A new international study has shed light on the role of heart rhythm abnormalities in exercise-associated sudden death (EASD) in Thoroughbred racehorses, highlighting the potential for earlier detection and prevention.
Published in the Equine Veterinary Journal, the research analysed electrocardiograms (ECGs) from horses that suffered fatal collapse during or shortly after exercise, a condition thought to account for up to 25% of racing fatalities.
Researchers examined 24 ECG recordings from 11 horses across major racing jurisdictions, including the UK, USA, France and Australia. In several cases, horses were wearing ECG devices at the time of death, offering rare insight into the cardiac events leading up to collapse.
Atrial fibrillation identified as a key risk factor
One of the most significant findings was the prevalence of atrial fibrillation (AF), a common but often undetected heart rhythm disorder.
- Three of the four horses monitored at the time of death were in AF throughout exercise
- In these cases, AF progressed into dangerous arrhythmias and ultimately cardiac arrest
- Another horse showed AF days before death
Overall, 17% of ECG recordings showed AF, and 70% revealed some form of arrhythmia.
The authors suggest that AF, particularly when present but undiagnosed, may play a far greater role in sudden death than previously recognised.
Warning signs may appear before collapse
The study also identified abnormal heart rhythms in the days and weeks leading up to fatal events.
In particular, arrhythmias during the late recovery period after exercise emerged as a recurring pattern. Some horses showed increasingly complex rhythm disturbances prior to death, even when earlier exercise phases appeared relatively normal.
These findings raise important questions about whether routine monitoring could help identify horses at risk before a catastrophic event occurs.
Screening could help reduce fatalities
The researchers conclude that improved cardiac screening, particularly for atrial fibrillation, could be a key step in reducing EASD.
Wearable ECG devices, now increasingly used in training, may offer a practical way to detect abnormalities earlier. The study also highlights the potential role of emerging tools such as smartphone-based ECGs and AI-driven rhythm analysis.
However, the authors caution that more research is needed, particularly to better understand which arrhythmias carry the greatest risk and how they should influence training or racing decisions.
Small study, but important insights
As a retrospective case series involving a limited number of horses, the study cannot yet define clear risk thresholds. But it provides rare, real-world data from fatal events and strengthens the growing evidence linking cardiac rhythm disturbances to sudden death in racehorses.
With EASD occurring far more frequently in horses than in human athletes, the findings underline the importance of continued research and the potential welfare and safety benefits of better cardiac monitoring.
