How is atrial fibrillation characterized on ECG and what is a major stroke risk concern?

• A. Regular rhythm with P waves before each QRS and a normal PR interval.
• B. Irregularly irregular rhythm with no distinct P waves and variable R-R intervals; risk of embolic stroke from left atrial thrombus.
• C. Regular rhythm with wide QRS complexes.
• D. Irregular rhythm with inverted P waves and no embolic risk.

Answer: (B)

Atrial fibrillation on ECG is defined by an irregularly irregular rhythm with no distinct P waves before each QRS and highly variable R-R intervals. This happens because the atria are fibrillating rather than contracting in an organized way, so the ventricles respond unpredictably through the AV node.

The major stroke risk comes from blood pooling in the left atrium (especially the left atrial appendage), which can lead to thrombus formation. If a piece of that thrombus breaks off, it can travel to the brain and cause an embolic ischemic stroke. This is why stroke prevention with anticoagulation is a central concern in AF management, guided by risk scores.

Other descriptions describe different rhythms that don’t match AF: a regular rhythm with P waves and a normal PR interval is sinus rhythm; a regular rhythm with wide QRS complexes points to a conduction abnormality like VT or bundle branch block; an irregular rhythm with inverted P waves suggests another atrial/ventricular conduction pattern and does not carry the same embolic stroke risk profile.