Cardiac arrhythmias during intense exercise in Thoroughbred racehorses: frequency and association with subsequent race performance

BACKGROUND: The frequency, characteristics, and relationship with performance of arrhythmias occurring during high-intensity exercise in Thoroughbred racehorses remain poorly understood. HYPOTHESIS/OBJECTIVES: Describe the frequency, characteristics, and association with subsequent race performance of arrhythmias occurring during intense exercise. ANIMALS: Seventy-one racehorses competing in races sanctioned by the Hong Kong Jockey Club. METHODS: Risk-based case-control study. Single-lead ECGs were recorded during high-intensity trials using wearable devices. Ectopic depolarizations identified during warm-up, maximal exercise, and fast and slow recovery were characterized by frequency and morphology. Horses with ECG recordings obtained before a race were included. Race performance was classified as poor (finishing in the last 3 positions) or good (finishing in the first 3 positions). Associations between ECG variables and race performance were assessed using odds ratios. RESULTS: A total of 405 ECGs were recorded within 21 days before 1 or more race starts (82 poor- and 142 good-performance starts). Arrhythmias were detected in 73% and 81% of ECGs preceding good and poor performances, respectively. Ectopic beats during maximal exercise occurred in 57% of ECGs before good and in 70% before poor performances. The odds of performing poorly increased with each additional ectopic beat (odds ratio [OR], 1.15; 95% confidence interval [CI], 1.04-1.28; P = .01), and horses with any ectopic beat were nearly twice as likely to underperform (OR, 1.8; 95% CI, 0.86-3.81; P = .02). CONCLUSIONS AND CLINICAL IMPORTANCE: Ectopic beats are common and adversely associated with athletic performance. Electrocardiographic monitoring during intense exertion might enable detection of clinically important arrhythmias.