Atrial Fibrillation in the Horse: Clinical and Histopathological Studies of Two Cases II. Formal Pathogenesis

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Histopathological studies were performed on two racehorses (Case 1 and 2) with atrial fibrillation. In Case 2, atrial fibrillation had been removed by treatment with quinidine sulfate. Each animal revealed focal myocardial fibrosis in the right atrium, multifocal loss of parenchyma in the cardiac nerves, and medial degeneration in the pulmonaryartery and aorta. Lesions of myocardial fibrosis showed an alteration of small and minute blood vessels (microvascular alteration). Affected arterioles were often accompanied by perivascular edema and stenosis. The microvascular alteration was severer qualitatively and quantitatively in untreated Case 1 than in treated Case 2. In Case 1, subacute endocarditis was seen in the mitral valve with fibrosis and edematous thickening. Histopathogenetically, it is considered that the focal myocardial fibrosis may have been brought about by a local circulatory disturbance (ischemia) due to microvascular alteration which may have occurred in close connection with cardiac nervous disturbances. The presence of qualitatively and quantitatively severe microvascular alteration should be taken seriously as a pathogenic factor of atrial fibrillation.
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