Cardiopathological Changes in Racehorses with Abrupt Deceleration of Speed during a Race

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.

Pathologic and Electrocardiographic Findings in Sudden Cardiac Death in Racehorses.

“…On the other hand, no relation has been demonstrated between AF and SCD in the horse [10,29]. We have found that horses that have recovered from paroxysmal AF frequently reveal exercise-induced VPCs on the radiotelemetric ECG [22]. By contrast, apparently healthy horses without a history of AF have no arrhythmias during exercise [35].…”

“…In particular, the occurrence of paroxysmal AF causes the horse to slow down during the race [2,22]. It is known that human AF can precipitate heart failure, angina and ventricular arrhythmia, and increases cardiovascular mortality and the rate of SCD [7,34].…”

“…We have made some observations on cardiac pathology in Thoroughbred horses under 10 years old that were affected with various arrhythmias, including atrial fibrillation (AF) [1,[16][17][18][19]. These cardiopathologic studies of arrhythmias have been used as the basis of a proposed correlation between the pathogenesis of arrhythmia and SCD in the racehorse [20,22]. However, as in the case of arrhythmic death in the athlete [26], we have had difficulty proving conclusively that equine SD is definitely caused by an arrhythmia.…”

“…However, as in the case of arrhythmic death in the athlete [26], we have had difficulty proving conclusively that equine SD is definitely caused by an arrhythmia. No Histopathologic examination of the 5 horses focused on the hearts in the same manner described in our previous reports [16][17][18][19][20][21][22]. Special care was taken to retain the correct sequence of tissue block during histologic processing so that both the topography of the conduction system and the arterial supply of the sinoatrial and atrioventricular (SA and AV) nodes could be reconstructed from the previous reports [17,20,21].…”

Cardiac Lesions in Dairy Cows with Idiopathic Atrial Fibrillation.

“…Moreover, the anatomical structure of the right atrium may be such that its upper portion of the anterolateral and medial regions would be most affected by this increased hemodynamic load and would develop most lesions. Interestingly, such location of myocardial fibrotic lesions in the right atrial wall has also been observed in racehorses with AF [1,19].…”