Which ECG pattern is characteristic of atrial fibrillation?

• A. Irregularly irregular rhythm with fibrillatory waves; atrial rate >250 bpm
• B. Regular rhythm with narrow QRS
• C. Wide QRS with block
• D. Peaked T waves

Answer: (A)

Atrial fibrillation on the ECG is defined by an irregularly irregular rhythm with no discrete P waves and chaotic atrial activity seen as fibrillatory waves. The atrial rate is very rapid, often in the 350–600 beats per minute range, but the ventricular response is irregular because the AV node conducts impulses variably. This combination of an unpredictable, uneven rhythm with fibrillatory baseline activity is the hallmark finding.

Other patterns don’t fit because a regular rhythm with a narrow QRS points to sinus rhythm or a regular Supraventricular Tachycardia, a wide QRS with block suggests a ventricular conduction issue or bundle branch block, and peaked T waves indicate electrolyte disturbance (hyperkalemia).