What is a Heart Murmur?


Why Should I Bring my Pet to Willows for Investigation of a Heart Murmur?

Willows is one of Europe’s leading small animal referral centres. Our state-of-the-art hospital is led by internationally renowned Board Certified Specialists committed to providing the highest standards of veterinary care. Our team of Specialist Cardiologists are supported by our multidisciplinary team of Specialists across a number of disciplines including; Anaesthesia, Diagnostic Imaging, Soft Tissue Surgery and Emergency and Critical Care.

In addition, Willows has a large dedicated team of Nurses and clinical support staff available 24 hours a day, every day of the year to provide the best possible care for your pet.


What is a Heart Murmur?

Being told that ‘your pet has a heart murmur’ can be very daunting. However, the significance of a murmur very much depends upon the situation. Just as a fever can be something or nothing, a murmur can be similar; it may be insignificant, however it could also be a symptom of a disease that requires treatment.

A heart murmur is a sound produced by a squirt of blood inside the heart when it pumps and there are many causes of a murmur.

What is the Significance of a Heart Murmur?

This depends upon what the cause of the murmur is, whether the defect is classified as: mild, moderate or severe, and whether or not it is resulting in heart enlargement. In some heart diseases, a louder murmur can suggest a more severe disease. However, a mild murmur does not rule out significant heart disease. In fact, some cardiac diseases may not even cause a heart murmur.


What are the Most Common Causes of a Heart Murmur?

Murmurs due to regurgitation

Each of the four valves in the heart act as non-return valves, permitting blood flow in one direction (forwards). If a valve becomes faulty and no longer prevents backflow, then there is a resultant backward squirt of blood through the gaps in the valve with each heartbeat. This backward squirt of flow through an incompetent valve results in an abnormal heart sound called a heart murmur. A murmur thus sounds like a ‘squirting’ or ‘gushing’ sound during each heartbeat. The most common cause of an incompetent valve in older dogs is Myxomatous Mitral Valve Disease, also known as Endocardiosis.

Murmurs due to narrowed valves

If a heart valve is abnormally narrowed, usually due to a congenital defect (e.g. Aortic Stenosis or Pulmonic Stenosis) then blood flow pumped out through the narrowed valve is pinched, resulting in an abnormal squirt of flow, i.e. a murmur. This can be likened to putting a thumb over the end of a hose pipe to make the water squirt, rather than pour.

Murmurs due to ‘holes in the heart’

A murmur can also occur through a hole in the heart, the murmur is caused by the squirt of blood going through the hole. A hole between the left ventricle and right ventricle results in a squirt of blood being pushed through the hole when the heart ventricles pump (this is called a Ventricular Septal Defect (VSD)). A hole between the aorta and pulmonary artery (the two major arteries leaving the heart) results in blood squirting through the hole (this is called a Patent Ductus Arteriosus (PDA)).

Puppy and kitten murmurs

In puppies and kittens less than six months of age, in addition to murmurs produced by congenital defects, puppies can have innocent or benign murmurs, often called a 'puppy or kitten murmur'. These occur due to the fast flow of blood in such a small heart. As the puppy or kitten and its heart grow and mature, the murmur gradually disappears. It is virtually impossible for a vet to discern the difference between a puppy/kitten murmur and a congenital defect. If a murmur persists beyond six months of age, it is more likely to be a congenital defect.

Puppy and kitten cardiac screening is available at the fixed package price of £400 and includes the consultation fee, ensuring transparency and reassurance when it is needed most. 

Puppy and kitten cardiac screening is available at the fixed package price of £400, and includes the consultation fee, ensuring transparency and reassurance when it is needed most.

Murmurs associated with illness

A murmur can also be produced when a dog is anaemic and the blood thin. In this situation thinner blood (less viscous) results in faster speed of flow and thus a murmur. A similar situation can occur when a dog is ill for other reasons, such as during a fever.


An ultrasound image showing a dog’s heart; each of the chambers are labelled. Blood flow from the left atrium (LA) is upwards through the mitral valve (MV) and into the left ventricle (LV). The mitral valve acts as a one-way valve.


The same image as above, with the computer colour coding showing the direction of blood flow. When the left ventricle (LV) pumps, the mitral valve should prevent any backflow. In this instance there is a jet of blood regurgitating back into the left atrium (LA).


How is a Heart Murmur Diagnosed?

Listening with a stethoscope is not easy and many Vets find this difficult. It is likely that a Specialist Veterinary Cardiologist will hear murmurs well, as a result of extensive experience and additional training. In such instances, it is possible to ask your general practice Vet to refer you to a Specialist who will listen to the murmur and determine which tests need to be carried out to reach a diagnosis and assess the severity of the murmur.

Most commonly, a heart scan (echocardiography) is the best way to diagnose a murmur. In some cases, additional tests are required, such as chest X-rays, ECG or blood tests. Echocardiography is a difficult and skilled examination that is best performed by a trained and experienced cardiologist.

What can I Expect if my Pet Under Goes a Heart Scan?

The heart scan allows the cardiologist to find the cause of the heart murmur, and evaluate the size and shape of the heart as well as its pumping ability. It is a non-invasive test that is performed most often without sedation. Dogs and cats may need to have a patch of fur clipped over both sides of the chest (in the region of the arm pit) and they will to lie down on a table specially designed for heart scans.

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Find out more

To assist owners in understanding more about the conditions related to and treatments available for patients with heart and lung problems, we have put together a range of information sheets to talk you through the some of the more common cardiology conditions seen by our Specialists.