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July 21, 2022 — The heart is always on duty. Beating away second by second, day by day and year by year. It’s so easy to take for granted, until something goes wrong. In fact, heart disease is a leading cause of death in people and a health concern for our pets, too – particularly myxomatous mitral valve disease, the most common heart health problem diagnosed in dogs.

Myxomatous mitral valve disease (MMVD) accounts for a whopping 75% of all heart problems diagnosed by veterinarians – that’s more than all other heart diseases, in both cats and dogs, combined.

Turn the beat around - heart function 101

The heart is a specialized muscle, designed to fill with blood then contract, ejecting blood to begin its journey to all parts of the body. Inside the heart, four valves help blood flow in the proper direction.

The following simple diagram is a good way to visualize the heart’s chambers and the flow of blood (by convention, the right side is on the left and the left side of the heart is depicted on the right):

blood flow diagram

Blood flows through the heart in a defined pathway:

  1. Blood enters the right atrium from body. This blood has low levels of oxygen and high levels of carbon dioxide.
  2. Blood flows through the tricuspid valve into the right ventricle and then leaves the right ventricle to go to the lungs through the pulmonic valve.
  3. In the lungs, carbon dioxide is released on expiration and oxygen enters the blood during inspiration.
  4. Blood returns to the heart and enters the left atrium.
  5. Blood then flows through the mitral valve into the left ventricle.
  6. Blood then leaves the heart through the aortic valve and is distributed to the rest of the body.

The classic lub-dub sound we associated with the heart beating is the sound of the valves closing. A murmur is caused by turbulence. Although we tend to think of valve problems as a common cause of murmurs – and they are – it’s also important to remember that murmurs are not always a sign of valve disease. However, a murmur suddenly detected in a dog with no previous history of one should be investigated further.

When good valves go bad – mitral valve degeneration

In some dogs, the mitral valve can degenerate, which ultimately affects the valve’s function. Instead of forming a tight seal, blood can leak back through the valve when the heart contracts. Instead of moving out of the heart and circulating through the body, blood moves backward through the valve. Over time, as the valve continues to degenerate and more and more blood flows backward, the heart tries to compensate for the backflow by enlarging. However, there is a limit to what the heart can do and ultimately, the heart begins to fail.

It’s unclear why the mitral valve changes in some dogs and not in others, but genetics probably plays a role, since certain breeds of dogs are much more likely than others to develop MMVD. The disease is more common in small and medium-sized dogs but can occur in any breed.

In the last 10 years, research on the exact mechanisms responsible for valve degeneration has increased. Scientists hope that by better understanding this process, new treatments could aim to stop the degeneration, rather than only focusing on treating heart disease symptoms.

Taking the stage – new system helps standardize diagnosis and treatment

Several years ago, cardiologists developed a classification system to standardize evaluation of human patients with heart disease, which in turn helped clinicians determine treatment plans and develop therapeutic guidelines.

A similar system was developed for dogs. The system consists of four stages:

  • Stage A – Dogs considered high risk for heart disease but with no clinical signs
  • Stage B – Dogs with murmurs detected but no signs of heart failure
  • Stage C – Dogs with signs of heart failure that need treatment
  • Stage D – Dogs with heart failure that is difficult to manage and is not responding to treatment

Many veterinary studies now use this system and you might hear these terms used if your dog is diagnosed with MMVD.

Early diagnosis key to a good quality of life for dogs with MMVD

Although serious, MMVD is treatable. Knowing the signs to look for and getting prompt veterinary care are essential to help your dog thrive with MMVD.

Common signs of MMVD include:

  • Coughing
  • Exercise intolerance
  • Increased respiratory rate and effort
  • Fainting

Physical examination findings can include:

  • Newly discovered heart murmur
  • Moist lung sounds
  • Poor pulse quality

A chest X-ray, echocardiogram (ultrasound of the heart) and an ECG (electrocardiogram) are all valuable tools for evaluating dogs for heart disease.

Owners of certain dog breeds at high risk of developing MMVD need to be especially vigilant for any signs of heart disease in their dog.

Dog breeds reported to be at higher risk for MMVD are:

  • Cavalier King Charles spaniels
  • Papillons
  • Miniature poodles
  • Yorkshire terriers
  • Chihuahuas
  • Dachshunds

Although veterinary cardiologists are learning more about how MMVD develops, as well as some of the genes responsible, treatment still relies on diet change, activity modification and medication. Veterinary cardiologists continue seeking the best combination and timing of medications to slow heart disease progression and improve quality of life and longevity in dogs with MMVD.

Morris Animal Foundation picks up the beat with MMVD-focused research

Our research projects are focused on new and exciting areas of MMVD research, including exploring the genetics of the disease and testing new treatments.

Learn more about our investment in canine heart health and how you can help give all animals have healthier lives.


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