Pathologic and Electrocardiographic Findings in Sudden Cardiac Death in Racehorses.

Abstract

ABSTRACT. Five racehorses in apparently normal condition succumbed to sudden cardiac death (SCD) during or shortly after intensive training exercise. Cardiopathologic examination was performed. In 1 of the 5 horses, the use of an electrocardiogram (ECG) recording taken continuously for 440 sec enabled us to analyze some of the arrhythmias in the terminal event of SCD. The ECG tracing exhibited the R-on-T phenomenon following a pair of ventricular premature contractions (VPCs). The phenomenon rapidly degenerated into ventricular fibrillation, which led to cardiac arrest. In all 5 horses cardiopathologic examination revealed the following lesions: (i) foci of myocardial fibrosis in the right atrium located close to the sinoatrial (SA) node, (ii) fibrotic and/or fibroplastic changes in the upper portion of the interventricular septum, including the atrioventricular (AV) conduction system, and (iii) arterio-and arteriolosclerosis of the SA and AV node vessels. Pathogenetically, the process by which the focal lesions of myocardial ischemia secondary to vascular sclerosis progressed into fibrosis and/or fibroplasia could play a major role in the genesis of arrhythmias. Presumably the fibrotic and/or fibroplastic changes in the area of the AV bundle and its bundle branches are closely related to the onset of fatal ventricular arrhythmias such as VPCs, deteriorating into ventricular fibrillation. SCD in training and racing Thoroughbred horses appears to be due to arrhythmia.-KEY WORDS: arrhythmia, equine, heart, pathology, sudden death.J. Vet. Med. Sci. 61(8): 921-928, 1999 matter what conclusions have been drawn thus far about human SCD, the precise relationship between the prodromal signs (including fatal arrhythmias) and the pathologic condition is still controversial [3,5,8,9,11,23,26,36]. This paper reports the cardiopathologic changes found in 5 racehorses that had been in apparently normal condition when they succumbed to SCD or instantaneous death during or shortly after intensive training exercise. It also describes the terminal electrocardiographic events in 1 of the 5 animals. MATERIALS AND METHODSNecropsy of the 5 racehorses in this study (Table 1) was performed as soon as possible after SD, usually within 3 hr. Fortuitously, when horse 1 fell and lay down suddenly shortly after a training exercise, it was possible to record an electrocardiogram (ECG). For the ECG recording, a baseapex lead was used [29]; the positive electrode was positioned over the apex-beat area of the heart and the negative electrode over the cardiac base. The recording was performed continuously over the 400-sec terminal event of the SD, at a paper speed of 25 or 50 mm/sec. So that we could analyze the arrhythmias, the onset of the R-on-T phenomenon was set up as a 0-sec point, and the beginning point of the recording was calculated back to 370 sec (Table 2). For measurements of the Q-T interval, 5 successive intervals with sinus rhythm were randomly taken from the ECG tracing, mainly at a paper speed of 50 mm/sec. The corrected Q-...

Cardiac Valvular and Vascular Disease in Bull Terriers

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