Understanding Myxomatous Mitral Valve Disease (MMVD) in Dogs

Written by: Marcella Zhang
Posted on: May 21, 2026

Key symptoms	Coughing, faster breathing at rest or during sleep, exercise intolerance, lethargy, appetite changes, weight loss, restlessness at night
Commonality	High in small and toy breed dogs, rare in cats
Transmission	Not contagious
Impact on health	Mild to severe (depending on stage)
Treatment cost	~ $1,500 to over $5,000 AUD (depending on disease severity and treatment)
Treatment duration	Typically life-long once medication is started
Ease of prevention	Not a preventable condition as it is associated with genetics and age

Most affected dog breeds

Cavalier King Charles Spaniel
Dachshund
Miniature
Toy Poodle
Chihuahua
Maltese
Pomeranian
Yorkshire Terrier
Shih Tzu
Miniature Schnauzer
Cocker Spaniel

If your vet has just told you your dog has a heart murmur and it’s likely Myxomatous Mitral Valve Disease (MMVD), it’s completely normal for your mind to jump to the worst-case scenarios. Hearing “heart disease” can feel overwhelming, and it’s easy to start thinking in short timelines. But in most cases, that’s not how this condition unfolds at all.

What exactly is MMVD and how bad is it?

MMVD is the most common degenerative heart condition in dogs, accounting for about 75% of all canine heart disease. It is especially common in small and toy breeds such as Cavalier King Charles Spaniels, Chihuahuas, Dachshunds, and Miniature Poodles. MMVD is a progressive disease of the mitral valve that causes blood to leak backwards instead of forwards. It usually progresses slowly over years, and many dogs can live happily and comfortably with the condition for a long time when it is properly managed. The key is catching the disease early, monitoring carefully at home, and starting treatment at the right moment. MMVD becomes dangerous when the leaking valve forces the heart to remodel, pressures rise, fluid backs up into the lungs, and congestive heart failure (CHF) develops.

What actually happens to the mitral valve?

Normal valve function

The mitral valve sits between the left atrium and the left ventricle, the two chambers on the left side of the heart. In a healthy heart, the valve leaflets shut like a well-cut door in a good frame, held in place by the chordae tendineae that act as the “strings” that anchor the valve to the ventricle. With every beat, blood moves forward into the aorta, and the body gets its supply.

What does wrong in MMVD

In MMVD, the valve tissue thickens and becomes nodular, and stops closing properly.

Inside the leaflets, waxy substances build up that cause the tissue to stretch and balloon back into the heart chamber above, instead of sealing shut as they should. The delicate strings holding the valve in place (the chordae tendineae) can stretch too, and in rare cases one can snap entirely, which is why an apparently stable dog can deteriorate overnight.

The result of all this is that the valve leaks. With every heartbeat, some blood flows backwards into the left atrium instead of being pushed forward around the body; this is called mitral regurgitation. Over time, the heart has to work harder to compensate: the left atrium stretches, then the left ventricle enlarges to keep pace. For years, and sometimes even many years, the heart manages this remarkably well, and your dog looks completely fine. That’s what makes this disease so deceptive.

Eventually, the pressure on the left side of the heart rises beyond what it can handle. That pressure backs up into the lungs, and fluid begins leaking into the air spaces. This is pulmonary oedema, and it marks the onset of congestive heart failure (CHF).

Which dogs are most at risk?

Breeds
MMVD is responsible for around 70–75% of all heart disease cases in dogs. It’s heavily skewed toward small and toy breeds, and the risk increases with age.

Cavalier King Charles Spaniels are the poster breed for MMVD. They develop the disease younger and far more commonly than other breeds. One prevalence study estimates their risk to be 20 to 30 times higher than that of mixed-breed dogs, and virtually all Cavaliers are expected to develop the disease by 10 years of age. Targeted screening programmes in places like Denmark and the UK have actually delayed the average age of onset in Cavaliers by up to two years, which proves that ethical breeding makes a real difference.

Other commonly affected breeds include Dachshunds, Miniature and Toy Poodles, Chihuahuas, Maltese, Pomeranians, Yorkshire Terriers, Shih Tzus, Miniature Schnauzers, and Cocker Spaniels.

In large breed dogs, MMVD is uncommon compared with small breeds, but it can still occur. When it does, it may be accompanied by other cardiac changes such as abnormal heart rhythms, which can make disease progression and prognosis more variable and less predictable.

Age & Sex
Age is the quiet driver. The older the dog, the more likely the valve has been quietly degenerating. Mild sex patterns appear in some studies, but in real life it’s less “male versus female” and more “how long has this valve been under strain, given the dog’s genetics and build.”

If you have a high-risk breed, this is the key takeaway: documented cardiac screening of breeding dogs is far more meaningful than general claims of “no heart problems in our lines” without supporting test results.

How is MMVD staged?

Vets follow a four-stage system by the ACVIM consensus guidelines to describe where a dog is on the MMVD journey. It is used as a guide to whether treatment is needed, when to start it, and which medications are most appropriate.

Stage	What's happening	What you might notice at home	Typical next move
A	At-risk breed, no murmur	Nothing	Annual check-ups, smart breeding choices
B1	Murmur present, no heart enlargement	Usually nothing	Monitor every 6–12 months
B2	Murmur and measurable enlargement	Often still nothing visible	Start pimobendan, closer follow-up
C	Current or past episode of CHF	Cough, fast breathing, fatigue	Diuretics plus core CHF meds
D	CHF that no longer responds to standard treatment	Breakthrough symptoms despite therapy	Specialist management, advanced options

Stage B1 vs B2
Stage B1 is often the “watch and monitor” phase of MMVD. A heart murmur is present, but the heart has not yet changed enough to justify starting certain medications. At this stage, many dogs still feel completely normal and can continue living happy, active lives. Your veterinarian will usually recommend regular rechecks, may suggest an echocardiogram to assess the heart more closely, and will monitor for any signs that the disease is beginning to progress.

Stage B2 is the “don’t ignore this” stage even if your dog looks completely normal. There is evidence of heart enlargement on imaging, and starting pimobendan now buys real time. The EPIC trial, a huge, multi-centre randomised study, showed that pimobendan in B2 dogs delayed the onset of CHF by a median of about 15 months, and extended overall survival by around five months.

Stage C and D
Stage C means CHF has happened, typically with pulmonary oedema. From here, treatment shifts from “delay” to “manage”: getting fluid out of the lungs, easing the workload on the heart, and keeping breathing comfortable.

Stage D is the advanced stage of MMVD, where CHF becomes difficult to control despite standard treatment. Dogs in this stage often need higher medication doses, additional drugs, and much closer monitoring to keep them comfortable. The focus shifts toward maintaining quality of life, managing breathing difficulties, and supporting both the dog and owner through a more fragile phase of the disease.

Typical timelines

Many owners want a clear timeline for how MMVD will progress, but the reality is that every dog follows a different path. Some dogs remain in Stage B1 for years with very little change, while others progress more quickly into Stage B2 or congestive heart failure. Sudden complications such as chordal rupture, tricuspid valve disease, or arrhythmias can also dramatically alter the course of the disease. The most important thing is not predicting exactly how the disease will progress, but monitoring closely enough to recognise changes early and respond before problems become more serious.

What clinical signs do I look out for?

The reality with the early stages of MMVD is that many dogs show no obvious symptoms at all. The heart is still compensating effectively, so most dogs continue to run, play, sleep, and behave completely normally. When something does start to show, here’s what you might notice:

Coughing: especially at night, with excitement, or with collar pressure. Cough is the sign people latch onto, and it’s also the sign that lies. It can be heart-related, but it can also be tracheal collapse, bronchitis, kennel cough or airway disease. Imaging is how vets sort that out.
Faster breathing at rest or in sleep: the single most reliable sign at home (more on this below)
Slowing down on walks: exercise intolerance, longer recovery, less interest
Restless sleep: moving around more, struggling to settle
Appetite changes or weight loss: late-stage signs
Fainting or collapse: uncommon but treat as urgent

Treat as an emergency if you notice:

Open-mouth breathing while at rest
Heavy, laboured breathing or visible distress
Blue, grey or pale gums
Collapse, fainting or sudden weakness
Sustained breathing rate well above 40 breaths per minute at rest

The single most useful thing to do at home – count sleeping breaths!

If you do one thing for a dog with MMVD, do this: track sleeping respiratory rate.

A sleeping dog can’t fake their breathing. While they’re resting or sleeping (not dreaming), count how many breaths they take in 15 seconds (one breath is one rise and fall of the chest), then multiply by four. That’s their breaths per minute. For most dogs, a normal breathing rate during sleep is under 30 breaths per minute (bpm), including dogs with well-controlled heart disease.

Call your vet if:

Resting or sleeping respiratory rate is above 30-35 bpm
The rate climbing consistently from your dog’s normal baseline
Breathing looks fast even when your dog is fully relaxed and asleep

Sleeping respiratory rate is the most validated, owner-friendly way to catch pulmonary oedema early, often a day or two before the cough and breathlessness arrive. Write the numbers down in a notes app or a simple log. Pattern matters more than any one reading.

How do vets diagnose and stage MMVD?

A murmur during a routine check-up is usually the first clue. Murmurs are graded 1 to 6 on loudness, and while louder generally means more leak, the relationship isn’t perfect. Staging depends on what’s happening inside the heart, not just what it sounds like.

From there, vets layer in:

Echocardiography: This is the gold standard. It measures chamber sizes, looks at the valve, quantifies the regurgitation, and checks for complications like pulmonary hypertension. It’s what definitively places your dog in B1, B2 or beyond.
Chest X-rays (thoracic radiographs): Provide an overall view of heart size and can determine whether there’s fluid in the lungs. This is how vets confirm CHF.
NT-proBNP: A blood biomarker that supports diagnosis and helps with borderline cases. It can also help differentiate cardiac from non-cardiac breathing problems in an emergency.
Blood pressure: Affects how hard the heart has to work and how much blood leaks back. It is also used to monitor the response to medications that reduce systemic blood pressure (such as ACE inhibitors).

Medical therapy according to the MMVD Stage

Treatment matches the stage. Doing too much too early causes problems and doing too little once CHF arrives causes a faster decline.

Stage	Typical medication	What it's doing
A and B1	None	No drug benefit yet, focus on monitoring
B2	Pimobendan (~0.25–0.3 mg/kg every 12 hours)	Delays the onset of CHF, reduces remodelling load
C	Pimobendan + diuretic (frusemide/furosemide) + ACE inhibitor + spironolactone	Clears lung fluid, stabilises pressures, supports function
D	Switch to or add torsemide, optimise dosing, add sildenafil if pulmonary hypertension is present	Manage refractory CHF, maintain quality of life

A note on diuretics

Frusemide (fru-see-mide) is the standard first-line diuretic used to manage fluid overload in MMVD, while torsemide (tor-seh-mide) is a more potent, longer-acting alternative often reserved for more advanced or difficult-to-control cases of heart failure. Both work by helping the body remove excess fluid and ease breathing. Dogs on diuretics need regular bloodwork to check kidney function and electrolytes.

Pulmonary hypertension

A subset of dogs with advanced MMVD develops secondary pulmonary hypertension (pul-muh-nair-ee hy-per-ten-shun) on top of everything else, which adds further strain on the heart and can make clinical management more complex. Sildenafil (sil-den-uh-fil; yes, the same drug humans take for a different reason) is the go-to treatment.

Diet

Diet plays a supportive but not primary role in managing MMVD in dogs. While it cannot stop the progression of the disease, veterinary guidelines recommend maintaining a balanced, high-quality diet that supports lean body condition and avoids excess sodium, particularly as heart disease advances. In later stages, moderate sodium restriction may help reduce fluid retention and ease the workload on the heart, while ensuring adequate protein and calories is important to prevent muscle loss. Omega-3 fatty acids are also commonly recommended for their anti-inflammatory and potential cardiac benefits. Overall, nutrition is used alongside medication to support quality of life rather than as a standalone treatment.

Surgical options for MMVD

Surgical options for MMVD in dogs do exist, but they are highly specialised and not commonly performed in Australia. There are two main procedures: open-heart mitral valve repair using cardiopulmonary bypass, and minimally invasive transcatheter repair (TEER), which works on the beating heart. Both aim to reduce the leaking through the mitral valve rather than just manage symptoms with medication.

In Australia, these procedures are only available in a small number of specialist referral centres and are typically reserved for carefully selected cases. Availability is limited, and not every dog is a suitable candidate due to the strict assessment criteria, surgical risk, and required cardiac anatomy.

Cost is also a major factor. Surgery is usually considered a very high-cost option, often ranging from tens of thousands of dollars (commonly around $40,000-$70,000 depending on the procedure and case complexity), which places it beyond the scope of standard treatment for most pet owners.

In terms of prognosis, successful surgical repair can significantly extend survival and may offer a near-normal quality and length of life in selected dogs, whereas medical therapy alone focuses on slowing progression and managing symptoms but cannot stop the underlying valve disease from advancing.

For most dogs in Australia, MMVD is therefore managed medically with long-term medication, monitoring, and quality-of-life support, with surgery remaining an uncommon but emerging option in select cases.

Living with MMVD at home

For most dogs on medical management, life looks pretty normal between vet visits. Your role becomes part-monitor, part-medicator, part-record-keeper.

A simple home routine that works:

Count sleeping breaths a few times a week: Note the number and the date.
Watch energy and appetite: Subtle drops over a week matter more than a single off day.
Weigh weekly if you can: Sudden weight gain can mean fluid retention; weight loss can indicate muscle breakdown, known as cardiac cachexia (kar-dee-ak kuh-kek-see-uh).
Give medications on time, every time: Most owners realistically manage twice-daily dosing well. Three-times a day medication can be hard to keep up with, so it’s important to let your vet know so the treatment plan can be adjusted or simplified where possible.
Keep a written list of medications, doses, and times: somewhere you can hand to an emergency vet if needed.

What you might pay and where insurance fits in

Heart disease management isn’t cheap, and the costs add up over years rather than hitting you all at once.

The medical management of MMVD is usually a long-term, ongoing cost rather than a single treatment expense. For dogs in the earlier stages, medication and routine monitoring typically cost a few hundred dollars per month, depending on the dog’s size and treatment plan. As the disease progresses, costs increase due to the need for multiple heart medications, more frequent vet visits, imaging such as x-rays or echocardiograms, and occasional emergency care if heart failure develops. Overall, the financial commitment tends to rise steadily with disease severity, particularly once clinical heart failure is present.

For surgery in Australia, the upfront cost is substantial and generally not covered unless your dog has comprehensive pet insurance with high annual limits. The commonly quoted $40,000-$70,000 AUD range typically includes the procedure and immediate specialist hospital care, but does not usually account for referral travel, pre- and post-operative diagnostics, or the cost of managing any complications that may arise afterwards.

This is where having pet insurance in place before a heart murmur is detected matters. Once MMVD is diagnosed, it becomes a pre-existing condition, meaning any policy taken out after that point won’t cover it. There is also a 14-day waiting period from when your policy starts, so any signs or symptoms that appear in that window will be excluded too.

Petsy covers heart conditions and heart disease like any other illness, with no sub-limits on what we’ll pay toward eligible vet costs, but only when the policy is in place before any clinical signs appear. If you’ve got a high-risk breed such as a Cavalier, Dachshund, Cavoodle, Maltese, or small mixed-breed, getting cover sorted while your dog is young and healthy is the single most useful financial move you can make.

Prognosis

Owners always want a timeline. Vets rarely give one because the variation is genuinely huge.

What we know:

Many dogs sit in Stage B1 for years with no medication and no symptoms.
Dogs that enter Stage B2 and start pimobendan get a median of about 15 extra months before CHF, compared to placebo.
Median survival from a first CHF episode (Stage C) on standard medication is around 9-12 months, with significant variability. Some dogs do years, some don’t.
Successful surgical repair can extend survival to several years in selected dogs.
Negative signs to know about: refractory CHF, atrial fibrillation with a fast ventricular rate, severe pulmonary hypertension, rapid heart enlargement despite treatment, and persistently elevated NT-proBNP.

These numbers are not a fixed outcome or a prediction of what will happen, but a general guide to help understand the disease.

Questions to ask your vet at each visit

Polite vagueness doesn’t help your dog. These are fair, useful questions:

What stage are we in, and what specifically put us there?
Do we meet Stage B2 criteria, and should we be starting pimobendan now?
How should I monitor my dog’s normal sleeping breathing rate, and what number should make me call you?
What side effects should I watch for with each medication, and what’s the plan if appetite drops?
When do you want the next recheck, and is it time to involve a cardiologist?
If we ever get to CHF, what’s the emergency plan: which hospital, which medication, what to do in the car?

Frequently asked questions about Understanding Myxomatous Mitral Valve Disease (MMVD) in Dogs

Can a dog live a long time with MMVD?

Yes. Many do, especially when caught early and managed properly by stage. The dogs that struggle are usually those whose disease isn’t detected until well into CHF, or whose owners can’t keep up with monitoring and medications.

Is coughing always from the heart?

No, and that’s a common misdiagnosis trap. Cough has many causes in older small-breed dogs (collapsing trachea, chronic bronchitis, kennel cough), and “cardiac cough” is genuinely overdiagnosed. Chest X-rays and an echocardiograph are how vets separate the two.

Does a louder murmur mean it's worse?

Sometimes, not always. Murmur intensity is a clue, but staging depends on heart enlargement and regurgitation severity, not just sound.

Should I restrict exercise?

In Stage B1 and most of B2, normal sensible activity is fine and probably helpful. Once a dog is in CHF, exercise should be adjusted to what they can comfortably handle, and your vet can help you draw that line.

Can the valve actually be fixed?

Yes, in a small number of carefully selected dogs with open-heart valve repair or newer transcatheter procedures. However, it’s not a routine treatment and it comes with significant cost and strict eligibility criteria.

Is MMVD inherited? Should I worry about my dog's siblings or parents?

The genetic contribution is strong, especially in Cavaliers. It’s polygenic (pol-ee-jen-ik), meaning multiple genes are involved and heritability estimates for murmur severity are high. If you’ve got a related dog and they are a predisposed breed, it is recommended to get them checked.

The bottom line

MMVD is common, slow-moving for most dogs, and very manageable when you’re paying attention. The take-home messages are:

Annual check-ups catch murmurs early
Stage B2 is the moment pimobendan changes outcomes, so it is important to get the echo done.
Sleeping respiratory rate is your most useful daily tool at home.
Open-mouth breathing or collapse means emergency vet now, not later.
Surgery exists for selected dogs and is increasingly accessible.

If your dog has just been diagnosed, take a breath. The work ahead is monitoring, medicating, and showing up for rechecks. Not glamorous work, but it’s the work that buys time and quality of life. Your vet is your team-mate in this, and with the right plan in place, most dogs do far better than that first scary diagnosis suggests.

Marcella Zhang

Pet Lover

Marcella is a final year Doctor of Veterinary Medicine student at The University of Sydney with a strong passion for small animal medicine and animal welfare. She enjoys helping pet owners better understand their pets’ health and wellbeing, with particular interests in preventative healthcare and dentistry. In her free time, Marcella likes to explore new food spots and binge Netflix shows.

LEARN MORE ABOUT MARCELLA

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Understanding Myxomatous Mitral Valve Disease (MMVD) in Dogs

What exactly is MMVD and how bad is it?

What actually happens to the mitral valve?

Which dogs are most at risk?

How is MMVD staged?

What clinical signs do I look out for?

The single most useful thing to do at home – count sleeping breaths!

How do vets diagnose and stage MMVD?

Medical therapy according to the MMVD Stage

A note on diuretics

Pulmonary hypertension

Diet

Surgical options for MMVD

Living with MMVD at home

Prognosis

Questions to ask your vet at each visit

Frequently asked questions about Understanding Myxomatous Mitral Valve Disease (MMVD) in Dogs

The bottom line

Marcella Zhang

Pet Insurance

Pet insurance Info

More Links

Our Socials

About us

Work With Us

Media

Our Socials

Members Benefits

Become a Partner

More

Our Socials

How We work with Vets

Work with your Clinic

Vet News

Our Socials

DOG

CAT

Pet Info

Our Socials

Pet Insurance Hub

Insurance

Policy Admin

Our Socials

Tail Talk Hub

Our Socials

Contact us

Opening Hours

Legal Documents

Our Socials

Optional Extra Benefits