A 15 years old patient with a history of syncope receives 24h ECG monitoring. Scrolling through the tracing you encounter the following:
Top line shows AVB II/2 with 3:1 block. In AVB II look at PR intervals before and after the blocked beat. If the PR interval before the blocked beat is longer than the PR interval after the blocked beat it is Wenckebach periodicity (AVB II/1). If the PR intervals are the same it is Mobitz type 2 (AVB II/2). The latter is considered more serious as the localization of block is distal to the AV node. Pacemaker implantation may be required in certain cases.
Middle and bottom line takings show change to 2:1 block, skipping every other beat. 2:1 block may also appear in AVB II/1. The key again is to look at the PR intervals!
Another feature that can be noticed on this ECG in the middle and bottom line is that the P-P interval alternates. It is shorter from the P-wave that conducts to the following P-wave that is blocked, and longer from the blocked P-wave to the conducting P-wave. The ventricular contraction seems to accelerate atrial depolarization. This feature is called "ventricle-phasic response", and may also be seen in other forms of AVB.