ACVIM staging for canine myxomatous mitral valve disease (MMVD) uses a five-stage system (A through D) based on the presence of a murmur, radiographic and echocardiographic evidence of cardiac enlargement, and clinical signs of heart failure. The critical clinical decision point is differentiating stage B1 from B2, because stage B2 triggers pimobendan therapy to delay the onset of congestive heart failure (CHF).Research in Vet…+1
Stage A dogs have no structural heart disease but are at high risk of developing MMVD — this stage carries no treatment indication and requires no diagnostic workup beyond recognition of breed predisposition.Research in Vet…
Stage B is the asymptomatic preclinical phase, subdivided by the presence or absence of cardiac enlargement. Stage B1 is defined as a dog with mitral regurgitation that does not meet the enlargement criteria below, including dogs with only minimal or unilateral remodeling (left atrial enlargement with normal left ventricle, or vice versa). Stage B2 requires all of the following to be met simultaneously: murmur intensity ≥3/6, vertebral heart size (VHS) >10.5, echocardiographic normalized left ventricular internal diameter in diastole (LVIDDn) ≥1.7, and echocardiographic left atrial-to-aortic ratio (LA/Ao) ≥1.6.Journal of Vete…+1 Pimobendan is recommended for all dogs meeting stage B2 criteria.Journal of Vete…+1
Stage C is defined as symptomatic disease — current or past clinical signs of heart failure (pulmonary edema, ascites, or syncope) secondary to severe MMVD. Stage D is stage C disease that is refractory to standard cardiac treatment.Research in Vet… The only evidence-based drug combination for chronic stage C management is a loop diuretic with pimobendan; 48% of surveyed practitioners correctly identified this combination.Animals
When echocardiography is unavailable, radiographic indices can support B1/B2 differentiation. Vertebral left atrial size (VLAS) is the most validated radiographic surrogate. A VLAS cutoff of 2.6 provides the greatest overall diagnostic accuracy for identifying stage B2 (sensitivity 95%, specificity 84%). A VLAS ≥2.5 maximizes sensitivity (sensitivity 100%, specificity 78%) and is the preferred screening threshold to identify dogs that warrant echocardiographic confirmation. A VLAS ≥3.1 maximizes specificity (sensitivity 47%, specificity 100%).Journal of Vete… The ACVIM consensus statement explicitly endorses VHS and VLAS as radiographic tools for staging when echocardiography is unavailable.Journal of Vete…
The pulmonary vein-to-pulmonary artery ratio (PV:PA) is a radiographic index that detects early hemodynamic changes. PV:PA is the only index that differs significantly between healthy controls and stage B1 dogs, suggesting it may reflect pulmonary venous pressure changes before LA and LV enlargement become measurable by conventional indices.American Journa…
Differentiating B1 from B2 is the most challenging step in practice — 60% of Dutch and Belgian practitioners report using the ACVIM guidelines, and the B1/B2 distinction is self-reported as more difficult than diagnosing stage C.Veterinary Scie… Thoracic radiographs are used by 98% of practitioners to confirm cardiogenic pulmonary edema in suspected stage C.Veterinary Scie…
Subclinical renal tubular damage is present across all MMVD stages and worsens with disease severity. Urinary neutrophil gelatinase-associated lipocalin (uNGAL) is elevated in MMVD dogs at all stages compared to healthy controls (median 1204 pg/mL vs. 584 pg/mL), and both uNGAL and uNGAL normalized to urinary creatinine (uNGALC) increase progressively from B1 through C/D. This damage is subclinical and occurs in the absence of azotemia, so serum creatinine alone underestimates renal injury in these patients.Journal of Vete…
Atrial fibrillation (AF) complicates MMVD in 2.7% of cases overall and is strongly associated with advanced stage, increased LA size, increased body weight, and decreased fractional shortening. AF is uncommon in early-stage disease and should prompt reassessment of staging and hemodynamic status when detected.Journal of Vete…
| Stage | Key Criteria | Treatment Indication | Key Caveat |
|---|---|---|---|
| A | No murmur; high-risk breed | None | No structural disease required |
| B1 | Murmur present; does not meet B2 enlargement criteria | None | Includes unilateral remodeling |
| B2 | Murmur ≥3/6 + VHS >10.5 + LVIDDn ≥1.7 + LA/Ao ≥1.6 | Pimobendan | All four criteria must be met |
| C | Current or past CHF signs | Loop diuretic + pimobendan | 16 drug combinations used in practice; only one is evidence-based |
| D | Refractory to standard treatment | Escalated/individualized therapy | Surgery not commonly recommended by practitioners |
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