Atypical Wenckebach

Vignette: An elderly with pneumonia. What is your ECG diagnosis?

The rhythm is sinus. After QRS # 7, there is a non-conducted P wave. The PRI of the first conducted P wave is shorter compared to the last conducted P wave. There is also a prominent increase in the PRI on the second conducted P wave (QRS # 9). These are all hallmarks of second degree AV block type I (Mobitz I or AV Wenckebach). An interesting feature here is the PRI prior to the dropped beat is seemingly the same (but prolonged). Also, if you measure the PRI in V1, the PRI of the second conducted beat is seemingly the same after that. This is a hallmark of atypical Wenckebach. Thus, this is atypical Wenckebach. When the conduction ratio is more than 5:4 or 6:5, the PRI remains the same but prolonged. 

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