What is the definition of atrial fibrillation (AF) under current published literature?

Guideline Definition of Atrial Fibrillation

Atrial fibrillation is defined as a supraventricular tachyarrhythmia characterized by three essential ECG features: absolutely irregular RR intervals without a repetitive pattern, absence of distinct P waves (replaced by rapid fibrillatory waves varying in amplitude, shape, and timing), and when visible, an atrial cycle length less than 200 ms (≥300 bpm). 1

Core Diagnostic Criteria

The most recent ESC guidelines (2010) establish three mandatory characteristics for AF diagnosis: 1

1. Absolutely irregular RR intervals - The surface ECG shows RR intervals that do not follow any repetitive pattern, earning AF the designation "arrhythmia absoluta" 1

2. Absence of distinct P waves - No clear P waves are visible on the surface ECG, though some apparently regular atrial electrical activity may be seen in certain leads, most often in lead V1 1

3. Rapid atrial cycle length - When atrial activity is visible, the interval between two atrial activations is variable and typically less than 200 ms, corresponding to an atrial rate of ≥300 bpm 1

Pathophysiologic Definition

AF represents uncoordinated atrial activation with consequent deterioration of atrial mechanical function. 1 The ACC/AHA/ESC guidelines describe this as replacement of consistent P waves by rapid oscillations or fibrillatory waves that vary in size, shape, and timing, associated with an irregular ventricular response when AV conduction is intact. 1

Minimum Duration Requirement

Any arrhythmia demonstrating the ECG characteristics of AF that lasts sufficiently long for a 12-lead ECG to be recorded, or at least 30 seconds on a rhythm strip, should be considered AF. 1 This 30-second threshold is clinically significant because even brief episodes carry the same risk of AF-related complications (particularly stroke) as sustained forms. 1

Ventricular Response Characteristics

The ventricular response in AF depends on multiple factors: 1, 2

• Electrophysiological properties of the AV node and other conducting tissues
• Level of vagal and sympathetic tone
• Presence or absence of accessory conduction pathways
• Action of drugs affecting AV nodal conduction

The ventricular rate is frequently rapid and irregular, though regular RR intervals are possible when AV block or ventricular/AV junctional tachycardia is present. 1

Critical Diagnostic Pitfalls

Extremely rapid ventricular rates exceeding 200 beats per minute strongly suggest the presence of an accessory pathway (pre-excitation syndrome) or ventricular tachycardia, not typical AF. 1, 2 This distinction is crucial because management differs significantly.

In patients with implanted pacemakers, diagnosis may require temporary pacemaker inhibition to expose underlying atrial fibrillatory activity. 1

A rapid, irregular, sustained, wide-QRS-complex tachycardia suggests either AF with conduction over an accessory pathway or AF with underlying bundle-branch block. 1

Differential Diagnosis Considerations

Several supraventricular arrhythmias can mimic AF, most notably: 1

• Atrial tachycardias and atrial flutter - These typically show longer atrial cycle lengths ≥200 ms, distinguishing them from AF 1
• Rare forms of frequent atrial ectopy
• Dual antegrade atrioventricular nodal conduction

A 12-lead ECG of sufficient duration and quality to evaluate atrial activity is essential for definitive diagnosis. 1 When the ventricular rate is fast, AV nodal blockade during Valsalva maneuver, carotid massage, or intravenous adenosine can help unmask atrial activity. 1

Clinical Detection

An irregular pulse should always raise suspicion of AF, but ECG recording is mandatory for diagnosis. 1 The heart rate in AF can be calculated from a standard 12-lead ECG by multiplying the number of RR intervals on the 10-second strip (recorded at 25 mm/s) by six. 1