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What do you need to know about heart murmurs?

Dr Ruth Morgan discusses the most common heart murmurs identified in event horses and also some of the less common but more concerning causes.

What is a heart murmur? 

A heart murmur is an abnormal sound occurring at an abnormal time in the cardiac cycle. When listening to your horse’s heart, your vet can hear two main sounds (often referred to as a loud ‘lub’ and a quieter ‘dub’). Commonly, two extra sounds can also be heard, especially in large, fit horses. These sounds are completely normal and correspond to blood flow through the valves of the heart. 
 
Heart murmurs are very common in sports horses – they have been identified in up to 60 per cent of all sports horses – but research has found no relationship between a low grade murmur and performance. 

What causes heart murmurs in horses? 

There are three main types of heart murmur: 

  • Flow murmurs – where abnormal blood flow through the valves of the heart is completely benign, causing the horse no problem at all and just a sign of a big heart in an athletic horse.

  • Physiological murmurs – can occur quite frequently if a horse is dehydrated, such as when it is suffering from colic or if it is anaemic.

  • Pathological murmurs – caused by disease of the heart. 
 
The most common cause is leaking of the valves between the heart chambers. Leaky valves are very common in horses and do not always cause a problem unless a significant amount of blood is leaking back through the valves. The most common forms of leaking valves are: 

  • Tricuspid regurgitation – the most commonly diagnosed leaking valve; it is not normally associated with severe disease of the heart and has not been associated with poor performance.

  • Mitral regurgitation – also common; this is more commonly associated with valve pathology and can result in abnormal rhythms, poor performance or heart failure if very severe.

  • Aortic regurgitation – the most common cause of a heart murmur in older horses; it is therefore of some concern if identified in horses younger than 15 years or if it is associated with an abnormal rhythm. 
 
A defect in the heart (a hole in the heart) present from birth can also occur in horses. The most common defect identified in horses is a ventricular septal defect. Infection of the heart valves (endocarditis) can also result in audible heart murmurs. 

A full examination of the horse’s cardiovascular system by your vet will include: 

  • Heart rate and rhythm: if your horse’s heart is functioning normally, it should be beating at a normal rate (30-40 beats per minute) and with a normal rhythm. 

  • Mucous membrane colour: looking at your horse’s gums to check their colour. They should be light pink; if they are pale, it may indicate that the heart is struggling to get blood to the body’s extremities. Pressing on the gums assesses the horses ability to refill the area with blood (capillary refill time). 

  • Pulse: the horse’s pulse can be felt in several places, but most commonly just under the lower jaw. The rate and strength of the pulse indicates the strength with which the heart is beating.  

  • Jugular pulse: the large jugular vein runs in the jugular groove. In cases of heart disease, it can pulse or fill abnormally. 

  • Lungs: heart disease can cause a horse to breathe more rapidly than normal and there can be more lung sounds than normal. 
 

When might a murmur be detected?

  • On routine examination prior to vaccination or sedation.

  • Routine examination prior to competition.

  • When your horse is unwell.

  • At a pre-purchase examination.

When is a murmur significant?

  • When it is very loud.

  • When it is heard in a very young horse.

  • When it is associated with poor performance or illness.

  • When there are other signs of heart disease such as an abnormal rhythm.