It's not uncommon for an owner of a particularly keen horse to affectionately say he has a “big heart.” But if that animal is a sport horse that completes intense workouts, he might, quite literally, have a huge heart.

Often called the “athlete’s heart,” an enlarged heart in a horse is often accompanied by murmurs and arrhythmias, said Rikke Buhl, PhD, exercise physiologist at the University of Copenhagen, in Denmark, at the 2014 International Society for Equitation Science (ISES), held Aug. 6-9 in Bredsten, Denmark. And while that might sound worrisome, Buhl said it’s still not clear whether athlete’s heart is actually related to the sudden equine deaths that sometimes occur during or just after exercise.

Scientists do know, however, that big hearts often mean big wins.

“Our studies actually proved what lay people have been saying for a hundred years: Horses with big hearts win more races than those with small hearts,” Buhl said.

Case in point: After 1973 Triple Crown winner Secretariat died in 1989, a post-mortem examination revealed that his heart weighed 10 kilograms (22 pounds; for comparison, the heart of an average 1,000-pound adult horse weighs about 10 pounds), approximately 2% of his entire body weight. Both breeding and training have something to do with that, Buhl said. Thoroughbreds are selected for speed, which is associated with genetically larger hearts, but “after a certain age the heart will only continue to grow if the horse is in intense training,” she said. The intense training leads to athlete’s heart.

Still, whether this big-heart syndrome might be related to sudden equine deaths remains a mystery, she added. In humans, increased heart size and other cardiac changes related to athlete’s heart appears to be associated with increased risk of sudden heart failure. But it’s important to remember that human hearts are quite different from equine hearts, Buhl said.

Human hearts are able to pump up their blood flow during exercise to about four or five times the regular output at rest, she explained. Horses, on the other hand, have an immense output capacity, reaching up to 16 times the resting rate.

“Certainly the horse is not the fastest animal in the world,” Buhl said. “But when it comes to cardiac capacity, it's just extraordinary compared to other mammals in the world.”

While high-profile horses like Harry Meade’s Wild Lone and Eric Lamaze’s Hickstead have made headline news due to sudden cardiac-related deaths, Buhl cautioned that we can't assume their heart failures resulted from training.

Leaking cardiac valves could be a source of concern, she said, as they can cause the blood to flow backwards into the heart. In a recent study, 19% of Standardbreds studied had leaking valves at age five; however, there was no correlation with their performance, Buhl said, so it appeared to have no negative effect on the horse.

[image imageid="4235" includeTitle="false" includeSummary="false"]An echocardiographic image of a 3-year-old Standardbred trotter's heart.[/image]

Cardiac arrhythmias might be dangerous, she added. But again, there seems to be little correlation with performance. Even so, a cardiac arrhythmia occurring in a ventricle appears to be “more suspicious,” she said. It’s possible that these arrhythmias are related to sudden death, but at this time that’s impossible to know since post-mortem examinations (hence, on a stopped heart) can give no information on past arrhythmias.

Currently Buhl’s group is studying the role of potassium channels in the heart, which might lead to a better understanding of the relationship between athlete’s heart and sudden death. However, conclusions from that research are still several years away, she said.

“Sudden deaths are a big concern, and they raise a lot of dilemmas for us in the industry–ethics, safety for the rider, safety for the horse, economic consequences, negative public relations for the equestrian sport," Buhl said. "Further research is necessary.”