A NEW international study has offered one of the clearest insights yet into the possible cardiac events behind one of racing’s most devastating outcomes: the sudden collapse and death of an apparently healthy horse during or shortly after exercise.

Known as exercise-associated sudden death (EASD), these tragedies are estimated to account for around a quarter of racing fatalities. They are rare, but when they happen they are shocking, often unexplained, and deeply distressing for all involved.

Now researchers analysing ECG recordings from thoroughbred racehorses in four major racing jurisdictions say two patterns stood out: previously undetected atrial fibrillation (AF) and abnormal heart rhythms appearing during the later stages of recovery after fast work.

Available recordings

The team reviewed cases from Australia, France, Britain and the United States, identifying 11 thoroughbred horses that suffered EASD. They then matched those horses with available recordings from wearable ECG technology used in training.

In total, 24 ECG recordings were assessed, taken during fast exercise and recovery periods in the days, weeks or months before death.

Although the numbers are small, the findings are significant, because opportunities to capture real-time heart rhythm data in these cases are exceptionally rare.

One of the strongest messages from the study was the frequency of atrial fibrillation (AF). AF is an irregular rhythm caused by disorganised electrical activity in the heart’s upper chambers. It is recognised in horses and can sometimes occur without obvious outward signs.

Three of the four horses wearing ECG devices on the day they died were already in AF before exercise began. A fourth horse was found in AF nine days before death. That does not mean every horse with AF is at immediate risk, but it does raise an important welfare question: should more horses be screened before high-intensity exercise?

The authors of the research published in April this year by Navas de Solis et al, Equine Veterinary Journal, suggest the answer may be yes.

In human sport, cardiac screening has helped reduce exercise-related sudden deaths. The researchers believe there may be scope for similar preventative thinking in equine sport.

They note that atrial fibrillation can often be detected by:

In practical terms, that could make routine checks more achievable in training yards than ever before.

For trainers and owners, the message is not panic, it is awareness.

Abnormal rhythms

The second pattern of interest was abnormal rhythms appearing not during the gallop itself, but several minutes afterwards.

In one horse, heart rhythm disturbances became progressively more serious during the late recovery phase before deteriorating fatally. Similar late-recovery rhythm changes were also seen in several earlier recordings from other horses.

That matters because cooling down is often seen as the safer part of exercise. This research suggests the recovery window may deserve closer attention.

The authors stress more work is needed before these rhythms can be reliably linked to risk, but they may prove an important part of future monitoring.

This was a small case series, and the researchers are careful not to overstate the conclusions. It does not mean all irregular rhythms are dangerous. Many horses can show transient rhythm changes during training without ever suffering clinical problems. It also does not mean every sudden death can be predicted. However, it does suggest that some warning signs may be detectable in advance, particularly AF present before fast exercise.

For a sport increasingly focused on horse welfare, data-led prevention matters. If relatively simple screening can identify horses that need further veterinary investigation, then the benefits are obvious:

Sudden death in any horse sport is emotionally charged and scientifically complex. But progress often starts with better evidence.

This study moves the conversation forward by showing that wearable technology may help reveal silent cardiac risk factors that would otherwise remain hidden. For racing, perhaps the challenge now is how to turn that knowledge into practical prevention?