Horses

Atrial fibrillation is the most common heart arrhythmia in horses, and it is one that sometimes affects performance for the equine athlete.

By Beth Mueller

Atrial fibrillation is the most common heart arrhythmia in horses, and it is one that sometimes affects performance for the 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 together to treat horses with this heart condition. Transvenous electrical cardioversion is one treatment option.

What Is A fib?

Atrial fibrillation 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 of the heart is required for elite athletes. They cannot function at a top speed or performance level without it.

“The most common clinical sign seen with A fib is a sudden change in performance,” Dr. Austin explained. “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. Sometimes, a nosebleed also can be seen.

Diagnosis

Whenever they notice a change in performance or attitude, owners should have their horse examined by a veterinarian.

“A veterinarian will start with a physical exam to determine any abnormalities and what could be causing the clinical signs,” Dr. Austin said. “This examination will include 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 on the horse. As in ECGs for people, electrodes are attached to the horse so a machine can record the electrical activity of the heart and produce an image that can be interpreted as a certain rhythm.

Horse owners often wonder what could have caused A fib in their horse.

“At this time, we don’t know the exact cause of A fib in horses,” Dr. Austin said. “Horses have a large heart with a comparatively slow heart rate, and this combination predisposes to a loss of electrical coordination.”

Treatment Options

“The medical treatment for A fib is a medication called quinidine,” Dr. Fries said. “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, acute laminitis (a painful inflammation of the tissue within the hoof) and even death. When no other underlying heart disease is present, the majority of horses given this treatment will respond by returning to the correct heart rhythm. The horse will need to be rechecked periodically and can be given the medication again if the A fib returns.

Dr. Fries does not recommend any of the other medical treatment options that have been suggested for A fib.

“There are too many side effects and expenses associated with them, and very little evidence that they are effective,” he said.

TVEC procedure

The other option for treatment of A fib is a procedure known as TVEC, which stands for transvenous electrical cardioversion. Horse owners who elect to go with this procedure usually are referred to a specialty hospital such as the Veterinary Teaching Hospital because this procedure 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,” Dr. Fries said.

First, the horse will have a full work up done, including an ECG, an echocardiogram and bloodwork, to ensure the patient is otherwise healthy. The procedure involves placing a catheter in the jugular vein of the horse. Through this catheter, Dr. Fries is able to advance two small electrodes to the heart and position them appropriately. A shock is administered to the heart 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 and still reach the heart,” Dr. Fries said.

Delivering the shock directly to the heart in a horse 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,” Dr. Fries said.

He first learned this procedure during his residency and has been doing it ever since. He will be passing on this knowledge to the cardiology residents in training at the Veterinary Teaching Hospital as well.

Risks and Success

With any major procedure, there are some risks. There is the risk associated with general anesthesia, which is mitigated by the thorough physical examination to ensure the horse is otherwise healthy. The potential for bleeding during the catheter placement also poses some risk. This complication occurs less than 2 percent of the time, and the anesthesia team closely monitors the patient for any changes during catheterization.

TVEC has a greater than 95 percent success rate at converting the heart back to a normal rhythm. If the abnormal rhythm returns at some point, the procedure can be safely performed again in most cases.

The specialists at the Veterinary Teaching Hospital offer a wide range of cardiac care services for horses. If you have questions about the cardiology of large animals, contact your local veterinarian.

Beth Mueller is a Doctor of Veterinary Medicine candidate in the Class of 2021 at the University of Illinois at Urbana-Champaign.