Mobitz I 2* AV Block

Mobitz I 2* AV block is also known as Wenckebach. It is the progressive lengthening of the PR interval until there is a dropped beat. Therefore, both the P-P interval will be constant and the R-R interval will be irregular; the R-R interval progressively shortens until the dropped beat. This causes ‘grouping’ of the QRS complexes (4 into group below and 5th beat would be in its own grouping). Evaluating the PR interval just before a dropped beat and comparing it to the first PR interval after a dropped beat may make this diagnosis more clear as those PR intervals will be the most different.

It is usually due to reversible block at the level of the AV node. Due to this, it is typically considered a benign rhythm without requiring treatment. If patients are symptomatic, they respond to atropine. Many of the causes of 1* AV block can also cause this. Additionally, some inferior OMIs may cause this as the AV node is typically supplied by the RCA. 

This lengthening can be very slow with only 1 dropped beat on an entire rhythm strip, making it difficult to identify compared to a Mobitz II 2* AV block. It is easily differentiated from a 3* AV block in that the R-R intervals are not regular with Wenckebach; in other words, the grouped beating helps establish this vs 3* AV block.

Examples:

Further Reading:

https://litfl.com/av-block-2nd-degree-mobitz-i-wenckebach-phenomenon/

http://hqmeded-ecg.blogspot.com/search/label/wenckebach

https://www.mayoclinicproceedings.org/article/S0025-6196(11)62080-6/fulltext