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.

 When trying to diagnose a heart murmur, the practitioner should evaluate the following characteristics:

Grade or Intensity--Murmurs are graded on a six-point scale. Grade 1 murmurs are only heard after careful auscultation and are usually short in duration. A grade two murmur is heard if the stethoscope is placed over the spot where the murmur has the highest intensity. Grade three murmurs are easily detectable. Beginning at grade five, a thrill is heard, and a grade six murmur can be heard when the stethoscope is not even touching the horse. In older horses which are not suffering from some other disease, murmurs of grade three or higher often relate to cardiac pathology, mainly valvular degeneration of the mitral or aortic valves.

Location in the Cardiac Cycle--Determining when the heart murmur occurs during the cycle can help determine where in the heart there is a problem. Mitral valve regurgitation produces a systolic murmur, aortic valve regurgitation a diastolic murmur.

Duration--This can also help give the practitioner more information as to what type of murmur he or she might be hearing. Pathological murmurs typically occupy all of systole or diastole.

Shape--This refers to the pattern of intensity change over time.

Character--Sometimes called pitch, this is a function of the pattern of frequencies within a murmur. Some murmurs have a harmonic pattern and can be described as honking (short with medium pitch), buzzing (longer with a low pitch), and musical (longer with a high pitch). Musical and buzzing harmonic murmurs are most likely related to aortic regurgitation, the abnormal back flow of blood through an incompetent aortic valve into the left ventricle. Honking murmurs seem to be equally split between aortic and mitral valve regurgitation. Those that are not harmonic are usually referred to as blowing. The majority of mitral valve regurgitation cases and about half the cases of aortic regurgitation are blowing in character, according to Naylor.

With these characteristics in mind, a practitioner can then begin to determine the type of murmur and find the underlying cause.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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