Myxomatous mitral valve disease (MMVD) is the most common heart disease in adult dogs, especially small breeds. The ACVIM consensus stages it A through D — from at-risk breeds with no murmur, to congestive heart failure requiring lifelong medication. Stage B2 is the critical inflection point where pimobendan delays heart failure by roughly 15 months on average.
Last reviewed: May 2026
What Is Canine MMVD?
Canine MMVD is a progressive degeneration of the mitral valve in which the valve leaflets thicken, distort, and stop sealing properly, allowing blood to regurgitate back into the left atrium with each heartbeat. It is the most common acquired heart disease in dogs, affecting roughly 75 percent of small-breed dogs over age 10 and an even higher proportion of Cavalier King Charles Spaniels, who can develop murmurs by age 5. The 2019 ACVIM consensus statement is the current standard-of-care framework for diagnosis, staging, and treatment (Keene et al., 2019, JVIM (ACVIM Consensus on MMVD)).
Stage A — At-Risk Breeds
Stage A dogs are predisposed breeds with no murmur and a structurally normal heart on echocardiogram. Examples include the Cavalier King Charles Spaniel, Dachshund, Chihuahua, Toy and Miniature Poodle, Shih Tzu, and Maltese. Stage A requires no medication — only annual cardiac auscultation starting at age 5 to 6 so a murmur is caught the moment it appears. Annual screening for predisposed breeds is also recommended by the AAHA canine life stage framework, which advises baseline cardiac auscultation by age 5 in toy and small breeds (AAHA Canine Life Stage Guidelines, 2019). Catching the first murmur is the single most important early-detection step, because a quiet stage A murmur can progress to symptomatic disease within a few years.
Stage B1 — Murmur Without Cardiac Enlargement
Stage B1 dogs have a heart murmur from mitral regurgitation but the left atrium and ventricle are still normal-sized on echo or x-ray. These dogs are asymptomatic and require no medication, but they should be rechecked every 6 to 12 months with imaging because they can progress to stage B2.
Some stage B1 dogs remain stable for years; others progress within months. There is no reliable way to predict the trajectory other than serial imaging.
Stage B2 — Cardiac Enlargement, Still Asymptomatic
Stage B2 is the most important decision point in MMVD. The murmur is present, the left atrium and/or ventricle are enlarged on echo (specific thresholds: LA:Ao greater than 1.6 and LVIDDN greater than 1.7), but the dog still feels and looks normal — no coughing, no exercise intolerance. The landmark EPIC trial showed pimobendan started at stage B2 delays the onset of congestive heart failure by a median of roughly 15 months compared with placebo. ACVIM now recommends pimobendan for all stage B2 dogs even though they are asymptomatic, as described in Bonagura's Kirk's Current Veterinary Therapy.
Stage C — Congestive Heart Failure
Stage C dogs have developed congestive heart failure — usually pulmonary edema causing cough, increased respiratory rate at rest, exercise intolerance, and sometimes fainting. Standard therapy is the 'quadruple combination': pimobendan, furosemide (or torsemide), an ACE inhibitor (benazepril or enalapril), and spironolactone. About 80 percent of dogs respond to initial CHF treatment, and median survival from first CHF onset on this protocol is roughly 9 to 12 months, with significant individual variation.
Stage D — Refractory Heart Failure
Stage D dogs no longer respond adequately to standard CHF therapy. Diuretic doses have been pushed to ceiling, additional drugs (such as sildenafil for pulmonary hypertension or amlodipine) have been added, and quality of life is becoming difficult to maintain. Median survival at stage D is weeks to a few months. Conversations about hospice care, in-home euthanasia, and quality-of-life scoring become central at this stage. Resting respiratory rate at home (target under 30 breaths per minute while sleeping) is the single best home metric for catching decompensation early. Pain control becomes a core part of stage D care because dogs with severe CHF often have musculoskeletal discomfort layered on top of cardiac fatigue, and modern multimodal pain plans can meaningfully improve quality of life (AAHA Pain Management Guidelines, 2022).
When to See a Vet
Not every symptom is a midnight emergency, but some warrant same-day attention and a few are true ERs. Use the lists below to sort which bucket you're in.
Call your vet today if:
- A new heart murmur detected at a wellness exam
- Mild cough that gets worse at night or after exercise
- Slowing down on walks in a small-breed senior dog
- Resting respiratory rate consistently above 30 breaths per minute
- Known MMVD dog whose dosing schedule needs adjustment
Go to the ER immediately if:
- Open-mouth breathing, blue or gray gums, or extreme respiratory effort
- Collapse, fainting, or sustained weakness
- Resting respiratory rate sustained above 40 breaths per minute
- Coughing up pink, foamy fluid (suggests pulmonary edema)
- Sudden inability to use the back legs (suggests aortic thromboembolism, more common in cats but possible in dogs)
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Frequently Asked Questions
How long can a dog live with MMVD?
Time from first murmur to congestive heart failure averages 3 to 5 years; survival after CHF onset on full medical therapy averages another 9 to 12 months. Many small dogs die of unrelated causes before reaching CHF. Starting pimobendan at stage B2 adds roughly 15 months of CHF-free time on average.
What does pimobendan do for dogs with heart disease?
Pimobendan is an inodilator — it strengthens heart muscle contraction while dilating blood vessels, reducing the workload on the failing heart. The 2016 EPIC trial showed it delays the onset of heart failure by a median of 15 months when started at ACVIM stage B2 (significant left-sided enlargement without symptoms).
How much does treating dog MMVD cost?
Initial cardiology workup runs $400 to $800 (cardiologist exam $150 to $300, echocardiogram $400 to $600, chest x-ray $150 to $400). Ongoing CHF medications (pimobendan, furosemide, ACE inhibitor, spironolactone) cost roughly $80 to $200 per month for a 20-lb dog. Recheck echos every 6 to 12 months add another $400 to $600 each.
What is the resting respiratory rate I should watch at home?
Count breaths per minute while your dog is sleeping or fully at rest — one breath equals one inhale plus exhale. A sustained rate above 30 breaths per minute over multiple readings often indicates early pulmonary edema and warrants a same-day vet call. Many cardiologists ask owners to log a weekly number.
Should I exercise my dog with a heart murmur?
Yes, moderate routine exercise is generally encouraged for dogs in stages A, B1, and B2 — it supports overall fitness and quality of life. Avoid extreme exertion in stage C and D. Watch for new coughing, slowing down, or exaggerated panting that does not resolve quickly; those signal it is time to scale back.
Still Not Sure if Your Dog Needs a Vet?
When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of your dog's resting breathing rate, gum color, or any cough video, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.