Targeted horse husbandry and evolution has produced an elite equine athlete capable of attaining fast speeds and exercising over long distances. The heart and cardiovascular system is key to this capacity. The equine heart can go from a resting rate of 36 beats/minute to a maximal rate of nearly 240 beats/minute during racing.

The stroke volume of a horse's heart (the volume of blood pumped out of the left ventricle with each beat) can increase from 1,000 ml at rest to 1,700 ml at maximal exercise. And the all-important cardiac output—the volume of blood pumped per minute and required to oxygenate and power this incredible athlete (stroke volume x heart rate)—can go from 36 L at rest to a staggering 400 L at maximal exercise.

Studies differ as to the number of cardiac abnormalities present in the equine population. Researchers at the Department of Veterinary Clinical Sciences at the University of Sydney, New South Wales, Australia, looked at 846 racing Thoroughbreds in 2000.¹ They found that 686 of these horses (81.1 percent) had some type of heart murmur. Systolic murmurs over the heart base were by far the most frequent, most of which were heard best over the pulmonary valve area (43.1 percent). Murmurs over the tricuspid and aortic valves represented the next most common murmurs detected (28.5 percent and 27.4 percent, respectively). Murmurs over the mitral valve were identified in 3.8 percent of the horses, and diastolic murmurs were rare. Despite the high incidence of flow abnormalities in this study, the researchers concluded that most of the murmurs did not seem to be clinically important.

A somewhat similar study done in 2010 in the Department of Veterinary Clinical Sciences, Equine Internal Medicine and Sports Medicine Services at the Large Animal Veterinary Hospital of the University of Milan in Italy investigated the cardiovascular systems of 752 Standardbreds presented for poor performance.² Of these, 233 horses were found to have heart murmurs, and color-flow Doppler and echocardiography evaluations were performed in many of the cases. Most of the murmurs were due to tricuspid valve regurgitation, but mitral valve, aortic valve and pulmonary valve-related murmurs were also identified.

No difference seen in performance

A team of researchers at Specialist Equine Cardiology Services in Suffolk, U.K., looked at 526 fit Thoroughbreds that were either jump or flat racing in 2008.³ The researchers were most interested in the possible association between murmurs and athletic performance, which was largely unknown. After evaluating the horses and correlating their conditions to racing performance, the researchers concluded that, essentially, horses with murmurs didn't seem to perform any differently from those without murmurs.

These findings suggest that the average practitioner is likely to encounter cardiac murmurs in practice, but their significance and relevance may be of less importance than previously thought. These points were well-illustrated at a recent Fédération Equestre International endurance competition held at Biltmore, Ashville, N.C. Of the nearly 200 equine starters, event veterinarians identified six horses with cardiac murmurs, but none of these competitors had problems with performance, and all finished normally.

The lack of association between the number of murmurs heard in performance horses and evidence as to their negative effect has led to a number of studies seeking the significance of equine heart murmurs. There's no debate that the common murmur of mitral valve regurgitation can be serious, performance-altering or even life-threatening if the defect is severe enough. Aortic valve insufficiency eventually can lead to heart failure in older horses. But time and again horses are identified as having significant murmurs without any apparent effect on performance. How should a clinician evaluate such a situation, and what advice/council should a client be given?