Clinicopathological correlation was made in 8 cases of various A-V conduction disturbances, which consisted of 2 cases of complete, 4 of advanced, I of 2: 1 A-V block and 1 of A-V nodal rhythm. Serial sections of the conduction system revealed 4 types in location of main lesions. Type I (necrosis and/or fibrosis of A-V node) was found in 2 cases with syphilitic endoarteritis and acute myocardial infarction, respectively. Type II (fibrosis and fatty metamorphosis of the penetrating portion of

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... bundle) was found in 1 case. Type III (fibrosis with increased acid mucopolysaccharide of the branching portion of A-V bundle) was found in 4 cases with special relation to aging and degeneration of the central fibrous body. Type IV (necrosis of bilateral bundle branches) was found in 2 cases with fresh myocardial infarction. In general, degrees of histological damage and A-V conduction disturbances were fairly correlated. Etiological analysis showed ischemic necrosis (Type I, II and IV) in 4 cases, and degenerative changes (Type III) in the remaining 4. Degeneration of the branching portion of the bundle of His strongly suggested the similar etiological background with the "right bundle branch block with left axis deviation".