Which type of block is characterized by progressively longer PR intervals until a drop occurs?

The type of block characterized by progressively longer PR intervals until a dropout occurs is known as 2nd Degree AV Block Type I, commonly referred to as Mobitz Type I or Wenckebach. In this arrhythmia, the PR interval progressively lengthens with each heartbeat until a beat is dropped, which means that a QRS complex is missing. This progressive lengthening is a key feature that differentiates this block from others.

The underlying mechanism typically involves a gradual increase in the time it takes for electrical impulses to travel through the AV node, resulting in the elongation of the PR interval. When the conduction delay becomes significant enough, the impulse is blocked entirely, resulting in the dropped beat. This pattern continues in a cycle, signifying a repetitive rhythm.

In contrast, other types of blocks have different characteristics. For instance, 2nd Degree AV Block Type II does not involve the progressive lengthening of the PR interval; the intervals remain constant until a beat is dropped. Similarly, 1st Degree AV Block has a consistently prolonged PR interval without any drops, and 3rd Degree AV Block presents with no effective communication between the atria and ventricles, resulting in dissociated atrial and ventricular rhythms. Understanding these distinctions is