ACVIM Consensus Staging for Mitral Valve Disease (A, B1, B2, C, D)

Last updated: May 23, 2026

Why ACVIM Mitral Staging Matters

If your dog has a heart murmur on routine exam, the next question your veterinarian asks is, “What stage?” That word — stage — is shorthand for a specific framework published by the American College of Veterinary Internal Medicine (ACVIM) and updated in 2019 by Keene and colleagues in the Journal of Veterinary Internal Medicine. The ACVIM mitral staging system organizes myxomatous mitral valve disease (MMVD) into five categories that map directly to treatment decisions, monitoring frequency, and prognosis conversations.

For adopters and owners alike, understanding this framework removes a lot of fear. Many small-breed dogs live entirely normal lives with Stage B1 disease for years. Stage B2 dogs benefit enormously from early pimobendan therapy. Even Stage C dogs — those who have already had heart failure — often respond well to quadruple medical therapy and continue enjoying walks, snuggles, and dinner. The stage label is not a verdict. It is a roadmap.

The system applies to dogs. Cats with heart disease use a separate framework — the 2020 ACVIM consensus on feline hypertrophic cardiomyopathy by Luis Fuentes and colleagues — which we cover separately.

Stage A: Predisposed but Asymptomatic

Stage A means your dog belongs to a breed at high risk for MMVD but has no audible murmur and no echocardiographic abnormality. Cavalier King Charles Spaniels lead the list, followed by Dachshunds, Miniature Poodles, Chihuahuas, Pomeranians, Yorkshire Terriers, and other small breeds. Larger breeds can develop MMVD but at lower frequency and often at older ages.

For Stage A dogs there is no medication recommended. The action item is annual auscultation by your primary veterinarian beginning around age 5 — earlier for Cavaliers, where murmurs can appear in the first few years of life. Recognizing the predisposition is itself a useful piece of information, because it sets the schedule for the next check rather than waiting for symptoms.

Stage B1: Murmur Without Cardiomegaly

Stage B1 is the moment many adopters first hear the word “murmur.” A grade I to IV systolic murmur over the left apex on routine exam confirms valve insufficiency, but echocardiography and thoracic radiographs show the heart chambers are still within normal size. The valve is leaking; the chambers have not yet remodeled.

Stage B1 dogs do not require cardiac medication under the current ACVIM consensus. The EPIC trial (Boswood and colleagues, 2016, JVIM) — the pivotal study that established Stage B2 pimobendan benefit — did not find a clinically meaningful advantage to starting medication before cardiomegaly. The recommendation is monitoring: a baseline echocardiogram if available, then recheck at six to twelve month intervals depending on murmur grade and individual progression.

If you are considering adopting a pet with a heart murmur, ask the shelter or rescue what stage was assigned and whether echocardiography has been performed. Stage B1 dogs make wonderful companions and live full lives. For dog-specific murmur background, see our dog heart murmur overview.

Stage B2: Cardiomegaly Meeting Echo Criteria

Stage B2 is the critical threshold. The dog still feels normal — no coughing, no exercise intolerance, no fluid in the lungs — but the heart has remodeled. The 2019 ACVIM consensus defined Stage B2 by specific echocardiographic criteria:

• Left atrial to aortic ratio (LA/Ao) of 1.6 or greater on the right parasternal short-axis view at the heart base.
• Normalized left ventricular internal diameter in diastole (LVIDDN) of 1.7 or greater, calculated by indexing LVIDD to body weight.
• Vertebral heart score (VHS) of 10.5 or greater on lateral thoracic radiograph as a supportive non-echo metric.

When all three criteria align, the dog qualifies for pimobendan initiation. The EPIC trial demonstrated that starting pimobendan at Stage B2 delays the onset of congestive heart failure by approximately fifteen months on average and extends overall survival. That is one of the largest benefits in any veterinary cardiology trial. Your DACVIM-Cardiology (board-certified veterinary cardiologist) or primary veterinarian, working with structured echo measurements, makes the dosing decision — never owners or the internet. For drug background, see pimobendan for dogs, also marketed as Vetmedin.

The misclassification problem cuts both directions. A false-positive B2 (a small dog whose LA/Ao reads above threshold but who does not truly meet the full picture) leads to unnecessary medication. A false-negative B2 (a dog whose echo was hurried or operator-dependent) delays a high-value drug. Structured echo by an experienced operator with attention to veterinary echocardiography interpretation reduces both errors. The detailed framework for measurement standards and reference ranges is covered there.

Stage C: Past or Current Congestive Heart Failure

Stage C means your dog has had clinical congestive heart failure (CHF) — pulmonary edema confirmed by radiographs, often with cough, tachypnea, exercise intolerance, or syncope — and is now being managed medically. The quadruple-therapy framework guides Stage C management:

• Pimobendan — positive inotrope and balanced vasodilator; cornerstone since EPIC.
• Furosemide — loop diuretic for fluid management; titrated by your veterinarian to the lowest effective dose.
• ACE inhibitor — benazepril or enalapril; blocks the renin-angiotensin-aldosterone system upstream.
• Spironolactone — aldosterone antagonist; adds neurohormonal protection and mild diuretic action.

Stage C dogs need regular rechecks — every two to four weeks initially, then every two to three months once stable — with attention to respiratory rate, kidney values, electrolytes, and quality of life. Many Stage C dogs do remarkably well for one to two years or more with appropriate care. If you want a deeper look at what Stage C management feels like day-to-day, see congestive heart failure in dogs for the broader clinical picture.

Stage D: Refractory Heart Failure

Stage D is the most challenging part of the journey. The dog has reached a point where standard quadruple therapy no longer controls clinical signs. Diuretic doses have escalated. Pulmonary edema recurs despite maximum medical management. This is where the ProVIDE-style sequential nephron blockade comes in — adding a thiazide diuretic like hydrochlorothiazide or considering a switch from furosemide to torsemide.

Stage D is also when the quality-of-life conversation becomes central. A DACVIM-Cardiology may be able to extend comfort meaningfully, but specialty access is uneven across the country. If a board-certified veterinary cardiologist is not within reasonable driving distance, your primary veterinarian can consult by telemedicine while managing day-to-day care. End-stage decision-making becomes a quality-of-life-first conversation in concert with the family.

What Adopters Should Know About MVD-Positive Dogs

Many adopters worry that a Stage B1 or B2 murmur means a dog is “too sick” to take home. That worry is misplaced. Stage B1 dogs often live the rest of their natural lifespan without progressing to clinical CHF. Stage B2 dogs on pimobendan frequently enjoy years of normal activity. Even Stage C dogs, when managed thoughtfully and budgeted realistically, can have a meaningful chapter ahead.

The honest framing for any adopter taking on an MVD-positive dog: ask the shelter for the most recent echo report or VHS, confirm what medications are currently prescribed, build a realistic monthly cost estimate, and identify a primary veterinarian comfortable managing cardiac disease. Pet insurance signed up before adoption — and aware of any pre-existing diagnosis caveat — can help defray monthly medication and recheck costs over the dog’s lifetime.

Cost Framing and Specialty Access Reality

Cardiology specialty pricing varies by region, specialist, and protocol depth. As of 2024-2025, a full echocardiogram with Doppler from a DACVIM-Cardiology typically falls in a several-hundred-dollar range, with telemedicine consultation in a lower bracket. Monthly medication costs for Stage B2 to Stage C management depend heavily on dog size and the specific combination prescribed. Your veterinarian and pharmacy can provide accurate quotes.

If a board-certified veterinary cardiologist is not located in your region, your primary veterinarian can consult with a specialist by telemedicine, share echo images, and follow specialist-guided protocols while managing day-to-day care. Many primary practices manage Stage B1, B2, and stable Stage C very competently. The push to a specialist becomes especially valuable for diagnostic uncertainty, refractory cases, and procedural questions when arrhythmia overlaps with mitral disease.

Where Biomarkers and Imaging Fit

The 2019 ACVIM consensus is built on echocardiography, radiographs, and clinical history. Biomarkers add useful supporting data — particularly NT-proBNP interpretation for distinguishing cardiac from non-cardiac causes of respiratory signs, and cardiac troponin I for monitoring myocardial injury. Tissue Doppler imaging and speckle-tracking strain identify subclinical dysfunction earlier than traditional measures and are increasingly used in surveillance protocols.

The single most important takeaway: staging is a living conversation between you, your veterinarian, and (where available) the cardiologist. Stages shift over months and years. Echos repeat. Medications adjust. None of that is failure — it is the system working.

Frequently Asked Questions

Can a Stage B1 dog skip echocardiography?

A baseline echo is ideal because it confirms B1 versus B2 — and the distinction matters enormously for medication decisions. If echo is not financially feasible, vertebral heart score on lateral radiographs is a reasonable surrogate that your primary veterinarian can perform. Recheck timing depends on murmur grade.

How often should a Stage B2 dog be rechecked?

Most cardiologists recommend echo and chest radiograph recheck every six months on Stage B2, with bloodwork to monitor kidney function before any future diuretic addition. Owners watch for resting respiratory rate above thirty breaths per minute at home, exercise intolerance, or cough.

Does Stage B2 mean my dog is in heart failure?

No. Stage B2 means the heart has remodeled but your dog does not yet have clinical heart failure. Pimobendan is started to delay that transition. Many B2 dogs never progress to Stage C, or do so years later than they would have without medication.

What is the EPIC trial and why does it matter?

The EPIC trial (Boswood et al., 2016) randomized Stage B2 dogs to pimobendan versus placebo and found pimobendan delayed onset of CHF by approximately fifteen months and extended overall survival. It is one of the foundational evidence pieces behind the 2019 ACVIM staging update.

Can a Stage C dog still be adopted?

Yes. Many prepared adopters take on Stage C dogs and provide outstanding care. The honest conversation covers monthly medication cost, recheck frequency, expected lifespan range, and quality-of-life-first values. Connection with a primary veterinarian or cardiologist is essential. For broader cardiac care orientation, see mitral valve disease in dogs for the disease overview.