Which feature best defines atrial fibrillation on ECG?

• A. Regular rhythm with discrete P waves and constant R-R intervals.
• B. Irregularly irregular rhythm, no discrete P waves, variable R-R intervals; QRS often narrow.
• C. Slow rhythm with inverted P waves.
• D. Wide QRS tachycardia with stable rhythm.

Answer: (B)

At atrial fibrillation, the atria fire chaotically and are not organized into clear P waves. The result is an irregularly irregular ventricular rhythm with variable R-R intervals and no discrete P waves on the surface ECG. The QRS complexes are usually narrow because the ventricles conduct normally, unless there’s an accompanying conduction abnormality.

The other patterns don’t fit AF: a regular rhythm with distinct P waves and constant R-R intervals is typical of sinus rhythm, not AF. A slow rhythm with inverted P waves suggests a junctional or retrograde atrial activation pattern. A wide QRS tachycardia with a stable rhythm points toward ventricular tachycardia or SVT with aberrancy, not AF.