The significance of second degree atrioventricular block and bundle branch block. Observations regarding site and type of block.

His bundle (H) electrograms were recorded in 15 patients with second degree atrioventricular (A-V) block and bundle branch block and these patients were prospectively followed. Site of block was proximal to H in four (BPH), distal to H in nine (BDH), and undetermined in two (studied during 1:1 conduction). Surface electrocardiographic features were retrospectively examined to determine the value of these recordings in predicting the site of block. Patients with type I block, with or without type II or 2:1 block, had BPH. Patients with type II block, 2:1 block, or type II combined with 2:1 block had BDH. Heart failure was more common in those with BPH (three of four patients as compared to three of nine patients with BDH). Syncope developed more commonly in patients with BDH (six of nine patients) as compared to those with BPH (one of four patients). Permanent pacing was indicated in three of four patients with BPH, nine of nine patients with BDH, and one of two patients with block at undetermined site be-