The Anatomical Basis of Bradycardia–Tachycardia Syndrome in Elderly Dogs with Chronic Degenerative Valvular Disease

The hearts of seven elderly dogs in which bradycardia–tachycardia syndrome (BTS) had been diagnosed electrocardiographically were examined post mortem. The clinical basis of the underlying heart disease was invariably mitral or mitral and tricuspid regurgitation. Microscopical examination of the sinoatrial (SA) node and the SA junctional region consistently revealed depletion of SA nodal cells, with a corresponding increase in fibrous or fibro-fatty tissue that interrupted contiguity between the SA node and the surrounding atrial myocardium. The left and right atrial walls showed an increased amount of fibrous tissue in the myocardium and disruption of the muscle bundle architecture (interstitial myocardial fibrosis) to varying degrees. Qualitatively, these changes in the SA node and the SA node region resembled those associated with ageing in elderly people with or without BTS. Thus, it is possible that the pathological process affecting the SA node in these dogs was fundamentally related to ageing and may have caused BTS, in combination with atrial myocardial lesions caused by mitral and tricuspid regurgitation.