Exercise-Associated Cardiac Rhythm Disorders in Thoroughbred Racehorses

Abstract

Cardiac arrhythmias negatively impact horseracing through concerns for horse and jockey safety and welfare. The most clinically relevant rhythm disorders in Thoroughbred racehorses (TBs) are atrial fibrillation (AF), which typically causes poor performance, and fatal arrhythmias which precipitate sudden cardiac death (SCD). These rhythm disorders are also important in human athletes. The horse has a high prevalence of spontaneously occurring arrhythmias and is a large animal model for the athlete’s heart. Emerging evidence indicates that cardiac remodelling is not entirely benign and that changes elicited by high volumes of strenuous exercise can promote AF in humans. Atrial fibrillation in racehorses is most often identified post-race following a poor performance. The studies reported in this thesis sought to evaluate the association between strenuous exercise, cardiac arrhythmias and myocardial remodelling in TBs. A mobile based electrocardiography (ECG) device was validated for recording post-race cardiac arrhythmias in TBs. The device algorithm was accurate in identifying AF episodes. In horses with AF, abnormal, wide QRS morphologies were observed at heart rates exceeding 150 BPM. Such complexes are potentially deleterious and could suggest a potential link between AF and fatal cardiac arrhythmias. Outcomes for horses with AF were evaluated and AF occurred in 4.9% of horses during their career. In TBs, AF was predominantly paroxysmal (pAF).There was a high rate of recurrence of AF. A recurrent episode occurred in 23% of TBs following pAF and 64% of TBs following persistent AF. Race performance was substantially impaired on the day of AF detection. However, career duration and overall earnings were not reduced. Exercise induced pulmonary haemorrhage (EIPH) was the most important comorbid condition in horses with AF and a mechanistic relationship between these two conditions is suggested. Sudden cardiac death occurred three times more often during training exercise than in racing, which demonstrates that exercise intensity is not critically important in precipitating these events. In horses with SCD, post-mortem findings are commonly negative or equivocal, which makes definitive diagnosis of this condition challenging. Lifetime race starts and age were used as surrogate measures of exercise volume. Thoroughbred racehorses identified with both AF and SCD were younger than their racing peers, and therefore an association between exercise volume and cardiac arrhythmias was not found in this species. In both TBs with SCD and TBs euthanised for other injuries, myocardial fibrosis within the atrial and ventricular myocardium was increased compared to a sedentary control group. This indicates that regular volumes of exercise training are associated with myocardial fibrosis. Myocardial fibrosis might serve as a pro-arrhythmic substrate and partly explain the high prevalence of cardiac rhythm disorders in TBs. Emerging ECG technology allows for improved detection and understanding of AF in horses and could prospectively be used to introduce screening programs for horses entering training and to monitor horses throughout their careers. Post-mortem examination is crucial for the ongoing investigation of SCD in racehorses and the potential role of inherited disorders in this condition is worthy of further investigation.