What are the electrocardiogram (ECG) characteristics of atrial fibrillation (afib)?

ECG Characteristics of Atrial Fibrillation

Atrial fibrillation on ECG is characterized by three key features: absence of distinct P waves replaced by fibrillatory waves, irregular R-R intervals, and an irregular ventricular response.1

Key ECG Findings in Atrial Fibrillation

• P waves are replaced by rapid oscillations or fibrillatory waves that vary in amplitude, shape, and timing, representing chaotic electrical activity in the atria 2, 1
• The ventricular response is completely irregular (irregular irregularity) with variable R-R intervals 2, 3
• The atrial cycle length is usually variable and less than 200 ms, resulting in an atrial rate of ≥300 bpm 3
• The ventricular rate depends on AV node properties, autonomic tone, presence of accessory pathways, and effects of medications 2, 3

Visual Representation of Atrial Fibrillation on ECG

Key features to note in the image: absence of P waves, irregular R-R intervals, and fibrillatory waves between QRS complexes.

Distinguishing Features from Other Arrhythmias

• Atrial fibrillation must be distinguished from atrial flutter, which shows a characteristic saw-tooth pattern of regular atrial activation (flutter waves), particularly visible in leads II, III, aVF, and V1 [2, @3@]
• In atrial flutter, the atrial rate typically ranges from 240-320 beats per minute with flutter waves that are more organized and regular than the fibrillatory waves of AF [@3@, 2]
• Regular R-R intervals may occasionally be seen in AF in the presence of AV block or when a junctional rhythm takes over, which can lead to misdiagnosis [@4@, @6@]
• Extremely rapid ventricular rates (over 200 beats per minute) in the setting of irregular rhythm suggest the presence of an accessory pathway [@4@, @5@]

Clinical Significance of ECG Findings

• The absence of P waves reflects uncoordinated atrial activation, which leads to ineffective atrial contraction and loss of "atrial kick" contribution to cardiac output [@6@]
• Loss of coordinated atrial contraction can decrease cardiac output by 5-15%, which is particularly significant in patients with diastolic dysfunction [@5@, 1]
• Irregular ventricular filling due to variable R-R intervals further reduces cardiac output compared to a regular rhythm at the same mean rate [@6@]
• Even short episodes of irregular supraventricular tachycardia without P waves are associated with increased risk of future sustained AF and ischemic stroke [@7@]

Common Pitfalls in ECG Interpretation

• AF may be misdiagnosed as atrial flutter when atrial activity is prominent on ECG in more than one lead [@1@, @5@]
• A 12-lead ECG of sufficient duration and quality is essential to properly evaluate atrial activity and confirm the diagnosis [@5@, 1]
• P-wave index (the standard deviation of P-wave duration across all leads) is one of the strongest predictors of future AF development, with a hazard ratio of 2.7 for a P-wave index >35 [@8@]
• The co-existence of AF with complete AV block can be challenging to diagnose, as it presents with absence of P waves but a regular ventricular rhythm [@9@]

Quantitative Assessment

• Irregularity of R-R intervals can be quantified using various indices, which helps in automated detection of AF [@10@]
• Modern machine learning approaches using convolutional neural networks can detect brief episodes of AF with high sensitivity (>97% for all episodes) by analyzing both rhythm irregularity and P-wave absence 4
• The P-wave dispersion (difference between widest and narrowest P waves on a standard ECG) is predictive of future AF development with a hazard ratio of approximately 2 [@8