Atrial fibrillation is the most common heart arrhythmia in horses, and it can sometimes affect the performance of an equine athlete.
At the University of Illinois Veterinary Teaching Hospital in Urbana, Dr. Ryan Fries, a board-certified veterinary cardiologist, and Dr. Scott Austin, who is board-certified in large-animal veterinary internal medicine, work to treat horses with this condition.
What is ‘A fib’?
Atrial fibrillation, also called “A fib,” is an electrical disorder of the heart rhythm. The top chambers of the heart — the atria — do not contract properly, so efficiency of blood being pumped to the rest of the body is impaired. Instead, they kind of quiver, or fibrillate.
Efficient pumping is required for elite athletes. They cannot function at a top speed or performance without it.
“The most common clinical sign seen with A fib is a sudden change in performance,” Austin explains. “The horse may have a slower race time or not be able to push themselves as far as they usually do. This can occur in racehorses, working or jumping horses.”
Whenever owners notice a change in their horse’s performance or attitude, they should have it examined by a vet.
“A veterinarian will start with a physical exam,” Austin says, including “listening to the heart. An abnormal rhythm such as A fib can usually be detected this way.”
Veterinarians can confirm this finding by performing an electrocardiogram. As with humans, electrodes are attached to the horse so a machine can record the heart’s electrical activity and produce an image that can be interpreted.
Owners often wonder about the cause of A fib in their horse.
“At this time, we don’t know the exact cause,” Austin says. “Horses have a large heart with a comparatively slow heart rate, and this combination predisposes to a loss of electrical coordination.”
“The medical treatment for A fib is a medication called quinidine,” Fries says. “This medication is given to the horse by mouth in a series of four or five doses.”
Unfortunately, quinidine has several side effects, including swelling of the nose, laminitis (a painful inflammation of the tissue within the hoof) and even death. When no other underlying heart condition is present, the majority of horses given this treatment return to the correct heart rhythm. The horse will need to be rechecked and can be given the medication again if A fib returns.
Fries does not recommend any other treatment options that have been suggested.
“There are too many side effects and expenses associated with them and little evidence that they are effective,” he says.
The TVEC procedure
The other option for treatment of A fib is a procedure known as transvenous electrical cardioversion. Owners who elect this procedure are usually referred to a specialty hospital like the UI’s, because it requires coordinated care from specialists in cardiology, equine medicine and anesthesia.
“The goal of the TVEC procedure is to convert the horse’s heart back into its normal rhythm using controlled electrical pulses,” Fries explains.
First, the horse will have a full work-up done, including an electrocardiogram, an echocardiogram and bloodwork, to ensure it is otherwise healthy.
The procedure involves placing a catheter in the jugular vein, through which two small electrodes are advanced to the heart, where they deliver a shock to convert the rhythm. In humans, a similar shock can be administered on the outside of the body using paddles.
“A horse is much too muscular to able to use the outside approach,” Fries says.
Delivering the shock directly to the heart is very effective.
“There are only half a dozen to a dozen institutions in the U.S. that can perform this procedure, which requires special training to complete successfully,” Fries says.
He first learned this procedure during his residency and has been doing it ever since.
Risks and success
With any major procedure, there are some risks, including those associated with anesthesia, which is mitigated by the thorough examination to ensure the horse is otherwise healthy.
The procedure has a greater-than-95-percent success rate at converting the heart back to a normal rhythm. If the abnormal rhythm returns, it can be safely performed again in most cases.