's Heart Disease (10th ed.), Tintinalli's Emergency Medicine, Harrison's Principles (22nd ed., 2025), Miller's Anesthesia (10th ed.), Fuster and Hurst's The Heart (15th ed.)
