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Updated August 31, 2023 — The heart is always on duty. It beats away second by second, day by day and year by year. It’s easy to take for granted until something goes wrong. In fact, heart disease is a leading cause of death in humans and a major 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. With this in mind, it’s important to know the signs of heart problems and understand the basics of MMVD in order to catch the disease early, leading to improved quality of life for dogs. 

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 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. 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 associate with a heartbeat is the sound of 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, 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. 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. 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 aim to stop the degeneration, rather than only focusing on the treatment of heart disease symptoms. 

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. 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 is 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 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 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 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 quality of life and longevity for dogs with MMVD. 

We Got the Beat – Morris Animal Foundation MMVD-Focused Research 
Although we know a lot about MMVD, there are still big 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 the quality of life and longevity of dogs with MMVD. 

Fresh Scoop podcast: Mitral Valve Disease in Dogs (MMVD) 

Press release: Mitral Valve Disease in Dogs Varies Between Breeds 

Don't Go Breaking My Heart - Understanding Heart Disease in Cats and Dogs 

ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs 



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