Atrial Fibrillation in Horses

Up to 4% of Standardbred racehorses suffer from cardiac arrhythmias known as atrial fibrillation. That’s eight times more than the general horse population. While results from a recent study confirmed that it’s hereditary in the breed, researchers looked at the extent to which atrial fibrillation affects heart function and performance. And the findings, they say, are concerning.

“I was extremely surprised, and actually also quite scared, about the very high heart rate and abnormal cardiac beats observed on the electrocardiogram (ECG),” said Rikke Buhl, DVM, PhD, a professor (mso) at the University of Copenhagen’s Department of Veterinary Clinical Sciences, in Taastrup, Denmark.

“Previously I hadn’t been too worried about people riding or driving horses suffering from atrial fibrillation, but after this study I am more concerned,” she said.

Buhl and colleagues carried out their groundbreaking performance study by working with a group of nine healthy recently retired Standardbred racehorses. They exercised the horses on a treadmill to evaluate their cardiac function and performance. Then, they surgically implanted pacemakers into the horses to induce atrial fibrillation.

The researchers repeated the treadmill test after pacemaker implantation, inducing atrial fibrillation in six horses. They did not induce atrial fibrillation in three horses, which served as a control group.

When they compared the two tests, the researchers said the results “confirm that atrial fibrillation is detrimental to athletic performance.”

Atrial fibrillation is “characterized by decreased velocity, an increase in heart rate to an extent far above the normal accepted maximum heart rate, and decreased V200 (a cardiac velocity measurement representing the cardiac capacity of the horse during exercise),” they stated.

Specifically, the researchers found a 12% reduction in velocity in horses with atrial fibrillation, which, when compared with human studies, probably corresponds to a 15% reduction in exercise capacity, she said. What’s more, they noted maximum heart rates as high 346 beats per minute in the study group, compared to a healthy exercising heart rate of around 200-240.

Inducing cardiac disease in healthy horses was a necessary step toward understanding the disease and its effects, Buhl said. The horses’ owners donated them to the scientific project with full knowledge of what would occur during the two-month study period, she said.

“There are ethical considerations for us as researchers to consider,” she said. “We believe it is relevant to study the disease in these horses as it will help other horses in the future. Our study was only minimally invasive, and the horses were fully pain-medicated in the days after the pacemaker implantation (which was conducted with local analgesia while the horse were sedated). The horses were taken to the field every day, groomed daily, and treated very well, so I believe they had a good and unstressful life at our place.”

But, such a study design allowed the scientists to clearly see the effects of atrial fibrillation, she said. And this allowed the team to collect data that had not been accessible before.

“It seems we are the first to measure and quantify performance before and after atrial fibrillation and thereby give a measure of the reduced performance capacity,” Buhl said.

“The results are disturbing,” she continued. “Based on what we’ve seen, I now always recommend several exercise tests for horses, equipped with ECG readers, before I recommend riding or driving the horse suffering from atrial fibrillation.”

The study, “Effect of induced chronic atrial fibrillation on exercise performance in Standardbred trotters,” was published in the Journal of Veterinary Internal Medicine.