Myxomatous mitral valve disease (MMVD) is the most common heart condition in dogs, especially small breeds, and progresses through four defined stages (A to D). Many dogs live for years in the early stages with no symptoms, while later stages bring coughing, exercise intolerance, and eventually heart failure. Knowing the stage guides when medication should start.
Last reviewed: June 2026
What Is Mitral Valve Disease in Dogs?
Mitral valve disease is a slowly progressive degeneration of the heart valve that separates the left atrium and left ventricle, causing it to leak. Over years the valve leaflets thicken and warp, so each heartbeat pushes some blood backward into the left atrium instead of forward to the body. This backward leak (regurgitation) is what a veterinarian hears as a heart murmur, and over time it forces the left side of the heart to enlarge to compensate.
MMVD is by far the most common acquired heart disease in dogs, accounting for roughly 75% of canine cardiac cases, and it disproportionately affects small and toy breeds such as Cavalier King Charles Spaniels, Chihuahuas, and Dachshunds. As described in the ACVIM consensus framework, the disease is defined by stages so that treatment can be matched to severity (Keene et al., 2019, JVIM (ACVIM Consensus on MMVD)).
What Are the Stages of Mitral Valve Disease?
Mitral valve disease is divided into four ACVIM stages (A through D) based on whether a murmur, heart enlargement, or heart failure is present. Understanding the stage is the single most important factor in deciding when to start medication.
- Stage A: High-risk breeds with no murmur and a structurally normal heart. No treatment needed—just monitoring.
- Stage B1: A murmur is present but the heart is not yet enlarged. No medication is indicated; rechecks track for progression.
- Stage B2: A murmur with measurable heart enlargement on x-ray and echocardiogram, but no clinical signs yet. This is the stage where starting medication (pimobendan) has been shown to delay heart failure.
- Stage C: The dog has developed clinical signs of congestive heart failure (coughing, rapid breathing, fluid in the lungs) and needs multi-drug therapy.
- Stage D: End-stage heart failure that no longer responds to standard doses, requiring advanced or specialist management.
A landmark clinical trial established that starting pimobendan in Stage B2 dogs extended the symptom-free period by roughly 15 months on average (Keene et al., 2019, JVIM (ACVIM Consensus on MMVD)).
What Are the Signs to Watch For?
The earliest sign is usually a heart murmur detected by your vet before any symptoms appear, which is why annual exams matter so much in older small-breed dogs. As the disease advances into heart failure, owners begin to notice physical signs.
Warning signs of progression include:
- A soft, persistent cough, often worse at night or after lying down
- Increased resting respiratory rate (more than 30–35 breaths per minute while sleeping)
- Reduced exercise tolerance or tiring quickly on walks
- Heavy or labored breathing
- Restlessness at night, or a swollen belly in advanced cases
- Fainting or collapse episodes
Tracking your dog's sleeping breathing rate at home is one of the most reliable early-warning tools, and routine wellness monitoring is recommended in the AAHA Preventive Healthcare Guidelines, 2011, since a sustained rise above 30 breaths per minute often signals fluid building in the lungs.
Why Does Mitral Valve Disease Happen?
MMVD is a degenerative, age-related process with a strong genetic component, particularly in predisposed small breeds. The valve tissue accumulates abnormal mucopolysaccharide deposits that make the leaflets nodular and floppy, a change described in Ettinger's Textbook of Veterinary Internal Medicine. There is no way to prevent the underlying degeneration, but early detection lets treatment begin at the optimal moment.
Routine veterinary exams matter because murmurs are usually found incidentally. The AAHA Canine Life Stage Guidelines, 2019 recommend at least annual physical exams for adult dogs and twice-yearly visits for seniors, which is how most early murmurs are caught.
How Is It Diagnosed and Treated?
Diagnosis begins with hearing a murmur, then confirming heart enlargement with chest x-rays and an echocardiogram (cardiac ultrasound). The echocardiogram is the key test for distinguishing Stage B1 from B2, because medication timing depends on whether the heart is enlarged.
Treatment is stage-driven:
- Stages A and B1: Monitoring only.
- Stage B2: Pimobendan to delay the onset of heart failure.
- Stage C: A combination of pimobendan, a diuretic (furosemide), and an ACE inhibitor to control fluid and support the heart.
- Stage D: Higher or additional diuretic doses and specialist cardiology care.
With modern multi-drug therapy, many dogs in Stage C live comfortably for a year or more after their first episode of heart failure.
When to See a Vet
Call your vet today if:
- Your dog has a known murmur and is coughing more or tiring easily
- Your dog's resting (sleeping) breathing rate climbs above 35 breaths per minute
- You notice reduced appetite or a swelling belly in a dog with heart disease
Go to the ER immediately if:
- Your dog is breathing rapidly or with great effort at rest
- Your dog's gums or tongue look blue, gray, or pale
- Your dog collapses, faints, or cannot catch its breath
- You see continuous open-mouth breathing or coughing up pink, frothy fluid
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Frequently Asked Questions
How long can a dog live with mitral valve disease?
Dogs in early stages (B1) often live for years with no symptoms and a normal lifespan. Once heart failure (Stage C) develops, median survival with treatment is commonly around 9–15 months, though many dogs do considerably better. Starting medication at the right stage and monitoring sleeping breathing rate strongly influence the outcome.
When should a dog with a heart murmur start medication?
Medication is recommended once the heart becomes enlarged—ACVIM Stage B2—even before any symptoms appear. An echocardiogram and chest x-rays confirm enlargement. Dogs with a murmur but a normal-sized heart (Stage B1) don't need medication yet, only periodic rechecks to catch the transition to B2 early.
How much does it cost to manage canine mitral valve disease?
An initial exam runs $50–150, chest x-rays $150–400, and an echocardiogram $300–600. Daily heart medications typically cost $40–120 per month combined. If a dog goes into acute heart failure, emergency stabilization and overnight hospitalization can run $1,000–3,000 or more, so early management is far cheaper than crisis care.
Is mitral valve disease in dogs painful?
The valve degeneration itself is not painful, but advanced heart failure causes real distress through breathlessness and fatigue. Dogs in congestive heart failure can feel like they cannot get enough air, which is frightening for them. Good medication control keeps most dogs comfortable, which is why prompt treatment of fluid buildup matters so much.
What is the first sign of heart failure in a small dog?
The earliest reliable sign is usually an increased resting or sleeping breathing rate, often climbing above 30–35 breaths per minute, sometimes paired with a new or worsening cough. Counting your dog's breaths during sleep and logging the number gives your vet an early, objective warning that fluid may be accumulating in the lungs.
Can mitral valve disease in dogs be cured?
In most dogs the disease cannot be cured because the valve degeneration is permanent and progressive; treatment slows progression and controls symptoms rather than reversing the damage. Surgical mitral valve repair exists at a few specialist centers worldwide but is expensive and not widely available, so medical management remains the standard of care for most dogs.
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