Which finding is characteristic of a first-degree AV block?

• A. Prolonged PR interval with a normal QRS duration.
• B. Shortened PR interval with widened QRS.
• C. Irregular PR intervals with dropped QRS.
• D. Absent P waves.

Answer: (A)

In first-degree AV block, the conduction through the AV node is slowed, but every atrial impulse still reaches the ventricles. The sign of this is a consistently longer PR interval on the ECG, while the QRS duration remains normal because the ventricular depolarization process is unchanged. So you see a prolonged PR interval with a normal QRS complex, and there are no dropped beats.

This pattern fits because the key feature of this block is delayed but complete conduction from atria to ventricles, not a failure of conduction on some beats or a change in ventricular depolarization. A short PR interval with a widened QRS would point to a different condition (like pre-excitation or a ventricular conduction issue). Irregular PR intervals with dropped QRS suggests a second-degree AV block. Absent P waves indicates a rhythm without atrial depolarization, not AV nodal delay.