Understanding the Stages of Mitral Valve Disease in Dogs

Learning that your dog might have mitral valve disease (MVD) can be overwhelming. MVD is a progressive condition where the mitral valve, a small flap inside the heart that controls blood flow between two of its chambers, begins to weaken and leak. When the valve doesn’t close properly, blood flows backward instead of forward, putting extra strain on the heart over time. While this may sound scary, understanding the stages of MVD and how it progresses can help you make informed decisions about your dog’s care.

What is Mitral Valve Disease (MVD)?

The heart has four chambers, and valves between them act like one-way doors, keeping blood moving in the right direction. The mitral valve sits between the left atrium and the left ventricle, the two chambers on the left side of the heart. In MVD, this valve gradually weakens and stops closing properly, allowing blood to leak backward with each heartbeat. That leak creates extra pressure, and over time the heart compensates by enlarging. If the disease progresses far enough, it can lead to congestive heart failure (CHF), where the heart can no longer pump blood efficiently enough to meet the body’s needs.

MVD is the most common heart disease in dogs, accounting for approximately 75% of all canine heart disease cases. It’s especially prevalent in older, smaller breeds like Cavalier King Charles Spaniels, Dachshunds, and Chihuahuas.

ACVIM Stages of MVD in Dogs

Veterinary cardiologists at the American College of Veterinary Internal Medicine (ACVIM) have developed a staging system that categorizes MVD into stages A through D based on risk and severity. Each stage helps guide monitoring and treatment decisions.

Stage A

This stage includes dogs who are at higher risk of developing MVD, usually because of their breed, but show no symptoms and have no detectable heart murmur yet. Breeds like Cavalier King Charles Spaniels often fall into this category. Monitoring, rather than treatment, is recommended at this stage.

Stage B1

A heart murmur is now present, which your vet will detect through a stethoscope, but there are no signs of heart enlargement and your dog has no symptoms. Most dogs in this stage don’t require treatment, but regular check-ups are essential.

Stage B2

The heart has started to enlarge, which can be seen on imaging, but your dog still has no outward symptoms. At this point, medication like pimobendan is recommended to slow the progression toward heart failure. Research has shown that pimobendan can delay the onset of congestive heart failure by around 15 months.

Stage C

Dogs in stage C are showing signs of congestive heart failure. Symptoms like coughing, labored breathing, and lethargy become noticeable. Medications including pimobendan, ACE inhibitors (drugs that reduce the workload on the heart by relaxing blood vessels), and diuretics like furosemide (which help the body remove excess fluid that builds up when the heart isn’t pumping efficiently) are typically prescribed to manage symptoms and improve quality of life.

Stage D

Stage D represents advanced heart failure where symptoms persist even with standard treatment. Adjusted or additional medications are often necessary, and ongoing monitoring is critical to provide the best possible care.

Symptoms of Mitral Valve Disease

The symptoms of MVD vary widely depending on the disease stage:

• Stages A–B1: No visible symptoms.
• Stage B2: No outward symptoms, but changes can be detected on imaging.
• Stage C: Coughing, increased resting respiratory rate (over 30 breaths per minute during sleep), breathing difficulty, reduced activity levels.
• Stage D: Persistent symptoms, severe difficulty breathing, or even collapse.

One of the simplest things you can do at home is monitor your dog’s resting respiratory rate, meaning how many breaths they take per minute while asleep. Count the breaths for 30 seconds and double it. If it’s consistently above 30, call your vet. It’s one of the earliest signs that heart function may be worsening.

Diagnosis of MVD in Dogs

A vet typically starts by listening for a heart murmur during a physical exam. Murmurs are graded on a scale from I (very faint, only heard in a quiet room) to VI (loud enough to feel with a hand on the chest). A higher grade doesn’t always mean more severe disease, but it does prompt further investigation. To confirm MVD and determine its stage, diagnostic tests may include:

• Chest X-rays to check for heart enlargement and fluid in the lungs.
• An echocardiogram, an ultrasound of the heart, which gives your vet a detailed picture of how the heart looks and how well the valve is functioning.
• Resting respiratory rate monitoring, which you can track at home between vet visits.

Treatment Options for MVD

Treatment depends on the stage of MVD and the severity of symptoms:

• Stages A and B1: No treatment needed, but regular monitoring is crucial.
• Stage B2: Medications like pimobendan are prescribed to slow disease progression.
• Stage C: A combination of medications, including pimobendan, ACE inhibitors, and diuretics, helps manage symptoms.
• Stage D: Adjusted treatments, possibly including additional drugs, with a focus on providing symptom relief.

In symptomatic stages, your vet may also recommend reducing sodium in your dog’s diet, since salt can contribute to fluid retention, and making lifestyle adjustments to reduce physical strain on the heart.

Does Pet Insurance Cover Mitral Valve Disease?

Yes, mitral valve disease (MVD) is covered under Lemonade Pet’s base accident and illness policy, as long as it isn’t pre-existing. That includes eligible costs like echocardiograms, medications, and the ongoing care that typically comes with managing this condition long-term.

For breeds that are prone to MVD, staying on top of annual wellness exams is one of the most practical things you can do. Adding a preventative care package to your policy can help cover the cost of those yearly check-ups, giving your vet the chance to flag any early changes before they develop into something more serious.

Getting a policy in place while your dog is symptom-free is the clearest path to making sure you’re covered. If signs of MVD appear before your policy is active, the condition could be considered pre-existing and won’t be eligible for coverage.

Before we go

Mitral valve disease is a serious condition, but understanding its stages and treatment options puts you in a much better position to care for your dog. Regular vet visits, home monitoring, and starting treatment at the right time all make a real difference. If you’re worried about costs, looking into pet insurance sooner rather than later is one of the most practical things you can do.

Frequently Asked Questions

A few quick words, because we <3 our lawyers: This post is general in nature, and any statement in it doesn’t alter the terms, conditions, exclusions, or limitations of the policies issued, which differ according to your state of residence. You’re encouraged to discuss your specific circumstances with your own professional advisors. The purpose of this post is merely to provide you with info and insights you can use to make such discussions more productive! Naturally, all comments by, or references to, third parties represent their own views, and Lemonade assumes no responsibility for them. Coverage may not be available in all states. Please note that statements about coverages, policy management, claims processes, Giveback, and customer support apply to policies underwritten by Lemonade Insurance Company or Metromile Insurance Company, a Lemonade company, sold by Lemonade Insurance Agency, LLC.  The statements do not apply to policies underwritten by other carriers.