There are four valves in the heart, one between each atrium and its respective ventricle and one between each ventricle and the major vessel it supplies. A valve is a thin flap of tissue that opens and closes to let the heart fill and empty at the appropriate time. The opening and closing of the valves generate sounds and those are the heart sounds your veterinarian will be listening for with a stethoscope. A ‘heart murmur’ is an extra sound audible when listening to a horse’s heart, next to the normal heart beat.
Most murmurs that we find are ‘flow’ murmurs, which means that these murmurs can occur in completely healthy hearts which no significant abnormality but are caused by normal flow of blood. These flow murmurs happen in horses because they have a large heart compared to their body size, so there is a lot of turbulence in blood flow that can be audible as a murmur. These murmurs have no clinical significance.
However, some murmurs do indicate an abnormality in either blood flow or heart function. They can either mean a leaking heart valve, thickening or narrowing of a valve or large blood vessel or an abnormal hole between different heart chambers. A valve can start leaking for a variety of reasons. The most common cause is degeneration of the valve leaflets with age. They can become thickened and shortened. When this occurs, the valve leaflets cannot come together properly. Another reason for the occurrence of a murmur can be altered blood viscosity.
A thorough examination and listening to your horse’s heart will give us a better idea what kind of murmur your horse has. By determining the location of the murmur, the length, the volume and when the murmur is audible, the murmur will be graded. The grades run from 1, which is a very quiet murmur, to a grade 6 where the murmur can be heard without even touching your horse’s chest with the stethoscope.
Next to the murmur itself, the presence of any other clinical signs is important. Clinical signs of heart problems/failure could include exercise intolerance, weight loss, oedema and enlargement of blood vessels, mostly visible as a very clear visible pulsation in the jugular vein. If the murmur is severe or there are any other clinical signs, further investigation is required.
An ultrasound examination of the heart would be the first step. This allows for the heart to be imaged while it is beating. This is a specialised technique and your horse will have to be referred to a specialist. An ultrasound examination is also typically recommended for the evaluation of some murmurs detected at purchase examination, especially for horses intended for athletic use. The ultrasound will show the nature of the lesion causing the murmur to be identified, next to measuring the size of the chambers of the heart and the contractility of the heart. All these factors combined will give an indication of the severity of the murmur and the likely effect it will have on your horse. The examination will also provide a baseline; we may recommend a repeat examination in 6-12 months to note any progress of disease. Some lesions remain static for a number of years, and the horse can go on to work for several more years with no problems. However, if the problem appears to be progressing, it may be advisable to retire the horse.
Other tests that could be performed would be a blood test to tell if there is any damage to the heart muscle and an ECG or electrocardiogram to look for abnormalities in heart size and heart rhythm.
Much research has been conducted into the frequency of heart murmurs and they are a common finding and most do not seem to be clinically significant. With a thorough examination by your vet, decisions can be made if there are further investigations needed, or if the murmur is likely to be of no clinical significance.