Which type of heart block is indicated by a progressive prolongation of the PR interval until QRS drops?

The type of heart block indicated by a progressive prolongation of the PR interval until a QRS complex is dropped is Second-degree AV block, Type I, also known as Mobitz Type I or Wenckebach phenomenon.

In this mechanism, the PR interval gradually lengthens with each successive heartbeat until the electrical impulse fails to conduct through the AV node, resulting in a dropped QRS complex. This pattern is characteristic of Type I block and is often seen in young, healthy individuals or during situations where there is increased vagal tone.

This specific sequence of events is what distinguishes Type I from other types of AV blocks. In contrast, first-degree AV block involves a consistently prolonged PR interval without any dropped beats; Type II features intermittent dropped beats without a progressive increase in the PR interval; and third-degree AV block presents with complete disassociation between atrial and ventricular rhythms, with no relation between P waves and QRS complexes.