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

• A. Prolonged PR interval with all P waves followed by QRS
• B. Absent P waves
• C. Constant PR interval with dropped QRS
• D. Wide QRS complexes

Answer: (A)

The main idea is a delayed transmission of the impulse from the atria to the ventricles, seen as a longer than normal PR interval on the ECG. In first-degree AV block, the PR interval is consistently prolonged (typically >200 ms in adults), but every P wave is followed by a QRS complex, so atrial activity is still connected to ventricular activity in a 1:1 relationship. This means there are no dropped beats and the rhythm remains regular, just with a prolonged conduction time through the AV node. The prolonged interval reflects slower conduction through the AV node or related pathways, and while it’s often benign and asymptomatic, it can be influenced by medications or autonomic tone. Other patterns involve dropped QRS complexes or progressively lengthening PR intervals, which are not characteristic of first-degree block. Wide QRS complexes point to a different conduction issue within the ventricles rather than an AV nodal delay.