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Updated February 8, 2024 — The heart is always on duty, beating away second by second, day by day and year by year. It’s easy to take for granted until something goes wrong. Heart disease is a leading cause of death in humans and a significant health concern for our pets, too. 

The most common heart health problem diagnosed in dogs is myxomatous mitral valve disease, which accounts for a whopping 75% of all heart problems found in dogs. Given how common MMVD is in dogs, it’s essential to know the signs of heart problems and understand the basics of MMVD to catch the disease early, leading to improved quality of life for dogs. 

Understanding Heart Function and MMVD
The heart is a specialized muscle that fills with blood and then contracts, ejecting blood to begin its journey to all body parts. Inside the heart, four valves help blood flow in the proper direction. 

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

(For more in-depth information, check out our latest blog on heart function).

blood flow diagram

Blood flows through the heart in a defined pathway: 

  1. Blood enters the right atrium from the 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. Carbon dioxide is released in the lungs 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 associate with a heartbeat is the sound of valves closing. Turbulence causes a murmur. Although we 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, consequently affecting the overall function of the valve. Instead of forming a tight seal, blood can leak back through the valve when the heart contracts. Blood moves backward through the valve instead of out of the heart and circulates through the body. Over time, as the valve degenerates and more blood flows back, the heart tries to compensate for the backflow by enlarging. However, there is a limit to what the heart can do; ultimately, the heart begins to fail. 

It’s unclear why the mitral valve changes in some dogs and not in others. It could be that genetics play a role since certain breeds 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 stop the degeneration rather than only focusing on treating heart disease symptoms. 

New Diagnostic and Treatment Frameworks
Several years ago, cardiologists developed a classification system to standardize the evaluation of human patients with heart disease, which in turn helped clinicians determine treatment plans and develop therapeutic guidelines.  Experts developed a similar system for dogs. Many veterinary studies now use this system, and you might hear these terms used if your dog is diagnosed with MMVD.

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 who need treatment 
  • Stage D: Dogs with heart failure that is difficult to manage and is not responding to treatment

Early Diagnosis: The Key to 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 in helping your dog thrive while managing 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 and an ECG 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 dogs.

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 linked to the disease, treatment still relies on diet change, activity modification and medication. Veterinary cardiologists continue to seek the best combination of timing and medications to slow heart disease progression and improve the quality of life and longevity of dogs with MMVD.

New Publication Sheds Light on Heart Disease in Dogs
A recent publication from the Dog Aging Project is getting a lot of buzz on the internet and in the popular press. The article's authors broke down the frequency of different disease categories by body size and age. 

What they found was:

  • Overall, 6% of the 27,541 dogs in their study group were reported to have some type of heart disease.
  • Heart disease incidence was low in dogs under 11 years of age but then spiked in dogs over 11 years of age.
  • Dogs under 22 pounds were likelier to have heart disease than dogs over 22.

The authors note that the survey used to generate this data asked owners if their dog had been diagnosed with a heart problem, not which heart diagnosis was confirmed. The team is still collecting data, but these findings are consistent with what we know about heart disease in dogs – that MMVD is the most common heart disease and that it most commonly affects small-breed dogs

Stay tuned for more information from this large study!

We Got the Beat: Morris Animal Foundation MMVD-Focused Research  
Although we know a lot about MMVD, there are still significant gaps in our knowledge when it comes to the genetics behind the disease. Even so, our ability to treat MMVD is light-years better today than it was 20 years ago. But we’re always looking to do better. Two MMVD-specific areas of research the Foundation has supported in recent years include genetics and new treatments. 

Learn more about our investment in canine heart health and how you can help improve dogs' quality of life and longevity with MMVD. 


Stopping Heart Disease Starts With Science