What is characterized by a prolonged PR interval on an EKG?

A prolonged PR interval on an EKG is a hallmark characteristic of first-degree atrioventricular (AV) block. In first-degree AV block, the conduction through the AV node is delayed, resulting in a longer time for the electrical impulse to travel from the atria to the ventricles. This delay is represented as a PR interval greater than 200 milliseconds. Even though the PR interval is prolonged, all atrial impulses are eventually conducted to the ventricles, which differentiates this block from more severe forms of AV block.

In contrast, second-degree AV block can present with a prolonged PR interval, but it is also characterized by occasional dropped beats, meaning not all atrial impulses are conducted. Bundle branch block affects the conduction pathways within the ventricles rather than the AV node itself, so it does not specifically result in a prolonged PR interval. Third-degree AV block, or complete heart block, features a complete dissociation between atrial and ventricular activity, leading to a significantly altered EKG appearance, and the PR interval is not applicable as the atrial and ventricular impulses do not correlate. Thus, the clear and defining characteristic of a prolonged PR interval aligns with first-degree AV block, making this the correct identification.