Mobitz classified second degree A-V block into two categories. Mobitz type I, or Wenckebach block, is characterized by a gradual prolongation of the P-R interval preceding the dropped ventricular beat. In Mobitz type II block, the dropped beat occurs without preceding prolongation of the P-R interval. Bundle branch block usually is present in patients with Mobitz type II block. The importance of distinguishing between these two types of A-V block lies in the usual irreversibility and higher mortality of patients with type II A-V block, as contrasted with type I or Wenckebach block. Also, Mobitz type II block frequently progresses to complete A-V block and Adams-Stokes attacks, thus necessitating a cardiac pacemaker
