Studies show that more than 80% of racehorses have cardiac (heart) murmurs, according to Jonathan Naylor, MRCVS, PhD, Dipl. ACVIM, Dipl. ACVN, of the Department of Large Animal Clinical Sciences at Western College of Veterinary Medicine in Canada. Naylor presented "How to Diagnose Aortic and Mitral Regurgitation in Older Horses: A Case-Based Approach" at the 2002 American Association of Equine Practitioners' (AAEP) convention. He described the various characteristics of heart murmurs and how to diagnose them.

Despite this high incidence of murmurs in racehorses, they are very hard to diagnose. Heart murmurs are usually low-frequency sounds at the lower limit of human hearing which are difficult for the human voice to reproduce accurately. In the past, the ability to describe and interpret heart sounds has been limited by the lack of a bank of standard sounds or comparisons of types of murmurs produced by different lesions. During one study, the ability to diagnose the underlying cardiac problem causing abnormal heart sounds was only 54%, 33%, and 29% for diplomates of the American College of Veterinary Internal Medicine, equine practitioners, and undergraduates, respectively. In another study, veterinarians used a wide variety of different names to describe the same murmur, indicating a lack of consistency in nomenclature and possibly in comprehension.

Heart murmurs are caused by turbulent blood flow, said Naylor. "This produces vibrations that are transmitted through the thoracic wall, where they can be detected as sound waves, or in very severe cases, as a thrill (a series of tremors or vibrations)." Naylor said the use of standard words to describe heart murmurs can help practitioners communicate more effectively with one another. The recent introduction of computer-based texts–such as CDs of heart sounds with descriptions and interpretations–should improve understanding and diagnosis.

Not all heart murmurs are caused by structural damage to the heart. Sometimes an underlying condition can be stressing the cardiovascular system, resulting in a murmur. If the condition is fixed, then the heart murmur might go away or decrease in intensity