Page 725 - The Veterinary Care of the Horse

tissues. To compensate for this, the heart rate may speed up. The increased flow of blood

with possibly reduced viscosity and density may lead to a murmur being heard.
VetBooks.ir • Narrowing of any valve as this restricts blood flow and causes turbulence.

HOW DOES THE VET TELL IF A MURMUR IS SIGNIFICANT?

Sudden collapse and death due to heart failure is uncommon in the horse. A horse rarely has a
heart attack in the same way as a person. When horses collapse and die during or

immediately after exercise, it is usually due to rupture of a major blood vessel. Significant
heart disease usually causes a loss of performance or reduced exercise tolerance, rather than

sudden collapse and death. Many horses with murmurs continue to perform normally
throughout their working lives. When a possible abnormality is discovered, the vet needs to
decide whether it is affecting the current performance of the horse or is likely to cause a

problem in the future. The following may indicate significant heart disease.

• The resting heart rate is often elevated with severe heart disease. A resting rate of 50 or
more is usually significant.

• A raised resting respiratory rate or uncharacteristic respiratory distress after exercise
needs further investigation.

• Unexplained loss of performance, apparent weakness or unsteadiness on their legs. These
horses should have a cardiac examination without delay.

• Abnormal distension of the jugular vein or ventral oedema are serious signs.

• With severe heart disease the horse may cough and have breathing difficulties at rest.
Weight loss is common if they reach this stage.

As a very rough guide, loud murmurs with a palpable thrill that can be heard and felt over a
wide area of the chest are likely to be significant. They are associated with serious abnormal
flow and even if there are no obvious clinical signs, these horses should be checked regularly.

However, there are no hard and fast rules. A minor lesion can cause a loud murmur and the
murmur from a serious lesion may be very faint.

The vet may be confident that the murmur is not of any significance following

auscultation and clinical examination. However, if there is any doubt, further investigations
including electrocardiography (ECG) and echocardiography will be recommended.

Cardiac arrhythmias

Cardiac arrhythmias arise from:
• abnormalities of conduction of the electrical impulse, e.g. as a result of damage to the SA