As researchers spend more time studying cardiac function in horses, they're learning more about how to diagnose common cardiac conditions like atrial fibrillation – but what does it mean if an equine athlete turns out to have the disorder?
“If you listen to your horse with a stethoscope and your horse has atrial fibrillation (“AFib”), the best analogy I've heard is it sounds like shoes in a dryer,” said Dr. William Gilsenan at Rood and Riddle Equine Hospital's client education seminar earlier this year. “You need an echocardiogram to confirm, but that's usually atrial fibrillation.”
AFib is characterized by a sometimes rapid, irregular heartbeat which can also be accompanied by shortness of breath and coughing. Some horses exhibit no symptoms at all and are only diagnosed when a veterinarian listens to the heart as part of a routine exam.
Gilsenan said there are two possible causes for AFib. One of them produces a temporary kind of AFib, most commonly studied in Standardbred racehorses, which is thought to stem from dehydration or electrolyte depletion. AFib can also occur as a result of an enlarged heart, particularly a heart with an enlarged left atrium. AFib can cause the mitral valve between the left atrium and left ventricle to function poorly, allowing blood to flow backwards during a heartbeat. The ventricles of the equine heart account for about 75 to 80 percent of the work involved in pumping blood during a heartbeat, while the atrium is responsible for significantly less. The reduced effectiveness of the mitral valve limits the amount of blood a heart can move during AFib, which in turn limits a horse's ability to work.
In Gilsenan's experience, horses with AFib will probably be ok at low to moderate levels of athletic work but the reduced blood pumping is more evident once they began working hard. Racehorses with AFib may begin a race without issue but peter out as it goes on and the heart cannot keep up.
“That's because, instead of running at 100 percent, they're only running at 75 or 80 percent,” he said.
Once a horse is diagnosed with AFib, Gilsenan suggests bringing in a cardiologist or internist to determine how much work is safe for that particular patient. An ultrasound of the heart can also determine whether it's safe for the horse to receive treatment with quinidine, a drug commonly used to treat the condition in humans.
“We really only want to use quinidine if the heart is structurally normal, and the reason for that is if there's abnormalities in the heart or leakages in the heart and you treat the horse with quinidine, they're probably going to go back into AFib eventually,” he said.
The longer a horse is in AFib before treatment, the more challenging it will be for a veterinarian to help the horse establish a normal rhythm or to maintain one.
“All those things being said, that horse may be able to regain their maximal cardiac output and go back to normal,” he said.
There are some horses for whom treatment isn't safe, however. Those with valve leakage or who have been suffering from AFib for a period of time (particularly older horses) may not be candidates for treatment. These horses can often stand light to moderate levels of riding but should only be exercised by adults with a full understanding of their condition because there is a slim chance the abnormal beats could happen together and cause sudden collapse.
“It's a pretty low risk of collapse, but the risk is still there,” Gilsenan said. “To assess that risk, the cardiologist may recommend an exercising EKG, which looks at the heart output during the intended level of work.”
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