Veterinarians at the University of Wisconsin-Madison School of Veterinary Medicine believe they’re the first to use an oral drug to resolve a chronic case of atrial fibrillation (irregular heartbeats) in a horse. 
 
"It’s too early to tell if there are fewer side effects, but flecainide is much easier to administer than quinidine," says Dr. Åse Risberg, one of the veterinary school’s large animal internal medicine doctors. Quinidine, the drug used in horses to date, is given via frequent stomach tube placement or an indwelling tube. It’s a complicated process with significant side effects including depression and colic.

Flecainide is used routinely in humans with atrial fibrillations, but to Risberg’s knowledge has never been used orally to treat atrial fibrillation in horses. It has been administered intravenously to horses in Japan and England, but she has not seen any reports of flecainide use in the U.S. 

"We read about the drug in the veterinary literature and decided to try it," says Risberg. 

Her colleague, Dr. Sheila McGuirk, a large animal internal medicine veterinarian who is noteworthy for her knowledge of cardiology in large animals, notes that the prognosis for atrial fibrillation is dependent on the horse’s age, heart rate, and how long the animal has had the disease. Signs of atrial fibrillation include exercise intolerance at maximum performance. As a result, the condition is most commonly diagnosed in equine athletes, including racehorses, as well as performance and show horses. While heart problems in horses are not common, atrial fibrillation is the most common heart problem that affects performance