The Complete Guide to MMVD (Mitral Valve Disease) in Dogs

Why this guide exists

If your dog has been diagnosed with Myxomatous Mitral Valve Disease — or if your veterinarian has mentioned a heart murmur and you are trying to understand what comes next — this guide is for you. MMVD is the most common acquired heart disease in dogs, and if you own a small breed, there is a meaningful chance you will encounter it at some point in your dog's life.

The good news, and there truly is good news here, is that MMVD is one of the most well-studied and well-understood heart conditions in veterinary medicine. We know how it progresses. We know when to intervene. We have a landmark clinical trial — the EPIC study — that proved a specific medication can delay heart failure by over a year. And we know that many dogs with MMVD live long, happy, comfortable lives with appropriate monitoring and care.

This guide covers everything: what the disease is, which dogs are affected, how it is staged and monitored, what treatments exist, and what the research actually shows about outcomes.

What is MMVD?

Myxomatous Mitral Valve Disease is a progressive degenerative condition affecting the mitral valve of the heart. The mitral valve sits between the left atrium (the upper chamber that receives oxygenated blood from the lungs) and the left ventricle (the powerful lower chamber that pumps blood out to the body). Its job is simple but critical: open to let blood flow from the atrium to the ventricle, then close tightly to prevent blood from flowing backward when the ventricle contracts.

In MMVD, the valve leaflets undergo myxomatous degeneration — the tissue gradually thickens, becomes irregular, and loses its structural integrity. The smooth, thin leaflets become nodular and distorted. The chordae tendineae, the tiny cord-like structures that anchor the valve leaflets, can stretch and eventually rupture.

The result: the valve no longer closes properly, and blood leaks backward from the ventricle into the atrium with each heartbeat. This backward flow is called mitral regurgitation, and it is what your veterinarian hears as a heart murmur.

A slow process, not a sudden event

One of the most important things to understand about MMVD is that it is almost always a slowly progressive disease. The valve does not suddenly fail. It degenerates over months and years. In many dogs, the leak remains small and hemodynamically insignificant for a very long time — sometimes for the rest of the dog's life. The heart compensates by enlarging slightly to handle the extra volume, and the dog remains completely asymptomatic.

Only when the regurgitation becomes severe enough to overwhelm the heart's compensatory mechanisms does congestive heart failure develop. Even then, treatment can often manage symptoms effectively for extended periods.

How MMVD differs from DCM

If you have read about Dilated Cardiomyopathy on this site, it is helpful to understand the key differences:

• MMVD is a valve problem. The heart muscle itself starts out healthy. The issue is a leaky valve. DCM, by contrast, is a disease of the heart muscle — the muscle weakens and the chambers dilate.
• MMVD primarily affects small breeds. DCM primarily affects large and giant breeds.
• MMVD progresses more slowly in most cases. Many dogs live years — even the rest of their natural lifespan — with MMVD before it causes any problems.
• The prognosis is generally more favorable. While both are serious conditions, MMVD dogs typically have more time and respond well to medication.

How common is MMVD?

MMVD is remarkably common. It is the single most prevalent acquired heart disease in dogs, accounting for approximately 75 percent of all canine heart disease seen in veterinary practice.

The prevalence increases dramatically with age:

• Roughly 10 percent of dogs aged 5 to 8 have some degree of mitral valve degeneration.
• By age 10 to 12, prevalence rises to approximately 30 to 35 percent of dogs.
• In dogs over 16 years of age, approximately 75 percent have evidence of MMVD on examination or echocardiography.

These numbers do not mean that all of these dogs develop heart failure — most do not. But they illustrate just how common the underlying valve degeneration is, particularly in aging small-breed dogs.

Which breeds are affected?

MMVD can occur in any breed, but it disproportionately affects small to medium-sized breeds. The breeds most commonly and severely affected include:

• Cavalier King Charles Spaniel — By far the most affected breed. Nearly 100 percent of Cavaliers will develop MMVD by age 10, and many show evidence by age 5.
• Dachshund — One of the most commonly affected breeds, with high prevalence by middle age.
• Chihuahua — Frequently affected, often presenting with murmurs in middle to later life.
• Miniature and Toy Poodle — Common candidates, particularly as they age.
• Yorkshire Terrier — High prevalence, often developing murmurs in their senior years.
• Pomeranian — Frequently affected, with murmurs often detected by mid-life.
• Maltese — Another small breed with significant MMVD prevalence.
• Miniature Schnauzer — Commonly affected among terrier breeds.
• Cocker Spaniel — Both American and English Cocker Spaniels show increased prevalence.
• Shih Tzu — Frequently affected as they age.

Key point: While MMVD is predominantly a small-breed disease, it can occur in larger dogs as well. When it does occur in large breeds, it tends to progress more quickly and more aggressively.

Male dogs are affected slightly more often than females, and males tend to develop more severe disease at a younger age, though the reasons for this sex difference are not fully understood.

The heart murmur: your first clue

For most MMVD dogs, the journey begins when a veterinarian detects a heart murmur during a routine physical examination. A heart murmur is simply the sound of turbulent blood flow — in this case, the sound of blood leaking backward through the mitral valve.

Heart murmurs are graded on a scale from I to VI:

| Grade | What it sounds like | What it suggests | |-------|-------------------|-----------------| | I | Very soft, barely audible, heard only in a quiet room with careful auscultation | Very mild regurgitation | | II | Soft but readily audible | Mild regurgitation | | III | Moderate intensity, easily heard | Moderate regurgitation | | IV | Loud, prominent | Significant regurgitation | | V | Very loud, with a palpable thrill (vibration felt through the chest wall) | Severe regurgitation | | VI | Audible with the stethoscope held slightly off the chest wall | Severe regurgitation |

It is important to understand that murmur grade does not perfectly correlate with disease severity, though there is a general relationship. A grade I murmur in a 12-year-old Chihuahua may never progress to anything clinically significant. A grade IV murmur in a 6-year-old Cavalier warrants prompt evaluation.

The murmur grade also does not tell you the stage of the disease. That requires imaging — specifically, an echocardiogram.

The ACVIM staging system

The American College of Veterinary Internal Medicine developed a standardized staging system for heart disease that is now universally used by veterinary cardiologists. Understanding these stages is essential because they determine when treatment starts and what monitoring is needed.

Stage A: at risk, no disease detected

Stage A includes dogs that are predisposed to MMVD but have no detectable disease. No murmur, no structural changes, normal echocardiogram.

Who falls here:

• Small breed dogs from affected breeds, particularly Cavalier King Charles Spaniels
• Dogs with a family history of MMVD

What to do:

• Be aware of the risk
• Ensure your veterinarian listens to the heart carefully at every visit
• For high-risk breeds like Cavaliers, consider baseline echocardiography

No medication is needed at this stage.

Stage B1: murmur present, no significant heart enlargement

Stage B1 means your dog has a detectable heart murmur and echocardiography confirms mitral regurgitation, but the heart has not yet enlarged significantly in response. The leak is there, but it is small enough that the heart is handling it without needing to remodel.

Echocardiographic criteria:

• Left atrial to aortic root ratio (LA:Ao) less than 1.6
• Normalized left ventricular internal diameter in diastole (LVIDdN) less than 1.7

What to do:

• No cardiac medications are recommended at this stage. This is important — starting medication too early has not been shown to provide benefit.
• Monitor. Your cardiologist will recommend rechecks, typically every 6 to 12 months depending on the murmur grade and clinical picture.
• Learn to count your dog's sleeping respiratory rate at home. This becomes your most valuable early warning system going forward.
• Continue normal activity. There are no exercise restrictions at Stage B1.

Prognosis at Stage B1: Excellent. Many dogs remain at Stage B1 for years — in some cases, for the remainder of their lives. Some will never progress to B2. This is a stage for monitoring, not for worrying.

Stage B2: murmur present WITH significant heart enlargement

Stage B2 is the critical threshold. The mitral regurgitation has become severe enough that the heart has enlarged in response — the left atrium is bigger because it is receiving blood from both the pulmonary veins and the leaky valve, and the left ventricle is bigger because it is handling a larger volume of blood with each beat.

Echocardiographic criteria defining B2:

• LA:Ao ratio of 1.6 or greater — The left atrium is enlarged relative to the aortic root
• LVIDdN of 1.7 or greater — The left ventricle is enlarged in diastole
• Some cardiologists also use vertebral heart score as an additional measurement

Why B2 matters so much:

This is where treatment begins, and for a very good reason — the landmark EPIC study proved that starting pimobendan at Stage B2 significantly delays the onset of congestive heart failure.

Treatment at Stage B2:

• Pimobendan (Vetmedin) — This is the cornerstone medication, supported by the strongest clinical evidence available. More on the EPIC study below.
• Monitoring frequency increases — Echocardiograms typically every 4 to 6 months, plus regular resting respiratory rate monitoring at home.
• Continue normal activity unless your cardiologist advises otherwise.

Prognosis at Stage B2: With pimobendan, many dogs at Stage B2 have well over a year — and in many cases several years — before progressing to heart failure. The EPIC study showed that treatment extended the pre-heart failure period by a median of 15 months.

Stage C: congestive heart failure

Stage C means the dog has developed — or has previously developed — signs of congestive heart failure. The heart can no longer compensate for the volume overload, and fluid begins to accumulate, most commonly in the lungs (pulmonary edema).

Signs of Stage C:

• Increased respiratory rate and effort, especially at rest or during sleep
• Coughing, particularly at night or when lying down
• Decreased appetite and energy
• Restlessness, inability to settle comfortably
• In severe cases, respiratory distress requiring emergency care

Treatment at Stage C:

• Furosemide (Lasix) — A diuretic that removes excess fluid from the lungs and body. This is the most important medication in acute heart failure.
• Pimobendan — Continued from B2, or started now if not previously prescribed.
• ACE inhibitors (enalapril or benazepril) — Help reduce the workload on the heart.
• Spironolactone — A mild diuretic that also counteracts some of the harmful neurohormonal activation in heart failure.
• Dietary management — Moderate sodium restriction and ensuring adequate caloric intake.
• Activity modification — Guided by your dog's comfort and ability. Gentle walks are usually fine; strenuous activity should be avoided.

Prognosis at Stage C: Variable, but with aggressive medical management, many dogs with MMVD live 12 to 18 months or longer after their first episode of heart failure. Some dogs do significantly better. Quality of life can be very good with proper medication adjustments.

Stage D: refractory heart failure

Stage D is reserved for dogs whose heart failure no longer responds adequately to standard medications. Symptoms persist or worsen despite maximum doses of furosemide, pimobendan, ACE inhibitors, and spironolactone.

Additional treatments that may be considered at Stage D:

• Increased or additional diuretics — Torsemide may replace furosemide as a more potent option. Hydrochlorothiazide may be added.
• Sildenafil — If pulmonary hypertension is present.
• Pimobendan dose increases — Some cardiologists will increase to three times daily dosing.
• Dietary support — Maintaining nutrition becomes critical as appetite often declines.
• Palliative care — At this stage, the focus shifts increasingly toward comfort and quality of life.

Prognosis at Stage D: Guarded. This is advanced disease, and while some dogs can be stabilized temporarily with medication adjustments, the trajectory is progressive. The emphasis should be on quality of life, comfort, and making the most of the time you have.

The EPIC study: the evidence that changed everything

The EPIC study (Evaluating Pimobendan In Cardiomegaly) is the single most important clinical trial in MMVD treatment. Published in 2016, it is the gold standard that guides how veterinary cardiologists manage Stage B2 MMVD worldwide.

What the study tested

The EPIC study was a prospective, randomized, double-blinded, placebo-controlled, multicenter clinical trial — the highest level of clinical evidence. It enrolled 360 dogs with preclinical MMVD and cardiac enlargement (Stage B2) across 36 sites in 11 countries.

Dogs were randomly assigned to receive either pimobendan or a placebo and followed until they developed congestive heart failure, died from cardiac causes, or the study ended.

What the study found

The results were striking:

Dogs receiving pimobendan reached the primary endpoint (onset of congestive heart failure, cardiac death, or euthanasia due to cardiac disease) at a median of 1228 days (approximately 3.4 years), compared to 766 days (approximately 2.1 years) for the placebo group.

That is a median delay of approximately 15 months — over a year of additional time before heart failure developed. The pimobendan group also had a significantly longer overall survival time.

To put this in perspective: for a disease where we are measuring time in months and years, gaining an additional 15 months of good-quality, symptom-free time is enormous. This is time where your dog is still active, comfortable, and enjoying life — not time spent managing heart failure symptoms.

What this means for your dog

If your dog is at Stage B2, the evidence is clear: pimobendan should be started. This is not a judgment call or a matter of clinical preference. The EPIC study provides the strongest evidence available that early intervention with pimobendan at Stage B2 meaningfully delays disease progression.

If your veterinarian diagnoses Stage B2 MMVD and does not discuss pimobendan, ask about it. If your dog is being managed by a general practitioner rather than a cardiologist, consider requesting a referral to confirm staging and ensure the treatment plan aligns with current evidence.

Monitoring: your role as a heart dog parent

Monitoring is central to managing MMVD at every stage. Some of it happens in the veterinary clinic, and some of it happens at home — and the at-home component is arguably the most important of all.

Resting respiratory rate

This is the single most valuable thing you can do at home. Your dog's resting respiratory rate (RRR) — the number of breaths per minute when relaxed or sleeping — is a remarkably sensitive early indicator of fluid accumulating in the lungs.

How to count it:

• Wait until your dog is relaxed, resting, or ideally sleeping.
• Count the number of times the chest rises and falls in 15 seconds.
• Multiply by 4 to get breaths per minute.
• Record it. Do this daily, or at least several times a week, so you know your dog's baseline.

What is normal:

• Most dogs breathe 15 to 25 times per minute at rest.
• A consistent resting rate above 30 breaths per minute in a dog with known MMVD is a warning sign and should prompt a call to your veterinarian.
• A sudden increase from your dog's baseline — even if still under 30 — is worth noting and monitoring closely.

There are smartphone apps, such as Cardalis RRR and others, that make counting and tracking respiratory rates easy.

Key point: Learning to count your dog's resting respiratory rate is the most impactful thing you can do as an MMVD dog parent. It can detect heart failure before obvious symptoms appear and allow early intervention that can be lifesaving.

Echocardiography

Echocardiograms — cardiac ultrasounds — are how your cardiologist monitors the structural progression of MMVD. They measure chamber sizes, valve function, blood flow patterns, and the overall performance of the heart.

Recommended frequency:

• Stage B1: Every 6 to 12 months, depending on murmur grade and clinical picture.
• Stage B2 (on pimobendan): Every 4 to 6 months.
• Stage C: As directed by your cardiologist, often every 3 to 6 months or as clinical status changes.

Radiography (chest X-rays)

Chest X-rays complement echocardiography by showing:

• Overall heart size (vertebral heart score)
• Presence of pulmonary edema (fluid in the lungs)
• Pulmonary vein distension

They are particularly valuable when evaluating whether a dog has developed congestive heart failure.

Blood work

Regular blood work — particularly kidney values and electrolytes — is important for dogs on cardiac medications, especially diuretics. Furosemide can affect kidney function and electrolyte balance, so monitoring these values helps your veterinarian adjust medications safely.

Medications: a summary by stage

| Stage | Medication | Purpose | |-------|-----------|---------| | A | None | No disease present | | B1 | None | Heart not yet enlarged | | B2 | Pimobendan | Delays onset of heart failure (EPIC study) | | C | Pimobendan + Furosemide + ACE inhibitor + Spironolactone | Manage heart failure symptoms | | D | All Stage C medications + dose adjustments, torsemide, potentially sildenafil | Manage refractory heart failure |

Prognosis: what the research tells us

The prognosis for MMVD is, on the whole, more favorable than for many other forms of heart disease:

• Stage B1 dogs can remain stable for years. Many will live out their natural lifespan without ever developing heart failure. Some will never progress beyond B1.
• Stage B2 dogs on pimobendan have a median time of over 3 years before developing heart failure (based on the EPIC study data).
• Stage C dogs with well-managed heart failure often live 12 to 18 months or longer after their first episode. Some dogs do considerably better, particularly with attentive monitoring and medication management.
• Stage D carries a more guarded prognosis, but even here, quality of life can often be maintained for a period with aggressive medical management.

Many MMVD dogs live to be quite old. A 10-year-old Cavalier diagnosed with Stage B1 MMVD may well live to 14 or 15 with appropriate monitoring. A 12-year-old Yorkshire Terrier with a grade II murmur may never need any cardiac medication at all.

Key point: A diagnosis of MMVD is not a death sentence. For many dogs, it is a manageable chronic condition that, with proper veterinary care and attentive ownership, allows for years of happy, comfortable life.

Living with an MMVD dog

Exercise and activity

At Stages A, B1, and B2, most dogs can continue their normal activities without restriction. MMVD dogs are not fragile — they can play, walk, and enjoy life as they always have. Your cardiologist will tell you if and when any limitations are warranted.

At Stage C and beyond, activity should be guided by your dog's comfort. Gentle walks and light play are usually fine and even beneficial. Let your dog set the pace. If they want to rest, let them rest. If they want to walk around the yard, let them.

Diet

There is no specific diet that prevents or reverses MMVD. However, some dietary considerations are worth discussing with your veterinarian:

• Sodium restriction — Moderate sodium restriction may be recommended at Stage C and beyond. Extreme sodium restriction is generally not necessary at earlier stages.
• Adequate nutrition — Dogs in heart failure can lose weight and muscle mass (cardiac cachexia). Ensuring adequate caloric and protein intake is important.
• Omega-3 fatty acids — Some evidence supports omega-3 supplementation for anti-inflammatory and anti-arrhythmic effects in heart disease.

The emotional reality

Living with a heart dog — whether their diagnosis is MMVD, DCM, or anything else — carries an emotional weight that is difficult to describe to someone who has not experienced it. There is the worry that accompanies every cough. The counting of breaths that becomes second nature. The knowledge that this disease is progressive, even when your dog seems perfectly fine today.

But there is also gratitude. Many MMVD dog parents describe the experience as one that deepens their bond with their dog. The daily monitoring becomes a ritual of care. The awareness of the disease makes each good day more precious. And the reality is that with modern veterinary cardiology, most MMVD dogs have many, many good days ahead.

The bottom line

MMVD is the most common acquired heart disease in dogs, predominantly affecting small breeds, and it is a diagnosis that many dog parents will face at some point. The disease is well understood, well studied, and highly treatable.

The key facts to carry with you:

• MMVD is a slowly progressive disease of the mitral valve. Many dogs live years without symptoms.
• The ACVIM staging system (A through D) determines when treatment starts. Stage B2 is the critical threshold.
• The EPIC study proved that pimobendan started at Stage B2 delays heart failure onset by approximately 15 months.
• Resting respiratory rate monitoring at home is the most valuable thing you can do as an MMVD dog parent.
• Prognosis is often very good, especially with early detection, proper staging, and appropriate treatment.

If your dog has been diagnosed with MMVD, take a breath. Learn the staging system. Find a veterinary cardiologist you trust. Count those resting respiratory rates. And know that with the tools available today, there is every reason to be hopeful.