Key result
Exercise period and RR interval associate strongly with QT interval and cardiac restitution ratio.
Why the study?
Sudden cardiac death is common in racehorses, but factors associated with the QT interval predisposing to fatal arrhythmias are unknown, and cardiac restitution has not been described during maximal intensity exercise.
What physiological, demographic, and pharmacological factors are associated with the QT interval and cardiac restitution ratio from rest to maximal racing effort in clinically normal Standardbred racehorses?
Observational (n=42)
What physiological, demographic, and pharmacological factors are associated with the QT interval and cardiac restitution ratio from rest to maximal racing effort in clinically normal Standardbred racehorses?
Automated cardiac restitution analysis is feasible in exercising Standardbred racehorses and reveals that the QT-RR relationship and cardiac restitution ratio are multifactorial across exercise periods, making simple formulaic QT correction inappropriate during equine exercise.
Answers are grounded in this paper and cite their sources.
Questions fixed QT corrections in equine exercise; extends restitution analysis to racehorses but leaves open clinical translation.
BACKGROUND: Sudden cardiac death is common in racehorses. Factors associated with the QT interval that could predispose to fatal cardiac arrhythmias are unknown. Cardiac restitution, expressed as a ratio of QT/TQ, has been used in humans to assess arrhythmia risk but has not been described in horses during maximal intensity exercise. HYPOTHESIS/OBJECTIVE: Describe factors associated with the QT interval and cardiac restitution ratio (CRR) in clinically normal Standardbred racehorses under race-day conditions. ANIMALS: Archival electrocardiograms from 42 Standardbred horses during live racing in Ontario. METHODS: Observational study performing an automated cardiac restitution analysis. Cardiac cycles were obtained from rest, non-race exercise, non-race recovery, live racing, and post-race recovery periods. Multivariable linear regression analysis was performed with both QT interval and CRR as outcomes of interest. RESULTS: Analysis of 3827 sequential pairs of cardiac cycles was performed. Exercise period and RR interval were highly associated with both QT interval and CRR. Other significant associations varied by exercise period and included: racing gait, sex, age, whether the horse received furosemide, and whether the horse experienced complex ventricular arrhythmias after racing. Interactions between gait and furosemide, and sex and gait were also significant. CONCLUSIONS AND CLINICAL IMPORTANCE: An automated cardiac restitution analysis is feasible in exercising racehorses. The QT-RR interval relationship is multifactorial, and there are numerous significant associations that must be considered to interpret changes in QT interval and CRR in horses.
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Avison et al. (2025) conducted an observational in Arrhythmia risk and exercise-associated sudden cardiac death in clinically normal Standardbred racehorses (n=42). Exercise period, RR interval, racing gait, sex, age, race-day furosemide, and post-race complex ventricular arrhythmias vs. Rest period and baseline reference categories across 3,827 sequential pairs of cardiac cycles was evaluated on QT interval and cardiac restitution ratio (CRR, QT/TQ) across five exercise periods (p=<0.001). Exercise period and RR interval were highly associated with QT interval and cardiac restitution ratio (p<0.001) across 3,827 cardiac cycle pairs in 42 Standardbred racehorses.
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