“It’s a pretty extraordinary thing when you think about it,” said Katharyn Mitchell, BVSc, DVM-PhD, Dipl. ACVIM-LAIM, assistant professor of large animal medicine at Cornell University, in Ithaca, New York. “The heart is a phenomenal pump and does great job of getting oxygen from the lungs into the blood and then into the tissues to allow them to work maximally.”

Mitchell described normal and abnormal heart function during a presentation on equine heart murmurs as part of Cornell’s Equine Seminar Series, held May 17.

The Horse’s Heart Anatomy

To understand how heart murmurs develop, it helps to understand how the heart works. Each side of the heart has two small upper chambers—the left and right atria—that operate like reservoirs storing blood, Mitchell explained. Each side also has two bottom chambers—the left and right ventricles—that are the pumps that move the blood to the lungs and body.

“The right side takes blood from the body into the right ventricle, which pumps blood into lungs,” she said. “Blood gets filled with oxygen, comes back to left side into the left ventricle, which pumps blood out into the body, where we get the oxygen delivered to our tissues. It’s a pretty complex system that has to work simultaneously, both sides together, to get the blood from the body to the lungs then back to the body.”

Between each chamber (atrium and ventricle) is a valve that stops the blood from flowing backward; these are the tricuspid and mitral valves. “In the big blood vessels going to the lungs or body, there are also heart valves—the aortic and pulmonary valves—that also stop blood from going back,” said Mitchell. “The idea is blood is always going forward.”

Electrical signals across the atrium tell the heart to contract. “When the signal goes haywire, that’s when we get an arrythmia—either not enough or too many beats or irregular beats,” she noted.

How Do You Know There’s a Problem With Your Horse’s Heart?

Mitchell listed factors that might set off alarm bells pointing to a heart murmur:

• Breed predispositions: Arabians, Friesians, Welsh ponies, and athletic breeds including Thoroughbreds and Standardbreds are most at risk.
• A history of abnormal findings: Has your veterinarian always said your horse has a heart murmur, or is it a new finding? This information can help him or her identify the reason for the murmur and its severity.
• Clinical signs: A lack of energy, prolonged recovery, change in performance, weight loss, and cough can all be side effects of murmurs.
• Not keeping up with pasture mates: Young animals that don’t grow or move as fast as other horses might have heart issues.

“Many horses can have very normal performance despite quite impressive heart disease,” said Mitchell. “The heart has a huge reserve capacity because we’ve bred them to be athletes. Just because we’ve found something wrong with your horse’s heart doesn’t mean it’ll have issues with performance.”

A thorough physical exam can paint a more detailed picture. You can help ensure your veterinarian can truly hear what’s happening inside that heart—that means no leaf blowing, sweeping, hosing, dog-barking, or talking while he or she listens. Mitchell said a horse’s body condition can also be an issue, as excess condition can muffle heart noises.

“It’s always important to listen to both sides of heart,” she added. “Things can happen on one side or the other that can tell you different problems. Look at subtle things that might change with the horse, such as muscle loss, breathing rate changes, the jugular veins starting to stick out, edema under the chest or abdomen or in the legs, weight loss, lack of energy. These are all subtle signs there’s a problem with the heart that needs to be checked out.”

Normal vs. Abnormal Sounds

A normal heartbeat has that “Lub dub, lub dub” sound. “Lub is the closing of the big valves, the mitral and tricuspid,” said Mitchell. “When they close, they make a little bit of noise. Dub is the closing of those smaller valves in the great vessels, the aortic and pulmonary valves. When you hear those normal sounds, the heart is closing those valves as it should.”

She said other heart sounds, such as “Balub dub” and “Lub dupduh,”—the noises you hear when blood fills in the heart—are also normal.

When blood flow becomes turbulent, that’s typically when you hear abnormal sounds. Mitchell likened it to standing by a peaceful, calm river versus loud rapids. “If those valves aren’t working or start to leak, it causes turbulent blood flow,” she explained.

Turbulent blood flow through the mitral or tricuspid valves might sound like “Lub shh dub,” with “shh” being the turbulent blood flow. A “Lub dub shhhhew” sound is a diastolic (during ventricular filling) murmur, where the aortic or pulmonary valve is now leaking and the blood is flowing back in the wrong direction, she said.

Leaks can occur because of damage to valves or degeneration—just like joints that get worn out with use, Mitchell said. Other causes include acute issues such as infection or inflammation.

Heart Murmur Implications

Different types of heart murmurs can mean varying consequences for the horse. Mitchell described these briefly:

Mitral regurgitation If the left atrium gets enlarged over time due to this defect, the horse’s chances of developing an arrythmia increase, she said, most commonly resulting in atrial fibrillation.

Aortic regurgitation With this aortic valve leak, the left ventricle becomes enlarged over time, potentially stretching and causing the mitral valve to start leaking, as well. “This is a more serious situation because you can end up with a couple of different problems,” said Mitchell. “We worry about horses with enlarged ventricles having collapse or sudden death when exercised.”

Tricuspid regurgitation Veterinarians see this murmur frequently, and their level of concern depends on the type of horse. It’s common in horses in intense training, such as racehorses and eventers, where, as the heart gets bigger, the valve starts to leak and you hear a murmur. This scenario isn’t as worrisome, said Mitchell, as it would be in a low-level horse. “A pleasure or pasture horse is not fit enough to have changes to the heart to make this murmur, so there’s something else going on—maybe with the lungs—that makes it harder for the right heart to pump through the lungs.”

Ventricular septal defect This is a congenital hole in the heart that veterinarians see most commonly in Arabians, Welsh Ponies, and Standardbreds, typically causing a loud murmur. Mitchell said it’s normal for newborn foals to have a heart murmur that typically disappears within the first two weeks of life. “If the foal is not acting normal and has a loud murmur beyond two weeks, this isn’t normal,” she added.

Why Examine the Horse’s Heart?

If your veterinarian picks up on a heart murmur, don’t ignore it. A heart exam that includes a resting (and possibly exercising or overnight) electrocardiogram (ECG), echocardiography (heart ultrasound), and blood pressure monitoring can help you determine whether:

• A heart issue is affecting the horse’s performance.
• The horse is suitable for sale.
• It’s safe to continue riding the horse.
• It’s safe to sedate the horse for certain procedures.
• The horse is at risk of developing heart failure.

An exam can also tell the veterinarian if the horse has structural issues like degenerative changes or congenital holes in the heart. It can identify arrhythmias, as well, which are “our biggest concern from a safety and performance standpoint,” said Mitchell.

Further, serial exams over months and years can indicate whether a murmur is stable or rapidly changing and how long until it potentially becomes a problem, she said. During a typical heart exam at Cornell, the horse comes into the clinic in the morning, spends a few hours undergoing procedures, then can return home that afternoon.

Take-Home Message

Mitchell emphasized that most horses tolerate heart disease well and have no outward signs of a problem. To assess risk, veterinarians can perform an ultrasound and ECG to determine how big the heart is and how it’s functioning.

“Monitor for changes over time,” she said, “because on one day we don’t see the whole story. If it stays the same six months later, you can be more confident the horse will be fine over a longer period of time.”