How do I apply the ACVIM staging criteria for myxomatous mitral valve disease in dogs?

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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).313 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.3 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) ≥1.6.213 Pimobendan is recommended for all dogs meeting stage B2 criteria.912 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.3 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.6 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