Cardiopathological Observation on a Case of Paroxysmal Ventricular Tachycardia in a Thoroughbred Colt: Formal Pathogenesis

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A Thoroughbred colt with paroxysmal ventricular tachycardia was studied cardiopathologically. It revealed focal myocardial fibrosis in several regions of the heart: the posterolateral part of the right atrium, the interatrial septum near the orifice of the caudal vena cava, the atrioventricular (AV) junction region, the root of the anterior leaflet of mitral valve, the posterior papillary muscle and the apex, and multifocal loss of parenchyma in the cardiac nerves. Medial degeneration was also seen in the pulmonary artery and aorta. The myocardial fibrotic lesion showed edematous loosening and swelling of the walls of small and minute blood vessels (microvascular alteration). The alteration was severer qualitatively and quantitatively in the myocardial fibrotic lesion of the AV junction region than in the lesion of the other regions. All the cardiac valves were edematous and slightly fibrotic. Histopathogenetically, myocardial fibrosis should be regarded as an apparent ischemic change due to the microvascular alteration. This alteration seems to occur in close connec tion with cardiac nervous disturbances. The existence of lesions in the right atrium and interatrial septum may have effected a certain disorder in the impulse conduction. The existence of lesions in the AV junction region may have made a certain ectopic pacemaker.
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