Second Degree AV Block Type I and 2:1 AV Block ECG
This ECG has both second degree AV block with 2:1 block and second degree type I AV block. Note that intermittent P waves conduct to the ventricles on this ECG tracing, however it is clear that some do not. There does appear to be a prolonging PR interval before some dropped beats (second degree type I AV block), however at times P waves are randomly not conducted (2:1 AV block). Note the irregular R-R intervals making 3rd degree heart block unlikely.
The rhythm on this ECG needs to be distinguished from high grade AV block which is a form of 3rd degree AV block that can occur when some P waves are conducted through the AV node to the ventricles, however quite a few are not. The R-R intervals will vary unlike in true 3rd degree AV block (complete heart block) where they are regular. There is no definite Wenkebach periodicity seen in high grade AV block.