How Does Mitral Valve Disease Progress in Dogs?

Stage A: At Risk but No Heart Changes

Stage A describes dogs at higher risk of developing MMVD but without structural changes to the heart. Certain breeds with a known predisposition to MMVD fall into this category. At this stage, there is no heart enlargement or other evidence that the disease has developed.

Stage B1: Valve Changes Without Significant Heart Enlargement

The mitral valve normally prevents blood from flowing backwards when the heart contracts. As the valve deteriorates, it may not close properly, allowing blood to leak from the left ventricle, the lower pumping chamber, into the left atrium, the upper receiving chamber.

This leakage, called mitral regurgitation, may produce a heart murmur. At Stage B1, the heart has not enlarged enough to meet Stage B2 criteria and the dog has no heart failure symptoms.

Your dog may therefore appear completely well despite the valve changes. A murmur signals a reason for assessment, but its presence alone does not establish the disease stage.

Stage B2: Heart Enlargement Before Symptoms Appear

Stage B2 describes dogs whose heart enlargement meets defined clinical criteria but who have not developed signs of heart failure. The heart is compensating for the valve problem, although its workload has changed.

As leakage becomes more substantial, the left side of the heart handles additional blood volume. The left atrium and ventricle enlarge in response, initially helping the heart accommodate that extra volume.

This is an important distinction: maintaining normal activity does not necessarily mean the disease remains mild. Imaging can reveal progression that is not yet apparent at home.

Stage C: Heart Failure Develops

Stage C includes dogs with current or previous heart failure. Even when medication improves breathing, the dog generally remains in Stage C because controlling symptoms does not remove the underlying valve disease.

As the disease progresses, rising pressure within the heart can cause fluid to build up in the lungs, resulting in congestive heart failure.

Your dog may breathe faster at rest or struggle to settle comfortably. Reduced stamina and coughing may also occur, although coughing alone does not confirm heart failure.

Stage D: Heart Failure Becomes Harder to Control

At Stage D, standard treatment no longer adequately controls heart failure. Fluid accumulation may recur despite medication, making sustained comfort harder to maintain.

Care becomes more complex as the veterinary team balances symptom relief against how well the dog tolerates treatment. Not every dog reaches this stage, and the time spent in earlier stages varies considerably.

How Do Vets Monitor Changes in Mitral Valve Disease?

Comparing findings over time helps the vet distinguish a stable condition from changes that warrant a different care plan.

Tracking Changes During Heart Assessments

Repeat echocardiograms, or heart ultrasound scans, allow the vet to compare chamber measurements and assess valve leakage. Chest X-rays may be needed to look for fluid in the lungs, particularly when breathing changes develop.

Heart enlargement does not automatically mean fluid is present, so these tests help guide different treatment decisions. The timing of ongoing heart assessments depends on your dog’s stage and previous results. New symptoms may therefore require an earlier review.

Recognising Changes at Home

Tracking your dog’s sleeping breathing rate can help identify changes between visits. Count each rise and fall of the chest as one breath over a full minute, while your dog is asleep and not panting.

Keep a record so you can recognise a sustained increase from their usual rate. Ask your vet what reading or pattern should prompt contact. Report a persistent increase promptly, along with new coughing or reduced activity.

Laboured breathing or collapse, however, requires immediate veterinary attention, rather than waiting to collect more readings.

How May Treatment Change as the Disease Advances?

Your dog does not necessarily need to develop symptoms before treatment begins. At certain stages, medication can help delay further progression even while your dog still seems well.

Medication Before Heart Failure

Dogs in Stage B1 generally do not need medication specifically for MMVD, as treatment has not been shown to delay progression at this stage. Monitoring allows the vet to identify when that position changes.

Once a dog meets Stage B2 criteria, medication that supports heart contraction and helps blood move forwards may be recommended to delay the onset of heart failure. Starting treatment at this point does not mean heart failure has already developed.

Treatment After Heart Failure Develops

After heart failure occurs, treatment usually includes diuretics, which help the kidneys remove excess fluid. Dogs with an acute breathing crisis may need hospital care and oxygen while treatment takes effect.

Longer-term medication is adjusted according to the response. Blood tests help check kidney function and electrolyte levels, as fluid-removing treatment can affect both.

If control becomes less reliable, the vet may change the medication plan and arrange closer reviews. Owners should follow the prescribed doses and discuss any changes before making adjustments themselves.

Mitral Valve Repair With TEER

For selected dogs, a Mitral TEER procedure may be considered to reduce leakage by bringing parts of the valve leaflets closer together.

Assessment may be appropriate in Stage B2 or early Stage C when leakage is significant and the valve anatomy is suitable. Advanced late Stage C or Stage D disease may limit suitability, so TEER should not be viewed solely as a last resort.

The procedure does not replace ongoing cardiac care. Medication and follow-up assessments are usually still needed afterwards.

FAQs About Mitral Valve Disease Progression in Dogs

Can mitral valve disease worsen suddenly?

MMVD often develops gradually, but complications can cause a sudden deterioration. For example, a cord supporting the valve may tear, allowing leakage to increase abruptly. This can overwhelm the heart’s ability to compensate, even after a relatively stable period. Sudden breathing difficulty requires emergency veterinary care.

Can medicines for my dog’s other conditions interfere with MMVD treatment?

Some medicines can interact with heart treatment. For example, certain anti-inflammatory pain medicines may reduce the effect of diuretics or increase the risk of kidney problems. It can be helpful to give every vet treating your dog an up-to-date medication list, including supplements, so prescriptions can be coordinated. Do not stop an existing medicine without veterinary advice.

Why might a dog with advanced MMVD lose weight?

In dogs with advanced MMVD, heart failure can trigger changes in the body that increase muscle breakdown. Reduced appetite can contribute, but muscle loss is not always explained by eating less. Your vet should assess these changes rather than attributing them solely to ageing, as other conditions may also contribute.

About Us

Dr Nicholas Woo
Consultant Vet and Surgeon
BVSc (Hons) MANZCVS (Small Animal Surgery)

Dr Nicholas Woo is a consultant vet and surgeon with advanced training in small animal surgery and oncology. His clinical interests include soft tissue surgery, orthopaedics, oncology, spinal surgery, hip replacement and critical care.

In 2017, Dr Woo founded Advanced VetCare with the aim of providing accessible, high-level veterinary care supported by modern medical facilities and round-the-clock emergency services in Singapore.

Dr Woo has also presented at numerous veterinary meetings, where he shares his knowledge and experience with fellow professionals while contributing to ongoing developments in veterinary medicine.

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