The anatomical basis of complete atrioventricular block in cats with hypertrophic cardiomyopathy.

The cardiac conduction system was examined histologically in 13 feline cases of hypertrophic cardiomyopathy (HCM) with complete atrioventricular (AV) block. Marked degeneration and fibrous replacement of the AV conduction system were consistently observed in the combined regions of the branching portion of the AV bundle and the upper portion of the left bundle branch. These changes were associated with extensive fibrosis of the central fibrous body and endocardial and myocardial fibrosis in the upper border of the ventricular septum. Chondrometaplastic lesions or osseous lesions, or both, present in the base of the central fibrous body, compressed the underlying penetrating or branching (or both) portions of the AV bundle, causing apparent reduction of the conduction fibres. The pathological process and the nature and predilection sites of the lesions resembled those associated with ageing in human patients with complete AV block. It is possible that the pathological process in the cats was fundamentally related to the normal ageing phenomenon and may have been exacerbated by mechanical forces created by the cardiac hypertrophy associated with HCM.