The equine heart: Murmurs in performance horses

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.

But for all of its amazing physiology, the equine heart is a pump and is susceptible to various problems and conditions that can affect athletic performance. The most common problems in exercising horses are arrhythmias and murmurs. A working knowledge of each is required for veterinarians dealing with performance horses. In this article, I concentrate on murmurs (see "What about equine arrhythmias?").

Most murmurs don't seem clinically important

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?

Client education needed

Dr. Marianne Sloet and colleagues at the Department of Equine Sciences of the University of Utrecht in the Netherlands looked at a number of horses during a five-year period and assessed the murmurs that were found at prepurchase examination or at long distance rides.4 They found that only 10 percent of the horses that had heart murmurs experienced reduced performance but that the presence of murmurs still affected the sale price of many of the horses.

During the study period, Sloet's team found murmurs in 62 horses at prepurchase examinations and in 15 horses competing at distance rides. Complete cardiac examinations were performed on these horses. Seven percent of the murmurs were loud, and there was good correlation between heart auscultation with a stethoscope and more in-depth echocardiography results in 68 of the 77 horses. Mitral valve problems were found in 63 horses, and tricuspid valve issues were noted in 40 (some horses had multiple problems).

The researchers did find that the left atrial diameter (LAD) was the most important feature to predict future performance. The heart's left atrium increases in size as blood leaks into this chamber through a compromised mitral valve. Over time, the wall becomes thicker so it can try to fully eject the volume of blood building up in the atrial chamber because of back pressure and a leaky valve. LAD in normal horses should be less than 14 cm. If LAD is normal in the presence of a heart murmur, then the flow problem is being well-tolerated by the equine heart and shouldn't cause performance concerns. If the LAD is increased, the heart is adapting to the pathology in the valves, and the horse must be removed from performance and carefully monitored.

Forty-three percent of the horses in Sloet's study sold for the original asking price, despite being identified as having a murmur; 21 percent sold for a lower price because of the presence of a murmur, and the sale was terminated in 36 percent of these cases. The team concluded that the perception of heart murmurs was such that clients viewed their presence negatively, despite overwhelming evidence that less that 10 percent of horses with murmurs demonstrate clinical significance.

Equine clinicians are in a unique position to be able to educate clients and horse owners as to the possible significance of murmurs and to perhaps clear up some of the confusion as to their importance.

Dr. Marcella is an equine practitioner in Canton, Ga.

REFERENCES

1. Kriz NG, Hodgson DR, Rose RJ. Prevalence and clinical important of heart murmurs in racehorses. J Am Vet Med Assoc 216(9):1441-1445.

2. Zucca E, Ferrucci F, Stancari G, et al. The prevalence of cardiac murmurs among standardbred racehorses presented with poor performance. J Vet Med Sci 2010;72(6):781-785.

3. Young LE, Rogers K, Wood JL. Heart murmurs and valvular regurgitation in thoroughbred racehorses: epidemiology and associations with athletic performance. J Vet Intern Med 2008;22(2):418-426.

4. Conference on equine sports medicine and science, Saumur, 2002.