What is the defining feature of a second degree AV block, type 1?

The defining feature of a second degree AV block, type 1, also known as Wenckebach phenomenon, is characterized by a repetitive sequence in which the PR interval progressively lengthens until a QRS complex is dropped. This results in an irregular heartbeat, where one or more signals from the atria are not conducted to the ventricles, leading to an intermittent failure of transmitting impulses. This dropped QRS complex is what differentiates it from first-degree AV block, where impulses are consistently conducted, and from other types of blocks where conduction patterns differ.

In contrast, a consistent PR interval before each QRS indicates first-degree AV block, where all impulses are transmitted successfully without any dropped beats. The notion of multiple impulses being blocked without any QRS drops describes third-degree AV block, where no impulses reach the ventricles, causing complete dissociation between atrial and ventricular activity, which is not applicable here. Lastly, the failure of electrical impulses to reach the bundle of His signifies a complete block of conduction which does not characterize second-degree type 1 block. Thus, the presence of a repeating sequence with intermittent drops in the QRS complex is what distinctly identifies a second degree AV block, type 1.