Exercise intolerance often is first sign that a performance horse has an abnormal heart rhythm, said Kelsey A. Hart, DVM, PhD, Dipl. ACVIM, internal medicine clinician and graduate fellow in the department of large animal medicine at the University of Georgia, at the American Veterinary Medical Association meeting in Atlanta.

"It can be acute exercise intolerance where the horse is racing and all of a sudden just quits and can't race anymore," she said, “or it can be more subtle–the horse just isn't performing at the owner’s desired level or not moving up the levels like they would like."

One of the most common cardiac problems associated with poor performance is atrial fibrillation, an abnormal heart rhythm that involves the upper two chambers (the atria) of the heart. "A fib" occurs when the muscles in the atria quiver (or fibrillate) instead of having a coordinated contraction or heartbeat, explained Hart.

The heart rhythm in paroxysmal (spastic) A fib, which is often diagnosed in the racehorse that just quits racing, almost always returns to normal on its own. "It usually only lasts a day or two, and the horse will spontaneously convert back to sinus (regular) rhythm, and you don't need to treat this horse," she said