What is the hallmark of a first-degree AV block on ECG?

• A. Dropped QRS beats with normal PR interval.
• B. Prolonged PR interval (>200 ms) with every P wave conducted to a QRS.
• C. Prolonged QRS duration (>120 ms) with normal PR.
• D. Variable PR interval with intermittent nonconducted P waves.

Answer: (B)

Conduction from the atria to the ventricles is slowed but not blocked. This shows up on the ECG as a consistently prolonged PR interval (greater than 200 ms) with every P wave followed by a QRS complex. The rhythm remains regular and there are no dropped QRS complexes. The PR interval reflects AV nodal conduction time, so a uniform prolongation across beats indicates first-degree AV block.

This differs from patterns with dropped beats (second-degree blocks) or with a widened QRS (intraventricular conduction delays), or with AV dissociation where P waves and QRS complexes don’t relate predictably.