Progressive prolongation of PR interval followed by missing of a QRS complex is the classical manifestation of Wenckebach Phenomenon or Wenckebach 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.
Highest grade of Wenckebach AV block possible is 2:1 AV block. In 2:1 AV block, 50% of the P waves are non-conducted, while in the other grades, higher percentages are conducted. But in 2:1 AV block one cannot be sure of the Wenckebach nature of the AV block because there is no progressive prolongation of PR interval.
In Wenckebach AV block, in each cycle, the number of QRS complexes will be just one less than the number of P waves. Hence the conduction ratio in Wenckebach AV block will be 3:2, 4:3, 5:4 etc. By the same analogy, there is progressive shortening of PP interval followed by missing of a P wave in Wenckebach sinoatrial block. The longest PP interval is less than twice the shortest PP interval.