MMVD Stages Explained: What B1 and B2 Mean for Your Dog

Why B1 vs B2 is the most important distinction in MMVD

When your dog is diagnosed with Myxomatous Mitral Valve Disease, you will hear a lot of terminology — murmur grades, valve degeneration, regurgitation. But there is one distinction that matters more than almost any other, and it is the one that determines whether your dog starts medication or simply continues monitoring.

That distinction is B1 versus B2.

These are substages within the ACVIM (American College of Veterinary Internal Medicine) staging system, and the difference between them comes down to a specific question: Has the heart enlarged in response to the leaky valve?

If the answer is no, your dog is B1. If the answer is yes, your dog is B2 — and the landmark EPIC study proved that starting pimobendan at B2 delays heart failure by a median of approximately 15 months.

Understanding where your dog falls on this spectrum is not just academic. It directly determines the treatment plan, the monitoring schedule, and the conversation you should be having with your cardiologist.

The ACVIM staging system: the full framework

Before diving deep into B1 and B2, it helps to see the complete staging system. The ACVIM developed this framework through consensus statements by panels of the world's leading veterinary cardiologists. It applies to all forms of chronic heart disease, but it is most commonly used — and was originally designed around — MMVD.

Stage A: predisposed but healthy

Stage A is not a diagnosis of disease. It is a designation for dogs that are at increased risk of developing MMVD based on their breed, genetics, or family history.

Dogs in Stage A include:

• Cavalier King Charles Spaniels, Dachshunds, Chihuahuas, Toy Poodles, Yorkshire Terriers, Pomeranians, and other small breeds with known high prevalence
• Dogs whose parents or siblings have been diagnosed with MMVD
• Any small-breed dog, given the general age-related prevalence of the condition

What happens at Stage A:

• No murmur is detected
• No echocardiographic changes
• No medications are needed
• Awareness and regular veterinary examinations are the strategy

Monitoring: Standard annual veterinary visits with careful cardiac auscultation. For Cavalier King Charles Spaniels, many cardiologists recommend baseline echocardiography starting at age 1 to 2, given the extraordinary prevalence in this breed.

Stage B: disease present, no symptoms

This is where most MMVD dogs will spend the majority of their cardiac journey. A murmur has been detected, echocardiography confirms mitral regurgitation, but your dog shows no clinical signs — no coughing, no labored breathing, no exercise intolerance. From the outside, your dog looks and acts completely normal.

Stage B is divided into two substages, and this division is where the real clinical significance lies.

Stage C: heart failure has occurred

Stage C means the dog has developed, or has previously developed, congestive heart failure — most commonly presenting as pulmonary edema (fluid in the lungs) due to the overwhelmed left side of the heart.

Stage D: refractory heart failure

Stage D is reserved for dogs whose heart failure symptoms persist despite standard medical therapy at maximum doses.

We will cover C and D in detail later in this article, but the most critical discussion for most MMVD dog parents centers on the B stages.

Stage B1: the monitoring phase

What B1 means

Your dog has a heart murmur. An echocardiogram confirms that the mitral valve is leaking. But the heart itself — the chambers, the muscle — has not significantly enlarged. The leak is present but small enough that the heart is handling the extra workload without structural remodeling.

The numbers that define B1

Your cardiologist uses specific measurements on echocardiography to determine whether your dog is B1 or B2. At B1, the key measurements fall below the thresholds for cardiac enlargement:

• LA:Ao ratio less than 1.6 — The left atrial to aortic root ratio. This compares the size of the left atrium (the chamber receiving the regurgitant blood) to the aorta. Below 1.6, the left atrium is not considered significantly enlarged.
• LVIDdN less than 1.7 — The normalized left ventricular internal diameter in diastole. This measures how large the left ventricle is when relaxed and fills with blood, adjusted for the dog's body weight. Below 1.7, the ventricle is not considered significantly enlarged.

These measurements must be interpreted by an experienced cardiologist using proper echocardiographic technique. This is one of many reasons why referral to a board-certified veterinary cardiologist — rather than relying solely on general practice assessment — is strongly recommended for staging.

Treatment at B1

No cardiac medications are recommended at Stage B1. This is an important point, and one that sometimes causes confusion or anxiety. Your dog has heart disease — should they not be on medication?

The evidence is clear: starting pimobendan or other cardiac medications before the heart has enlarged (i.e., at B1) has not been shown to provide benefit. The EPIC study specifically enrolled dogs with cardiac enlargement (B2), and the benefit demonstrated was in that population. Extrapolating the results to B1 dogs is not supported by the data.

Starting medication too early can also complicate monitoring, as changes that might indicate natural disease progression could be masked or confused by drug effects.

What to do at B1

• Establish a resting respiratory rate baseline. Count your dog's breaths per minute when sleeping or resting deeply. Record these regularly. A normal resting rate is typically 15 to 25 breaths per minute. This baseline will become invaluable if the disease progresses.
• Schedule regular cardiology rechecks. Your cardiologist will typically recommend echocardiographic monitoring every 6 to 12 months, depending on the murmur grade and how close the measurements are to the B2 thresholds.
• Maintain normal activity. There are no exercise restrictions at B1. Let your dog run, play, and enjoy life as usual.
• Watch for changes. While symptom development at B1 would be unusual, be aware of increased respiratory rate at rest, new coughing, or decreased energy.

How long can a dog stay at B1?

This varies enormously. Some dogs remain at B1 for years — in some cases, for the rest of their lives. A 10-year-old dog diagnosed with a grade II murmur and B1 staging may never progress to B2. Others, particularly breeds like Cavalier King Charles Spaniels with aggressive disease, may progress to B2 within months to a year or two.

There is no reliable way to predict the rate of progression for an individual dog, which is why regular monitoring is essential.

Key point: Stage B1 is a stage for watchful waiting, not active treatment. It is understandable to feel anxious about the diagnosis, but the evidence-based approach is to monitor, establish baselines, and wait for the specific thresholds that trigger treatment.

Stage B2: the treatment threshold

What B2 means

Stage B2 is the stage that changes everything. The mitral regurgitation has progressed to the point where the heart has enlarged — the left atrium is dilated from receiving excess blood, and the left ventricle is dilated from handling the increased volume. The heart is remodeling in response to the chronic volume overload.

Your dog still has no symptoms. They still look and act normal. But their heart is now working harder than it should, and without intervention, the progression toward heart failure continues.

The numbers that define B2

The transition from B1 to B2 is defined by echocardiographic measurements crossing specific thresholds:

• LA:Ao ratio of 1.6 or greater — The left atrium is enlarged. This means the chamber is dilated from receiving both normal blood flow from the lungs and the regurgitant blood leaking back through the mitral valve.
• LVIDdN of 1.7 or greater — The left ventricle is enlarged in diastole. The ventricle is dilating to accommodate the increased blood volume it must pump with each beat.

Additionally, some cardiologists incorporate:

• Vertebral heart score (VHS) — A radiographic measurement of overall heart size relative to vertebral bodies. A VHS exceeding breed-specific norms supports the diagnosis of cardiac enlargement.
• Vertebral left atrial size (VLAS) — A newer radiographic measurement specifically assessing left atrial size.

Both criteria — left atrial enlargement AND left ventricular enlargement — should be met to classify a dog as B2. A dog with an enlarged left atrium but normal left ventricular size, or vice versa, may be in a borderline zone that requires careful clinical judgment and closer monitoring.

Why B2 triggers treatment: the EPIC study

The reason B2 is the treatment threshold is not based on clinical intuition or theoretical reasoning. It is based on the strongest clinical evidence in veterinary cardiology: the EPIC study.

Published in 2016, this landmark trial enrolled 360 dogs with preclinical MMVD and cardiac enlargement (Stage B2) across 36 sites in 11 countries. Dogs were randomized to receive either pimobendan or placebo and followed until they developed heart failure, died from cardiac causes, or the study concluded.

The results:

• Dogs on pimobendan reached the primary endpoint at a median of 1228 days (approximately 3 years and 4 months).
• Dogs on placebo reached the primary endpoint at a median of 766 days (approximately 2 years and 1 month).
• The difference: approximately 15 months of additional time before the onset of heart failure.

This is not a small effect. Fifteen months in a small-breed dog's life can represent a significant proportion of their remaining lifespan. And this is 15 months of pre-heart failure time — time where the dog is asymptomatic, active, and living normally.

Treatment at B2

Pimobendan (Vetmedin) is the cornerstone — and currently the only — medication recommended at Stage B2 for MMVD. It is given twice daily, typically at 0.2 to 0.3 mg/kg per dose, on an empty stomach (at least one hour before food).

Pimobendan works through two mechanisms:

• Positive inotrope — It strengthens the heart's contractions, improving cardiac output.
• Vasodilator — It relaxes blood vessels, reducing the workload on the heart.

This dual mechanism makes it particularly well-suited for MMVD, where the heart needs to pump more effectively while dealing with a leaky valve.

Other considerations at B2:

• ACE inhibitors are not routinely started at B2 in the absence of heart failure, though some cardiologists may add them based on individual clinical judgment.
• No dietary changes are typically needed beyond maintaining a high-quality, well-balanced diet.
• Activity remains unrestricted unless your cardiologist advises otherwise.

Monitoring at B2

Once your dog is on pimobendan, monitoring frequency increases:

• Echocardiography every 4 to 6 months — To track chamber sizes, valve function, and disease progression.
• Resting respiratory rate daily — This is your frontline detection system. Any sustained increase above 30 breaths per minute warrants a call to your cardiologist.
• Blood work periodically — Kidney values and electrolytes, as a baseline before any additional medications may be needed.
• Chest radiographs as clinically indicated — Particularly if there is concern about early fluid accumulation.

Key point: The B1 to B2 transition is not a crisis — it is a planned intervention point. You knew it might come, you have been monitoring for it, and there is a proven treatment ready to deploy. This is veterinary cardiology working as it should.

Stage C: when heart failure arrives

What Stage C means

Stage C means congestive heart failure has developed. The heart can no longer compensate for the severe mitral regurgitation, and fluid is accumulating — most commonly in the lungs (left-sided heart failure), though occasionally in the abdomen (if right-sided failure develops).

Signs that may indicate Stage C:

• Resting respiratory rate consistently above 30 breaths per minute
• Increased breathing effort, especially when resting or sleeping
• Coughing, particularly at night or when lying down
• Restlessness, difficulty settling
• Decreased appetite or energy
• In acute cases, open-mouth breathing or respiratory distress requiring emergency care

First episode management

The first episode of congestive heart failure is often the most alarming for dog parents. If it is severe, it may require hospitalization with oxygen therapy and intravenous diuretics. Once stabilized, dogs transition to oral medications.

Treatment at Stage C

The standard medical protocol for Stage C MMVD includes:

• Furosemide — The primary diuretic. Removes fluid from the lungs and body. Dose is adjusted based on respiratory rate and clinical status.
• Pimobendan — Continued from B2, or started now if the dog was not previously on it.
• ACE inhibitor (enalapril or benazepril) — Reduces harmful neurohormonal activation and decreases afterload.
• Spironolactone — A potassium-sparing diuretic with additional beneficial effects on cardiac remodeling.

Prognosis at Stage C

With good medical management, many MMVD dogs live 12 to 18 months or longer after their first heart failure episode. Some do significantly better. The key factors influencing prognosis include how quickly heart failure is recognized (another reason resting respiratory rate monitoring is so critical), how well the dog responds to medication, and whether complications develop.

Stage D: refractory heart failure

What Stage D means

Stage D is reached when standard medications at appropriate doses no longer adequately control heart failure symptoms. The dog continues to experience fluid accumulation, respiratory difficulty, or other signs of heart failure despite being on maximum therapy.

Treatment adjustments at Stage D

• Torsemide — May replace furosemide as a more potent loop diuretic with longer duration of action.
• Hydrochlorothiazide — An additional diuretic that works on a different part of the kidney.
• Pimobendan dose increase — Some cardiologists will move to three times daily dosing.
• Sildenafil — If pulmonary hypertension is contributing to symptoms.
• Dietary support — Maintaining adequate nutrition becomes increasingly challenging and important.

Prognosis at Stage D

Stage D carries a guarded prognosis. The focus shifts increasingly toward quality of life, comfort, and making the most of each day. Some dogs can be stabilized for weeks to months with medication adjustments, but the overall trajectory is progressive.

What progression looks like

Understanding how MMVD progresses helps you know what to watch for and what to expect:

B1 to B2: Often gradual. Murmur grade may increase. Echocardiographic measurements slowly approach and then cross the B2 thresholds. This can take months to years. You may notice no outward changes in your dog.

B2 to C: This transition can be gradual or sudden. Gradual onset involves slowly increasing resting respiratory rates over days to weeks. Sudden onset — often triggered by a ruptured chordae tendineae — can cause acute, severe heart failure requiring emergency care.

C to D: Progressive. Despite stable medication doses, heart failure episodes become more frequent or more difficult to control. Diuretic doses need to be increased. Quality-of-life days become increasingly interspersed with difficult days.

Key point: Not every MMVD dog progresses through all stages. Many dogs diagnosed at B1 will never reach B2. Many dogs at B2 on pimobendan will live years before reaching C. Progression is not inevitable at every stage.

Questions to ask your cardiologist

Understanding your dog's stage empowers you to have informed conversations with your veterinary cardiologist. Here are questions worth asking:

• What are my dog's current LA:Ao ratio and LVIDdN measurements?
• How close are we to the B2 thresholds?
• What is the recommended monitoring interval for my dog's current stage?
• When should I become concerned about the resting respiratory rate?
• Are there any changes since the last echocardiogram?
• At what point would you recommend starting pimobendan?

The bottom line

The ACVIM staging system for MMVD is not just a classification scheme — it is a treatment roadmap. Each stage carries specific recommendations for medication, monitoring, and lifestyle, all grounded in clinical evidence.

The most critical distinction is between B1 and B2:

• B1 means monitoring. The heart has not enlarged. No medications are needed. Regular echocardiographic rechecks and resting respiratory rate monitoring at home are the strategy. Many dogs remain at B1 for years.
• B2 means treatment begins. The heart has enlarged (LA:Ao of 1.6 or greater, LVIDdN of 1.7 or greater), and the EPIC study proved that pimobendan at this stage delays heart failure by approximately 15 months.

If your dog has been diagnosed with MMVD at any stage, knowing where they fall in this system gives you clarity, a plan, and the ability to work as an informed partner with your veterinary cardiologist. That knowledge is powerful, and it is the foundation for giving your dog the best possible outcome.