When Do Heart Murmurs Affect Horse Performance?
With the heart being the engine that powers a horse around a cross-country course or a barrel pattern, any abnormality affecting its function can limit the animal’s athletic capacity—or worse. Heart murmurs are fairly common cardiac abnormalities in horses, yet trying to determine if and how they might affect a horse’s performance can be challenging.

Katharyn Jean Mitchell, BVSc, DVCS, DVM, PhD, Dipl. ACVIM (LAIM), walked practitioners through recognizing, investigating, and monitoring heart murmurs in athletic horses during the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee.

“Many horses can have quite impressive heart disease and perform completely normally,” said Mitchell, an assistant professor in the section of Large Animal Internal Medicine at Cornell University’s College of Veterinary Medicine, in Ithaca, New York.

When veterinarians identify a murmur on physical exam, therefore, they must consider several factors—including breed disposition, horse history, and signs of reduced performance—before deciding its importance.

Heart murmurs are sounds that occur between the normal heart sounds, caused by the physiological flow of blood or turbulence created by pathological (related to disease or damage) abnormalities, Mitchell explained. Veterinarians categorize murmurs on a scale of 1 to 6, with 6 being most severe. “Grades 3 and higher are typically more concerning,” she said.

When examining affected horses, veterinarians generally perform a physical, ultrasound, and electrocardiogram (ECG) exam. “The physical exam tells us what’s leaking, the ultrasound scan tells us what the consequences are, and the ECG tells you what happens electrically during exercise,” said Mitchell.

Common Murmurs

Mitchell listed types of heart murmurs and their potential effects on performance:

Mitral valve regurgitation This murmur is fairly common in equine athletes. It’s typically slowly progressive, she said, with the heart’s left atrium getting bigger over time, which can predispose a horse to atrial fibrillation (AF, an irregular heart rhythm or arrhythmia). “If AF develops, reduced athletic capacity and poor performance are common,” said Mitchell.

Signs that mitral regurgitation is affecting performance include a high resting heart rate, prolonged heart rate recovery post-exercise, and an irregular heart rhythm in conjunction with a left-sided systolic murmur.

Aortic valve regurgitation (AR) This generally slowly progressive murmur is common in older horses but rare and concerning in younger ones, said Mitchell. In affected horses the left ventricle can begin to remodel and enlarge to the point they can develop potentially life-threatening arrhythmias. The pulse pressure can increase so much you can feel a bounding pulse, which is worrisome, she said.

“Ventricular arrhythmias post the biggest concern over safety and as a cause of poor performance in horses with AR,” she said.

Affected horses might show signs of reduced performance, exercise intolerance, prolonged heart rate recovery after exercise, weakness, or collapse.

Ventricular septal defect The significance of this murmur depends on the size and location of the defect, the pressure gradient between the left and right ventricles, and any concurrent problems or cardiac abnormalities, said Mitchell. Horses with small defects can live and perform normally, while those with more severe defects might have poor performance or respiratory issues, weakness, and a predisposition for developing arrhythmias, she said.

Tricuspid regurgitation The significance of this murmur depends on the horse and its workload. “It’s a common finding in athletic animals (such as racehorses) and can worsen with training,” she said. “This is usually harmless and often considered a ‘normal’ adaption to training. It’s more worrisome (and occasionally indicative of pulmonary hypertension, or abnormally high blood pressure affecting arteries in the lungs and heart) in a pleasure horse.”

How To Assess Risk

Mitchell said she asks the following questions when determining whether a heart murmur poses a safety risk for horse and rider:

  • What job does the horse do? Is it a high-risk sport—over fences or at speed?
  • Who rides the horse? Is it a professional, amateur, child (is the rider willing or able to understand and accept the risk)?
  • What kind of heart disease does the horse have?
  • Has anything happened previously? “I’m going to be more risk averse if it sounds like something dangerous has happened before, or if the horse has a history of feeling a little wobbly or a little weak during exercise,” she said.
  • How risk-averse is the rider? “Our risk assessment is going to change depending on the person’s acceptance of the risk,” Mitchell noted.

Take-Home Message

Horses tolerate most common heart murmurs, such as valvular regurgitations, well, with little effect on their performance or life span, said Mitchell. Athletic horses with aortic regurgitation are more worrisome.

“As practitioners, the key is to recognize cardiac abnormalities on a thorough clinical examination, investigate those meeting the criteria for additional diagnostic evaluation, and then closely monitor the horse for any change in performance,” she said.