Impact of Valvular Heart Disease on Performance

Results of a four-year prospective study designed to determine the influence of training and heart size on atrioventricular (AV) valvular regurgitation (backflow of blood from the lower to the upper heart chambers) in Thoroughbred racehorses, and to determine any association between AV valve regurgitation and performance in Thoroughbreds, were presented by Lesley Young, BVSc, PhD, DVA, DipECVA, DVC, MRCVS, of the Animal Health Trust (AHT) in Newmarket, England, at the 43rd British Equine Veterinary Association Congress.

She said the causes of poor performance in racehorses include:

  • Unreasonable owner expectations;
  • Lack of genetic ability;
  • Lack of enthusiasm;
  • Orthopedic problems (most common);
  • Respiratory disease (close second); and
  • Cardiovascular dysfunction (consider after above causes).

"How important is cardiac disease to racehorse performance?" she asked the audience. "It's believed to be the primary cause of, or a significant contributor to, poor or loss of performance in less than 6% of horses referred for performance investigation on the treadmill to AHT."

While that seems like an insignificant number of horses affected, all equine veterinarians know that many racehorses have cardiac abnormalities, though in the majority of cases these abnormalities do not affect the horse's performance.

Via cardiac auscultation (listening to the heart), confirmed by color flow Doppler electrocardiography (CFM) of the study horses, which ran in flat or National Hunt races, the researchers found murmurs of mitral valve regurgitation of various grades (backflow of blood through the mitral valve) in 7% of flat racehorses and 21% of fit chasers. They also found murmurs of tricuspid valve regurgitation in 12% of flat racehorses and 46% of chasers, while murmurs of aortic valve regurgitation are less common and were not present in this group of flat racehorses and in only  5% of the National Hunt group. In race-fit steeplechasers, significant (grade of murmur 2.5/6 or greater) mitral valve murmurs were associated with lower Timeform ratings, and moderate to severe mitral regurgitation imaged by Doppler echocardiography also resulted in lower win-to-run ratios.

In race-fit hurdlers, only aortic valve regurgitation decreased performance. Flat racehorses seemed to be unaffected by valve regurgitation or murmurs, but there were few flat horses with these problems, so the power of the study may have been limited, Young noted.

"Many affected horses are commercially successful racehorses," she commented. "It therefore seems very unlikely that compensated cardiac valve regurgitation has any noticeable effect on performance in horses engaged in less strenuous disciplines, unless secondary left atrial enlargement results in the development of atrial fibrillation (disorganized electrical conduction and pumping of the atria).

Training Effects
Young described findings of significant variation between horses at different stages of training in both probability and severity of a heart murmur. She also noted that murmur prevalence increases with age and training.

Regarding heart size changes, she stated that, "Racehorses respond to training like human athletes in power/endurance group--they show significant increases in left ventricular mass and mean wall thickness. However, there is no change in relative heart wall thickness (compared to width of the heart chamber), which is used to assess cardiac training adaptations to power or endurance exercise. Sprinters/power athletes tend to develop thicker heart muscles with minimal change to the width and volume of the heart chamber, while endurance athletes increase the heart chamber size with only moderate increases in wall thickness. Horses do a mixture of both just like human athletes like cyclists or middle distance runners.

"Valve regurgitation is clearly related to training in performance horses, and the high prevalence suggests that it's almost normal in some groups," she said. "It seems like all valves leak, some just a bit more than others! But regurgitation is not always physiological. It's sometimes progressive and associated with myxomatous valve degeneration (degeneration and deposition of mucin-like tissue within the valve). This distorts and thickens it, and makes it leak. In very severe cases, the heart gets bent out of shape by the leak and eventually the mechanical function of the heart can fail. This is very rare, though, and most horses tolerate even moderate valve dysfunction very well."

"Many horses with murmurs are successful racehorses," she stated. " As a result, you should be careful about attributing poor performance to valve leaks, as you might be easily misled."

Atrial Fibrillation
Atrial fibrillation (AF) is the most common cardiovascular cause of poor performance in horses, Young said, adding that it's present in about 1% of National Hunt horses and in about 2.5% of the general population.

"Horses' hearts are so big, they're an atrial fibrillation accident waiting to happen," Young commented. "Is this just another downside of two millennia of breeding for speed?

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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