Second-degree AV block, Mobitz I (or Wenckebach type), primarily occurs at the AV node. This condition is harmless in most cases. However, in the setting of an acute inferior wall MI or digitalis overdose, it may progress to complete AV block. 

Mobitz I has a Wenckebach period, which is characterized by a succession of gradually increasing PR intervals, followed by one completely blocked atrial impulse and a ventricular pause (also known as a dropped beat). For the diagnosis, the Wenckebach period must reoccur and includes three or four atrial impulses (including the one with the dropped beat).

Related Video – ECG Rhythm Review – Second-Degree Heart Block type 1 (Wenckebach)

Case 2

71-Year-Old Man With Second-Degree AV Block ECG