What is the characteristic feature of Mobitz I Second-Degree AV Block on an ECG?

The characteristic feature of Mobitz I Second-Degree AV Block, also known as Wenckebach, is the progressive prolongation of the PR interval on an ECG. This phenomenon occurs because the conduction through the AV node gradually slows down with each successive atrial impulse until a beat is eventually dropped, resulting in a missing QRS complex.

In this type of block, you will see that the PR intervals get longer with each heartbeat until a QRS complex does not occur after the P wave corresponding to the longest PR interval. This pattern then restarts, leading to the characteristic cyclical nature of the waveform in Mobitz I. The progressive nature of the PR interval prolongation is essential for distinguishing this block from others, as it signifies a specific underlying electrophysiological behavior within the heart's conduction system.