THE TERM �murmur� describes an additional or unusual sound heard during auscultation of the heart with a stethoscope. It is not unusual to hear these sounds and surveys suggest that murmurs are present in as many as 60% of horses.

THE TERM murmur' describes an additional or unusual sound heard during auscultation of the heart with a stethoscope. It is not unusual to hear these sounds and surveys suggest that murmurs are present in as many as 60% of horses.

Fortunately, the majority of murmurs are innocent or functional in nature, and reflect the normal flow of blood through the heart. In some cases, however, they are caused by abnormalities of the heart and major blood vessels. It is these so-called pathological murmurs that are potentially associated with poor performance and ill health.

The challenge to us vets is to decide whether a murmur is functional or pathological in origin, and this can be a particularly onerous task when a murmur is detected during a pre-purchase examination. With careful clinical assessment, and the judicious use of specialist tests, it is usually possible to predict the likely significance of a murmur and these criteria are discussed in this article.

Using a stethoscope, a horse's heart can be heard on both sides of the chest, under and around the area of the elbow. The normal equine heart beat consists of two loud sounds, which are best described by saying out loud "lub-DUB".

Two other, much quieter sounds can also be heard in some horses. Together these four sounds represent the closing of valves within the heart as blood moves through the heart.

Murmurs typically sound like a whooshing or swishing noise. They can range in intensity from very faint to very loud, and can vary in pitch from low to high. The character of the murmur may change within each heart beat, for example rising in pitch to a crescendo and then dying away. These features are important since they give clues to the likely origin of the murmur.

The intensity or loudness of a murmur is graded on a scale of 1 to 6, where 1 represents a murmur that can only be heard after careful auscultation of a localised area. Grade 2 murmurs are heard as soon as the stethoscope is placed on the appropriate part of the chest, where grade 3 and 4 murmurs are described as moderately loud and loud, respectively.

A grade 5 murmur is a loud murmur that is accompanied by a palpable pulsation called a precordial thrill. Grade 6 murmurs are sufficiently loud that they can still be heard when the stethoscope is lifted just off the chest surface.

Other key characteristics of murmurs are their timing within the heart beat, especially in relation to the major heart sounds, and the location on the chest wall where they are loudest. Murmurs may be audible on the left, the right or both sides of the chest, and can be louder high up towards the shoulder or low down by the elbow.

Several criteria are typical of innocent or functional murmurs. They are usually of low intensity - grade 1 or 2 - are short in duration, with a soft pitch, and are localised to a small area. These murmurs can come and go, being present some days and not others.

Functional murmurs can be heard on both sides of the chest, which is contrary to the long-held assumption that all murmurs detected on the right side of the chest are significant in nature. Another widely-held belief is that murmurs that disappear with exercise are always innocent in nature.

Yes, it is true that many functional murmurs do disappear as the heart rate increases with exercise. However, there are other functional murmurs that become more obvious with exercise.

The situation is also complicated by the fact that some pathological murmurs may be quieter after exercise. The effect of exercise is perhaps a poorer guide to the significance of murmurs that the rule of thumb enshrined in veterinary folklore.

Pathological murmurs are caused by structural changes in the heart, such as leaky valves, narrowing of valve openings, disease of the heart muscle or defects in the heart wall.

Anaemia or low blood protein levels can also result in pathological heart murmurs.

These murmurs are often associated with poor performance, especially in athletic horses. One study found evidence of reduced performance in 10% of horses with heart murmurs.

One of the most common causes of pathological murmurs is a leaky mitral valve (often referred to as mitral valve insufficiency). This is present in about 4% of the general horse population and typically one-half of these horses will have a history of poor performance.

Some types of murmur are more common in Thoroughbred horses and there is evidence that training as well as genetic factors may be responsible for this increased incidence.

Triscuspid valve insufficiency, for instance, occurs in over 16% of Thoroughbred racehorses compared with just 9% of non-Thoroughbred horses.

The true significance of any murmur is best defined by a cardiac ultrasound examination - called echocardiography - which will identify the origin of the murmur and provide an assessment of its likely impact upon performance.

Echocardiography is a specialist procedure, requiring skill and experience and should always be recommended when a murmur is detected at a pre-purchase examination. One might argue that it is unnecessary for an inexpensive hack or child's pony, but is essential when the purchase of high value or high performance horses is being contemplated.

Unfortunately, it is a fact that murmurs detected at vettings do have a significant effect on the sale. In one study of horses with murmurs identified at pre-purchase examinations, 21% were sold for a lower than asking price and the sale did not proceed in 36% of cases. Only 43% of the horses sold for the asking price. All of these horses had a full cardiac ultrasound examination - we can only wonder how many would have failed' or been sold at a lower price had the precise nature of the murmur not been investigated fully?