One of the most common causes of a heart murmur in dogs is a disease called Canine myxomatous or degenerative mitral valve disease (MMVD). It primarily affects older, small to medium size dogs, however, any dog can be affected. 

What is Myxomatous Mitral Valve Degeneration (MMVD)?

Let’s learn some basic anatomy and physiology to understand what causes a heart murmur in dogs. The heart is responsible for pumping blood around the body. The blood only travels in one direction through the heart. The heart has four ‘chambers’ called the right atrium, right ventricle, left atrium, and left ventricle. 

To enter each chamber there is a ‘one-way door’ (valve). The mitral valve is the valve between the left atrium and the left ventricle.


In MMVD, the mitral valves are thickened and too short. This means that the one-way door fails to form a complete seal, allowing blood to leak back into the left atrium. This abnormal flow of blood causes the turbulence we hear like a heart murmur in dogs.

Over time, MMVD may lead to congestive heart failure (CHF). This happens when the heart becomes too weak to push the blood in the correct direction, leading to excess fluid accumulating in the lungs.

What causes mitral valve disease?

The causes of mitral valve disease are unclear. However, there are a number of implicated genetic factors that make mitral valve disease a heritable cardiac disease. What essentially happens is the degeneration of the mitral valve, which compromises its ability to work like a one-way door.

Dogs at high risk for developing MMVD include the following. However, any small to medium-sized breed dog can develop MMVD:

  • Cavalier King Charles Spaniel
  • Cocker Spaniel
  • Chihuahua
  • Miniature Schnauzer
  • Maltese

How is myxomatous mitral valve degeneration (MMVD) diagnosed?

If your vet detects a heart murmur on a physical exam, you will be recommended tests to diagnose what is causing the abnormal sound in the heart.

An echocardiogram is recommended. This is an ultrasound of the heart performed by a specialist. An echocardiogram is extremely valuable because it will diagnose the cause of the heart murmur. It will also examine the health of the heart valves, measure the blood pressure to the lungs, and examine secondary changes to the heart, including abnormal thickness.

This is an echogram being performed on a patient. 


A chest X-ray may also be recommended because it helps us examine the lungs for any fluid and the size of the heart. Sometimes, if the heart gets too big, it can put a lot of pressure on the windpipe.

This is a chest x-ray of a patient. The arrows that you can see on the x-rays are measuring the patient’s Vertical Heart Score. 

What are the signs of a heart murmur?

A heart murmur on its own is often accompanied by NO signs. That’s why every year your veterinarian will check your dog’s heart for any murmurs. If the heart murmur is detected too late, there may be evidence of heart disease instead.

What are the signs of heart failure?

A heart murmur can progress into congestive heart failure. This is a serious condition where their lungs get congested with fluid. You may notice your fur-baby has a cough, exercise intolerance, or difficulty breathing.

Other signs include:

  • Lethargy
  • Fainting
  • Collapsing
  • Coughing
  • Gagging
  • Trouble breathing
  • Exercise intolerance
  • Reduced appetite
  • Swelling in the abdomen
  • Muscle loss

Coughing can be caused by heart disease for a couple of reasons. First of all, the heart getting bigger can push on the windpipe in the chest causing coughing. Secondly, if the leaky mitral valve is letting too much blood flows backward, fluid accumulates in the lungs, causing coughing.

That said, not all coughing means heart disease. In other cases, coughing may be caused by a respiratory issue rather than a heart issue. This includes bronchitis, allergies, pneumonia, asthma, or kennel cough.

What do I do now that my dog has a heart murmur?

According to the American College of Veterinary Internal Medicine (ACVIM), an echocardiogram needs to be performed every 6 months. This specialist panel of cardiologists stresses the importance of monitoring the disease very closely because it’s easier to prevent heart disease than play catch-up with congestive heart failure (CHF).

A heart check-up with your veterinarian is also recommended every 6 months once a heart murmur is detected.

Why is it so important to repeat the echocardiogram and/or chest x-rays?

There are different stages of heart disease:

  1. Stage A – Dogs at high risk of developing heart disease. No disease is present yet.
  2. Stage B1 – A murmur is heard but there are no visible signs of heart failure or heart enlargement.
  3. Stage B2 – A murmur is heard but there are no visible signs of heart failure. There is evidence of heart enlargement. Heart medication needs to be started at this stage. However during this stage, owners do not think their pet is deteriorating because they are still not showing any signs (e.g., coughing, lethargy, inappetence). The only way to tell if your dog needs to start medication is a heart echocardiogram every 6 months.
  4. Stage C – Heart disease is worsening and signs of heart failure are apparent (eg. coughing, lethargy, fainting). More medication is usually required at this stage.
  5. Stage D – End-stage heart disease which is not responding to treatment. Signs of heart failure.


How can do you treat Mitral Valve disease?

There are different treatment plans depending on what stage heart disease your dog has.

  • Stage A and B1 do not require any heart medication.
  • Stage B2 requires daily heart medication tablets to slow down the progression of congestive heart failure.
  • As heart disease progresses, multiple medications daily may be required to help control heart failure.

What is the prognosis of Myxomatous Mitral Valve Degeneration?

The prognosis of newly diagnosed MMVD varies considerably. A heart murmur can be big news for a paw-parent. But the earlier the diagnosis is made, the better the prognosis for your dog. It is very important to investigate, stage and treat a heart murmur properly because heart murmurs can lead to rapid deterioration.

At My Vet Animal Hospital, we work closely with Specialists in Small Animal Medicine and advanced training in Cardiology. We are able to organise a specialist to come to our clinic for an echocardiogram, rather than travelling to a specialist hospital. We also offer diagnostic x-rays and blood testing in house for immediate results. If your dog needs special heart medication and doesn’t like tablets, we can also offer personalised compounded medicine with added flavour including chicken, beef or fish. This is our way to ensure your dog has an accurate course of diagnosis, access to gold-standard monitoring tests and the best treatment.

Ready for an appointment?

Our team is here to answer your questions and get an appointment scheduled for you.

Pin It on Pinterest

Share This