Atrial Fibrillation is Irregularly Irregular
Atrial fibrillation is characterized by an irregularly irregular rhythm, meaning the RR intervals show absolutely no repetitive pattern. This fundamental characteristic distinguishes AF from other arrhythmias and is sometimes referred to as "arrhythmia absoluta" 1.
Defining Characteristics on ECG
The irregularly irregular nature of AF manifests through specific ECG findings:
- The surface ECG shows 'absolutely' irregular RR intervals that do not follow any repetitive pattern 1
- P waves are replaced by rapid fibrillatory waves that vary in amplitude, shape, and timing 1
- The ventricular response is completely irregular when AV conduction is intact 1
Understanding "Irregularly Irregular" vs "Regularly Irregular"
This terminology distinction is clinically important:
- Irregularly irregular means there is no pattern whatsoever to the RR intervals—this is atrial fibrillation 1
- Regularly irregular describes rhythms with a predictable pattern of irregularity (such as atrial flutter with variable block or sinus rhythm with frequent premature beats)
Important Clinical Caveat
Regular RR intervals ARE possible in AF under specific circumstances, which can create diagnostic confusion 1:
- Presence of complete AV block 1
- Concurrent ventricular or AV junctional tachycardia 1
- Patients with implanted pacemakers (may require temporary pacemaker inhibition to expose underlying atrial fibrillatory activity) 1
Nuance in the Evidence
While classic teaching emphasizes total irregularity, one older study found that approximately 30% of AF patients showed some non-random patterns in consecutive RR intervals, though the overall rhythm remained irregular 2. However, the authoritative ACC/AHA/ESC guidelines consistently define AF by absolutely irregular RR intervals without repetitive patterns 1, which should guide clinical diagnosis.
Practical Diagnostic Approach
When evaluating rhythm irregularity:
- An irregular pulse should always raise suspicion of AF, but ECG confirmation is mandatory 1
- Record at least 30 seconds of rhythm strip or a full 12-lead ECG to properly assess the pattern 1
- Differentiate from atrial flutter with variable block, which may appear irregular but shows underlying flutter waves with a saw-tooth pattern 1, 3