Abstract
Five racehorses were selected at the time of necropsy during a 4-year period involving about 500 thoroughbred horses. The 5 horses suddenly weakened during races and trailed in. Electrocardiographically, 3 of the 5 animals exhibited paroxysmal atrial fibrillation (AF) at rest, shortly after the races. Ten days to three months later, the 5 revealed exercise-induced ventricular premature contractions (VPCs) on the radiotelemetric electrocardiogram. Cardio pathologically focal lesions of myocardial fibrosis were observed in each of the horses. The fibrotic lesions in all 5 cases were located in a vicinity of the sinoatrial node with the anter o lateral region of right atrium. Fibroplastic lesions also were observed in the atrioventricular junction including the upper portion of the interventricular septum. In addition, there were sclerotic changes of the intramyocardial microvasculature within and around both the fibrotic and fibroplastic lesions. Cardiopathologically, there were no absolute criteria for differential diagnosis to make the 3 horses with paroxysmal AF from the 5 with exerciseinduced VPCs. However, the fibrotic and fibroplastic lesions secondary to myocardial ischemia might play an important role, presumably in creating a suitable background for the initiation of ectopic arrhythmias via automaticity and reentry mechanism, leading to an arrhythmogenic condition that gives rise to paroxysmal AF associated with exercise-induced VPCs.
