What is the hallmark of a first-degree heart block seen on an ECG?

The hallmark of a first-degree heart block on an ECG is characterized by a consistently prolonged PR interval that remains stable throughout the heart rhythm. In first-degree AV block, every atrial impulse (P wave) is conducted through the AV node to the ventricles, but there is a delay, resulting in a longer PR interval than normal. This prolonged interval is generally greater than 200 milliseconds, but it does not vary; it is constant for each beat.

This constant prolongation distinguishes first-degree heart block from other types of heart blocks. For example, in a second-degree block, you would see varying PR intervals or dropped beats, while a complete block would show a complete dissociation between atrial and ventricular activity. An irregularly irregular rhythm typically suggests conditions like atrial fibrillation, which is unrelated to first-degree heart block. Thus, the prolonged and constant nature of the PR interval is a defining feature of this specific type of heart block.