What is Atrial Fibrillation (AF)?

What is Atrial Fibrillation (AF)?

Atrial fibrillation is a supraventricular tachyarrhythmia characterized by chaotic, uncoordinated electrical activation of the atria that results in ineffective atrial contraction and appears on ECG as absent P waves with irregular R-R intervals. 1

Electrocardiographic Features

The diagnosis requires ECG documentation showing three cardinal features 1:

• Irregularly irregular R-R intervals (when AV conduction is intact)
• Absence of distinct, repeating P waves
• Rapid oscillations or fibrillatory waves that vary in amplitude, shape, and timing

The ventricular rate depends on AV nodal properties, autonomic tone (vagal and sympathetic), presence of accessory pathways, and medications 1. Extremely rapid rates exceeding 200 bpm suggest accessory pathway conduction or ventricular tachycardia 1.

Epidemiology and Burden

AF is the most common sustained cardiac arrhythmia, affecting 1.5-2% of the general population 1. The prevalence increases dramatically with age: from <0.5% at ages 40-50 years to 5-15% at age 80 years 1. The lifetime risk for individuals over 40 years is 25% 1. Men are affected more frequently than women 1.

The incidence is projected to double over the next 25 years due to population aging, with current estimates of 2.7-6.1 million affected American adults 1.

Clinical Consequences and Mortality

AF doubles the death rate independently of other mortality predictors 1. The arrhythmia is associated with severe morbidity 1:

• Stroke risk increases 5-fold, with AF causing approximately one in five strokes 1
• Heart failure risk increases 3-fold 1
• All-cause mortality increases 2-fold, primarily from thromboembolic events and ventricular dysfunction 1
• AF contributes to over 99,000 deaths annually in the United States 1

Strokes associated with AF are typically more severe and result in long-term disability or death 1. Paroxysmal AF carries the same stroke risk as persistent or permanent AF 1.

Hemodynamic Impact

The hemodynamic consequences result from multiple mechanisms 1:

• Loss of coordinated atrial contraction (atrial kick)
• Suboptimal ventricular rate (either too rapid or too slow)
• Beat-to-beat variability in ventricular filling
• Sympathetic activation

These factors can lead to tachycardia-induced cardiomyopathy when ventricular rate remains uncontrolled 1. The irregular rhythm itself may decrease cardiac output even with adequate rate control 1.

Clinical Presentation

Symptoms range from none to severe 1:

• Fatigue (most common symptom)
• Palpitations
• Exertional dyspnea
• Chest pain
• Hypotension or syncope
• Heart failure exacerbation

Many patients remain asymptomatic, leading to delayed diagnosis ("silent AF") 1. The true prevalence is likely closer to 2% when accounting for undiagnosed cases 1.

Associated Conditions and Risk Factors

Over 70% of AF patients have underlying cardiac or systemic diseases 1. The most common associations include 1, 2:

Cardiac structural causes:

• Hypertension (present in 83% of Medicare beneficiaries with AF, conferring 1.8-fold increased risk) 1, 2
• Heart failure (present in 51-59% of AF patients) 1, 2
• Coronary artery disease (present in 64% of patients) 1
• Valvular heart disease, particularly mitral valve disease 1, 2
• Cardiomyopathies (hypertrophic, dilated, restrictive) 1, 2

Reversible/acute causes:

• Hyperthyroidism (requires evaluation in all new-onset AF) 1, 2
• Acute alcohol intake ("holiday heart syndrome") 1, 2
• Cardiac surgery (common early postoperative complication) 1, 2
• Myocardial infarction, pulmonary embolism, pericarditis 1, 2

Non-cardiac systemic conditions:

• Obesity (present in 25% of AF patients, increases risk through left atrial dilation) 1, 2
• Diabetes mellitus (present in 20% of patients) 1, 2
• Sleep apnea (especially with concurrent hypertension and structural heart disease) 1, 2
• Chronic kidney disease (present in 32-40% of patients) 1, 2
• COPD (present in 10-15% of patients) 1, 2

Pathophysiological Mechanisms

The arrhythmia develops through structural and electrical atrial remodeling 1, 2:

• Atrial fibrosis and enhanced connective tissue deposition
• Atrial dilation and hypertrophy
• Local conduction heterogeneities
• Changes in atrial action potential duration and refractory period
• Abnormal intracellular calcium handling

The renin-angiotensin-aldosterone system promotes AF through fibrosis stimulation, growth factor production, and cellular hypertrophy 1, 2.

Healthcare Burden

AF accounts for one-third of all cardiac arrhythmia hospitalizations 1. Patients with AF are hospitalized twice as often as those without AF and are three times more likely to have multiple admissions 1. The condition adds approximately $8,700 per patient annually, contributing $26 billion to US healthcare costs 1.

Drug-Induced Atrial Fibrillation

An increasing number of cardiovascular, non-cardiovascular, and anticancer drugs can induce or exacerbate AF 1. Drug-induced AF is often asymptomatic and paroxysmal, with spontaneous resolution, but may persist requiring cardioversion 1. Risk increases in elderly patients, those on polypharmacy, and those with existing AF risk factors 1.