Page 724 - The Veterinary Care of the Horse

the murmur be heard throughout the whole of systole or just in the early, mid or late part

of it?
VetBooks.ir • Whether the murmur is very localized, i.e. heard in only a small area or can it be heard

over a wide area of the horse’s chest?
• The location on the chest wall where the murmur is most easily heard. This helps the vet

decide which valve is leaking.
• Some murmurs can actually be felt as a vibration through the chest wall. This is known

as a ‘thrill’. The area over which this thrill can be felt is noted.

• Their intensity. Murmurs are graded as follows:

Grade 1: the murmur is very faint and can only be heard with very careful listening
in a localized area

Grade 2: the murmur is faint but can be readily heard after a few seconds of

auscultation

Grade 3: the murmur is obvious as soon as the stethoscope is placed on the chest but
is restricted to a localized area

Grade 4: the murmur is loud and can be heard over a large area of the chest

Grade 5: the murmur is loud, radiates widely and there is a palpable thrill Grade 6:
the murmur can often be heard without a stethoscope or with the stethoscope lifted off

the chest wall and there is a palpable thrill over a wider area of the chest wall.

• The response to exercise. Murmurs may be more obvious when the horse is exercised
because the flow of blood through the heart increases. Some murmurs are less audible or

disappear after exercise.

CAUSES

Murmurs may be caused by the following.

• Normal blood flow. In fit horses, the sound of blood accelerating as it is pumped out of
the heart can often be heard. This is known as a physiological, or flow, murmur and it is

of no clinical significance.
• Turbulence as blood escapes and flows in a backward direction through a leaking valve.

This may result from degenerative changes on the atrioventricular valves or nodules

forming on the semilunar valves.
• Deposits on the heart valves caused by bacterial infection are fortunately uncommon in

horses, but when they are present, a significant murmur is often heard.
• Anaemic horses have an abnormally low number of red blood cells to carry oxygen to the