In which type of AV block does the PR interval lengthen until a beat is dropped?

The correct answer is related to Second-degree Mobitz I AV block, also known as Wenckebach. In this type of block, the key characteristic is the progressive lengthening of the PR interval with each successive heartbeat until a QRS complex is dropped, meaning one heartbeat does not lead to ventricular contraction. This phenomenon is a result of the atrioventricular node progressively becoming less able to conduct electrical impulses, and after several beats, it fails to conduct the impulse altogether, leading to the dropped beat.

This pattern of gradual lengthening of the PR interval followed by a drop in the QRS complex is an essential diagnostic criterion for identifying this specific type of AV block, distinguishing it from other types of blocks such as first-degree AV block, which features a constant PR interval, and second-degree Mobitz II AV block, where the PR interval remains constant until a beat is dropped without the progressive lengthening. In third-degree AV block, there is a complete dissociation between atrial and ventricular activity, and the PR interval is not progressively lengthening but rather absent, making it a different scenario entirely.