MMVD in Dogs: Testing Mitral Valve Disease
· By Dr. Tang
TL;DR — MMVD (myxomatous mitral valve disease) is the most common acquired heart disease in dogs. It is a disease of aging and size: up to 85% of small-breed dogs under 20 kg show the valve lesion by age 13, and males are affected about 1.5× more often than females. Here is the twist that changes how you manage it: only about 30% of affected dogs ever progress to advanced disease. The entire point of testing is to separate the slow progressors from the fast ones — and that is the ACVIM B1→B2 decision, where a quantitative NT-proBNP trend, not a single murmur, is what justifies starting lifelong pimobendan.
In Plain Terms
Think of the mitral valve as a one-way door between the heart’s receiving chamber (left atrium) and pumping chamber (left ventricle). In MMVD, that door slowly warps and stops closing, so blood leaks backward with every beat. The heart compensates for years — which is why the dog looks normal — until the leak is too big and fluid backs up into the lungs.
The disease has two faces: very common (most old small dogs have it) and usually slow (most never get sick from it). Testing is how you tell which face your patient has.
Who Gets It
| Factor | Pattern |
|---|---|
| Size | Small breeds under ~20 kg (large breeds get it too, and often faster) |
| Age | Prevalence climbs steeply after 8–10 years |
| Sex | Males ~1.5× more than females |
| Breeds | Cavalier King Charles Spaniel (youngest onset), Dachshund, Chihuahua, Poodle, small terriers |
The headline number — 85% by age 13 — comes from the ACVIM consensus guidelines. It means that in a senior small-breed dog, MMVD is not a rare zebra; it is closer to the default.
The 30% Problem
If 85% of small dogs have the lesion but only ~30% progress to advanced disease, then a positive diagnosis of “MMVD” is nearly useless on its own. What you actually need to know is: how fast is this particular valve degenerating?
That is what staging is for. The ACVIM system classifies dogs into:
| Stage | Definition | Action |
|---|---|---|
| A | At risk, no disease | Monitor |
| B1 | Murmur, no enlargement | Monitor, no meds |
| B2 | Murmur with left heart enlargement | Start pimobendan (EPIC trial) |
| C | Current/past heart failure | Full therapy |
| D | Refractory end-stage | Palliative/intensive |
The B1→B2 line is the one that matters. It is the difference between “watch and wait” and “start a lifelong drug.” And it is not decided by how loud the murmur is — it is decided by objective evidence of enlargement.
The Testing Sequence
1. Auscultation — the entry point
A left apical systolic murmur is the classic first sign, often audible years before failure. Its intensity roughly tracks disease severity, but not reliably enough to stage. Limitation: a soft murmur in a panting or anxious dog is easy to under-grade.
2. Thoracic X-ray + vertebral heart score (VHS)
Screens for left atrial enlargement and general cardiomegaly. A VHS above about 10.5–11.5 (depending on breed) signals enlargement worth quantifying. X-ray shows size and fluid, not function.
3. NT-proBNP — the progression tracker
Because NT-proBNP rises with wall stress, it gives a numeric read on how hard the heart is working. In dogs with MMVD, an NT-proBNP above ~1500 pmol/L carried an odds ratio of about 5.76 for predicting congestive heart failure. More useful in practice: the trend. A value that climbs across serial visits — even inside the “normal” range — is objective evidence the valve is progressing, independent of who held the probe.
4. Echocardiography — the gold standard
Measures left-atrial-to-aortic ratio (LA/Ao) and left-ventricular dimensions to stage B1 vs B2 precisely. The B2 threshold is defined by specific enlargement criteria, which is why echo — not auscultation — is the definitive staging tool. Limitation: it needs equipment and a trained operator, so most practices use it periodically and lean on NT-proBNP between echoes.
The B1→B2 Controversy Nobody Prints
The EPIC trial established that starting pimobendan at B2 delays the onset of heart failure — a genuine survival gain. But “significant enlargement” is a moving target: the B2 threshold depends on breed-specific and operator-dependent echocardiographic measurements, and two cardiologists can occasionally disagree on the same dog.
Two practical consequences:
- Most practitioners get B1 right. In a published survey, 93% correctly recommended no medication for B1 dogs — so the common error is not under-treating, it is starting drugs on a murmur alone.
- A quantitative trend is the tie-breaker. When the echo is borderline, a climbing NT-proBNP across 3–6 months is the objective, reproducible signal that pushes a B1 dog into B2 territory — and it does not require a specialist on every recheck.
What the Numbers Mean for Prognosis
Stage matters enormously, and the survival spread is wide:
- B1: median survival ~1,854 days (≈5 years)
- B2: ~1,508 days
- C/D combined: ~930 days
- Stage D: median C→D progression 245 days, and median survival after reaching D only ~155 days
The takeaway for owners: a dog caught at B1 and monitored has years of good quality life ahead; a dog that first presents at C or D has already lost most of that runway. This is the whole case for screening before symptoms.
How the Test Runs
- Auscultate — note murmur grade and location.
- X-ray for VHS and any pulmonary changes.
- Draw blood, run NT-proBNP on a quantitative immunofluorescence analyzer — numeric result in 10–20 minutes.
- Refer for echo when enlargement or progression is suspected; repeat NT-proBNP serially to track the trend between echoes.
Related products: Canine NT-proBNP Test · FIA680 Analyzer · FIA880 Analyzer
FAQ
What is MMVD in dogs?
1 leaky valve, 2 chambers — MMVD is degeneration of the mitral valve that lets blood leak backward from the left ventricle into the left atrium, eventually enlarging the atrium and causing heart failure. It is the most common acquired heart disease in dogs.
How common is MMVD?
85% by 13, 30% progress — up to 85% of small-breed dogs under 20 kg show the lesion by age 13, and males are 1.5× more affected. But only ~30% ever progress, so the goal is finding the progressors, not just the lesion.
What are the ACVIM stages?
5 stages, 1 decision — A (at risk), B1 (murmur, no enlargement), B2 (murmur + enlargement), C (heart failure), D (end-stage). The B1→B2 call decides when to start lifelong pimobendan — it is the single most consequential decision.
What tests diagnose and stage MMVD?
4 steps — auscultation finds the murmur, X-ray/VHS screens enlargement, NT-proBNP quantifies strain, echo measures LA/Ao to stage B1 vs B2. NT-proBNP provides the objective trend between specialist echoes.
How long do dogs with MMVD live?
5 years to 5 months — median survival ~1,854 days at B1, ~1,508 at B2, ~930 at C/D. Stage D is the worst: ~245 days from C to D, then only ~155 days median survival.
Does every dog with a murmur need treatment?
No — 93% of practitioners correctly withhold meds from B1 dogs. Treatment starts at B2 based on the EPIC trial. A murmur is a reason to stage, not a reason to prescribe.
Key Takeaways
- Common ≠ aggressive — up to 85% of small dogs have the lesion by 13, but only ~30% progress. A diagnosis of “MMVD” without a stage is nearly useless.
- The B1→B2 line is the whole game — B1 = watch (median ~5 years survival), B2 = start pimobendan (EPIC trial). Get this wrong and you either under-treat a progressor or drug a dog that didn’t need it.
- Enlargement, not murmur loudness, decides it — stage by echo (LA/Ao) and X-ray (VHS >10.5–11.5), never by how loud the murmur sounds.
- NT-proBNP is the trend machine — >1500 pmol/L carried a 5.76 odds ratio for CHF; but the serial climb matters more than any single value, and it’s reproducible without a specialist echo every visit.
- Stage D is the cliff — once a dog reaches D, median survival is only ~155 days. The entire value of screening is catching the disease at B1/B2, where there are years, not months, left.
References
- Keene BW, et al. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. J Vet Intern Med. 2019. PMID 31389076
- Boswood A, et al. Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly (EPIC). J Vet Intern Med. 2016. PMID 27678080
- Raffan E, et al. NT-proBNP in canine mitral valve disease. J Vet Intern Med. 2009.
- Clinical findings and survival in dogs with MMVD at ACVIM stage D. J Vet Sci / companion literature. 2026.
- Prevalence, risk factors and survival in dogs with myxomatous valve degeneration. Arq Bras Med Vet Zootec. (SciELO)
This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any dog with suspected heart disease should be evaluated by a veterinarian. Reference ranges are platform-dependent; always use the intervals validated for your specific analyzer. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.