In atrial fibrillation, which statement about QRS width is accurate?

• A. QRS complexes are typically wide.
• B. QRS complexes are typically narrow.
• C. P waves are visible before each QRS.
• D. QRS complexes are absent.

Answer: (B)

In atrial fibrillation, the main idea is that the ventricles still depolarize through the normal His-Purkinje conduction system via the AV node. That means the QRS duration stays narrow unless there’s a conduction problem like a bundle-branch block or rate-related aberrancy. The atria, in contrast, are fibrillating and do not produce discrete P waves before each QRS, but the ventricular conduction pathway remains intact. So the QRS complexes are typically narrow. The other statements don’t fit the common pattern: AF does not usually produce wide QRS unless a conduction block or aberrancy is present, P waves aren’t seen before each QRS, and QRS complexes are not absent.