Abstract
Ilistopathological observation was performed mainly on the heart and cardiacnervous system in 8 horses affected with atrioventricular (AV) block. Main histopathologi-cat lesions emphasized on the 8 horses were as follows: (l) Focal myocardial fibrosis andedematous loosening and/or fibrous swelling of the walls of small and minute blood vessels, to be put forward microvascular alteration, in the posterolateral region of the rightatrium, as well as in the interatrial septum. (2) Microvascular alteration in the proximalportion of the AV junction, hydropic degeneration of the AV nodal fibers, and focalmyocardial fibrosis close to the AV node. (3) Lesions in the intrinsic cardiac nerves in theinteratrial septum, right atrium and AV junction. (4) Multifocal lesions in the extrinsiccardiac nervous system.Histopathogenetically, myocardial fibrosis should be regarded as a result of local circulatory insufficiency (ischemia) due to microvascular alteration. The following lesions mayalso have closely taken part in the occurrence of AV block through the hypofunction and/or circulatory insufficiency (hypoxemia or ischemia) in the tissue concerning impulse con-duction: myocardial fibrosis and microvascular alteration in the posterolateral region ofthe right atrium, as well as in the interatrial septum, and/or the microvascular alterationand hydropic degeneration of specialized fibers in the AV node, and organic disturbancesin the cardiac nervo.ts system.
