Not all Equine Heart Abnormalities are Abnormal

Ever since the death of Olympic Champion Hickstead at a Fédération Equestre Internationale World Cup event on Nov. 5, there has been an increased amount of public interest in the secret lives of horse's hearts. Luckily, not all murmurs or rhythm abnormalities are career- or life-threatening.

During the 12th Congress of the World Equine Veterinary Association, held Nov. 2-6 in Hyderabad, India, Rikke Buhl, DVM, PhD, associate professor in the Department of Large Animal Sciences, Faculty of Life Sciences at the University of Copenhagen in Denmark, was scheduled to review some of the most common arrhythmias and murmurs in horses and indicate which ones could potentially influence athletic horses' performance. Unfortunately, illness kept Buhl from participating in the conference, but caught up with her shortly after to learn about cardiac abnormalities. Buhl began by reminding that blood flows through the heart and blood vessels in a "laminar" pattern, which means the blood flows in parallel layers with no interruption between them.

"When laminar flow is altered, turbulent flow ensues that causes vibrations of cardiac structures, such as the valves that exist between the different parts of the heart to ensure one-way flow of blood," she said in the post-Congress interview. "These vibrations can be heard by the veterinarian when auscultating (listening via stethoscope) a horse's heart. Some veterinarians can become so adept at auscultation they can pinpoint the exact cause of the murmur, but in many cases an ultrasound of the heart is necessary."

In racehorses murmurs stemming from the tricuspid valve (between the right atrium and right ventricle) are common. These are typically caused by the growth of the heart muscle during athletic training that causes the valves' individual "leafs" to not meet as perfectly as normal.

"This murmur seldom causes problems in athletic horses," advised Buhl.

Unlike tricuspid regurgitation, when the bicuspid or mitral valve (located between the left atrium and ventricle) isn't perfect, horses can show signs of heart disease, including decreased performance, reduced recovery after work, an increased heart and respiratory rate, and coughing.

"Increased musculature of the left side of the heart (like in tricuspid regurgitation) can contribute to this murmur; however, with moderate or severe leakage of the bicuspid valve itself, the valve is diseased and becomes degenerative and thickened," Buhl noted.

Like murmurs, arrhythmias (changes in heart beat pattern) are also common; fortunately most do not impact performance. Innocuous arrhythmias include sinoatrial block, atrioventriucular block, and sinus pauses that all slow down the heart rate and are diagnosed easily using an electrocardiogram.

"The most common and more important arrhythmia in athletic horses is atrial fibrillation (AF)," explained Buhl. "This is caused by an 'electrical remodelling' of the atrium that causes the atria to contract very rapidly and irregularly. As a result, blood pools in the ventricles and does not get ejected from the heart normally."

Thus, atrial fibrillation is a performance-limiting condition but in short-standing cases is treatable.

According to Buhl, "Both the administration of quinidine sulphate and a technique called 'electroconversion' are very successful in horses with an acute onset of atrial fibrillation (less than three months)."

She cautioned, however, that only horses with no other underlying heart conditions are candidates for therapy. Mitral regurgitation is commonly diagnosed in horses with AF, thereby precluding their conversion candidacy.

A full summary of Buhl's presentation will be available for free on the International Veterinary Information System.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.