Progressive prolongation of PR interval followed by missing of a QRS complex is the classical manifestation of Wenckebach’s atrioventricular (AV) block. The increments in PR interval in each successive beat is decremental. This produces progressive shortening of RR intervals. The shortest RR interval is in the beat preceding the pause. The longest RR interval is less than twice the shortest RR interval.
By the same analogy, there is progressive shortening of PP interval followed by missing of a P wave in Wenckebach sinoatrial (SA) block. The longest PP interval is less than twice the shortest PP interval.