A heart murmur is a noise related to abnormal blood flow in or out of the chambers of the heart. This is usually only audible with a stethoscope and can be heard in addition to, or instead of, the normal heart beat sounds. While murmurs in growing puppies are normal and often harmless (“innocent” or “puppy” murmurs), in adult dogs it usually indicates an underlying problem.

What signs might I see if my dog has a heart murmur? 

This is very variable. Some dogs may show signs of a heart problem, such as lethargy, reduced ability to exercise, coughing or collapsing episodes. However, in many cases it might not be outwardly obvious that there is any issue. Quite often, a heart murmur is picked up by your vet at a routine appointment. 

What causes a heart murmur? 

The abnormal blood flow that results in a heart murmur can have several causes. 

Valve disease

Especially in smaller breed dogs, murmurs can be caused by ‘leaky’ valves in the heart. The 4 chambers of the heart are separated by a system of one-way valves. Over time, these valves can become damaged and their parts no longer fit together perfectly to make a tight seal. This means that they are leaky. As a result, blood flows back through them into the chamber from which it has just been pumped out. This abnormal blood flow causes the sound of the murmur. This is sometimes called a murmur of regurgitation. 

Narrowed vessels

The valves of the heart can also become narrowed (the veterinary term for this narrowing is “stenosis”). This means there is more resistance to blood flowing through them, resulting in abnormal blood flow. 

Heart muscle disorders

Some large breed dogs (some examples of affected breeds are the Great Dane, Wolfhound, Doberman and Boxer) can develop a condition in which the heart chambers become abnormally enlarged and the heart muscle is weak and does not contract as efficiently as normal (Dilated Cardiomyopathy). This means when the heart pumps out blood, some ends up going to the lungs instead of being pumped effectively to the rest of the body. The stretching of the chambers also means that the valves can become damaged and leaky, resulting in a murmur. 

Congenital heart problems

Murmurs are often audible in dogs with congenital heart defects. If there is for example an abnormal ‘hole in the heart’ this will cause abnormal patterns of blood flow and hence a murmur. However, it can be a completely normal finding for a young puppy to have a heart murmur; it does not necessarily mean a congenital heart defect is present. Some puppies have an ‘innocent’ murmur which is heard at early puppy checks but then goes away by a few months old. If your vet is concerned that a murmur may be caused by a congenital problem, then your puppy may be referred for further tests. 

Changes to the blood

When dogs are unwell, for example with anaemia or a fever, the blood can become abnormally thin. In some cases, this can result in a heart murmur. This will go away once the underlying disease is resolved. 

How is the cause of a heart murmur diagnosed? 

To confirm what is causing the heart murmur, some diagnostic tests will often be required. Some of these may be performed at your veterinary practice or they may refer your dog to a specialist in heart disease (cardiologist) to perform some of these tests. These may include: 

  • X-rays of the chest – this will show fluid build-up on the lungs or an abnormally enlarged or misshapen heart. 
  • ECG – pads are placed on your dog’s skin to take a reading of the electrical activity from the heart. This will show up any problems with the rhythm of the heart. 
  • Ultrasound of the heart (echocardiography) – this scan gives an accurate picture of the size and shape of heart chambers, condition of the valves and any abnormal structures or patterns of blood flow. This will provide more detailed information than an x-ray. 

What happens next? 

The treatment and prognosis (predicted outcome) for a dog with a heart murmur very much depends on the condition causing it. Therefore, establishing an accurate diagnosis is crucial. 

In some cases, a surgical operation may be required, for example to fix a congenital heart defect in a puppy. In many middle aged or older dogs with heart disease, treatment consists of medications given by the owner in tablet form. Your vet will also need to monitor your dog’s condition carefully with regular check-ups to listen for any changes to the heart murmur and check for any other signs developing. 

As with many conditions, early diagnosis and treatment of heart problems often results in a better outcome.