What is Atrial Fibrillation (AF)?

What is Atrial Fibrillation (AF)?

Atrial fibrillation is a supraventricular tachyarrhythmia characterized by chaotic, uncoordinated atrial electrical activity that results in ineffective atrial contraction and an irregularly irregular ventricular response. 1

Electrocardiographic Features

The diagnosis of AF requires identification of three cardinal ECG characteristics:

• Irregular R-R intervals when atrioventricular conduction is intact, creating the characteristic "irregularly irregular" rhythm 1, 2
• Absence of distinct, repeating P waves - instead, consistent P waves are replaced by rapid oscillations or fibrillatory waves that vary in amplitude, shape, and timing 1
• Irregular atrial activity with atrial cycle lengths typically less than 200 milliseconds, resulting in atrial rates ≥300 beats per minute 3

Pathophysiology and Ventricular Response

The ventricular rate in AF is determined by several key factors:

• AV nodal conduction properties serve as the primary gatekeeper, with L-type calcium channels responsible for the major depolarizing current 1
• Autonomic tone balance - sympathetic stimulation (beta-adrenergic receptors) accelerates AV conduction and increases ventricular rate, while vagal stimulation (muscarinic receptors) slows conduction 1
• Concealed conduction occurs when atrial impulses partially penetrate the AV node, rendering those cells refractory and preventing subsequent impulses from conducting, which paradoxically helps limit ventricular rate 1
• Accessory pathways, when present, can bypass normal AV nodal regulation and produce extremely rapid ventricular rates exceeding 200 beats per minute 1, 3

Hemodynamic Consequences

AF produces multiple adverse hemodynamic effects:

• Loss of atrial contraction ("atrial kick") reduces cardiac output by 5-15%, with more pronounced effects in patients with diastolic dysfunction, mitral stenosis, hypertrophic cardiomyopathy, or left ventricular hypertrophy 2
• Rapid ventricular rates limit diastolic filling time due to shortened diastolic intervals 3
• Beat-to-beat variability in ventricular filling further compromises cardiac output compared to regular rhythms at equivalent mean rates 1, 2
• Tachycardia-induced cardiomyopathy can develop when ventricular rates persistently exceed 120-130 beats per minute 1, 3

Clinical Significance and Complications

AF represents the most common sustained cardiac arrhythmia encountered in clinical practice:

• Prevalence affects 2.7-6.1 million American adults, with numbers expected to double over 25 years, and occurs in 3-5% of individuals over age 60 1, 4
• Stroke risk increases 17-fold compared to age-matched controls due to thrombus formation, primarily in the left atrial appendage 1, 2
• Mortality impact contributes to over 99,000 deaths annually in the United States 1
• Symptom spectrum ranges from asymptomatic to fatigue (most common), palpitations, dyspnea, hypotension, syncope, or heart failure exacerbation 1, 5
• Healthcare burden adds approximately $8,700 per patient annually, totaling $26 billion to US healthcare costs 1

Differential Diagnosis Considerations

Several conditions can mimic AF and must be distinguished:

• Atrial flutter displays a saw-tooth pattern of regular flutter waves (particularly in leads II, III, aVF, V1) with atrial rates of 240-320 beats per minute, rather than the chaotic fibrillatory waves of AF 1
• Regular R-R intervals in AF can occur with complete AV block, ventricular tachycardia, or junctional tachycardia, potentially causing diagnostic confusion 1
• Pacemaker patients may require temporary pacemaker inhibition to expose underlying atrial fibrillatory activity 1
• Wide-QRS tachycardia with irregular rhythm suggests AF with either bundle branch block or conduction over an accessory pathway 1

Common Diagnostic Pitfalls

• AF with prominent atrial activity on ECG may be misdiagnosed as atrial flutter 3
• A 12-lead ECG of sufficient duration and quality is essential to properly evaluate atrial activity and confirm the diagnosis 3, 2
• Extremely rapid rates over 200 beats per minute should prompt evaluation for accessory pathway conduction (pre-excitation syndrome) or ventricular tachycardia rather than typical AF 1, 3