Second degree AV block is where the AV conduction is partially intact and is subdivided into Types I and II, based on the behavior of the PR intervals preceding the blocked P wave. Typing 2nd degree AV block is important because of the different prognosis of Type I vs. Type II, and this is, in turn, related to the usual site of block, AV node (Type I) vs. the His-Purkinje system (Type II).
Type I is characterized by progressive lengthening PR intervals before the non-conducted P wave or dropped QRS (Wenckebach phenomenon), and the QRS is usually narrow, since the Wenckebach phenomenon occurs almost exclusively in the AV node. Type II has a constant PR interval before the dropped QRS, with usually a wide QRS, as the origin is frequently in the bundle branches with trifascicular block.