Canine myxomatous mitral valve disease and ACVIM staging

Myxomatous mitral valve disease is by a wide margin the most common acquired cardiac disease in dogs. It is prominent in small breeds — Cavalier King Charles Spaniel, Dachshund, Poodle and Yorkshire Terrier especially — and its frequency rises with age.

When a routine examination turns up a systolic murmur at the left apex, the question that follows is: which stage is this dog in? The answer directly determines whether medication starts today.

The ACVIM stages

The American College of Veterinary Internal Medicine staging runs on four letters.

Stage A — No disease, but at risk by breed. The Cavalier King Charles Spaniel is the archetype. No treatment; regular auscultation.

Stage B — A murmur is present, clinical signs are not. This stage splits in two, and the whole question lives in that split.

• B1 — No significant cardiac enlargement. No indication to treat; monitor.
• B2 — Left atrial and left ventricular enlargement is significant, but the dog remains clinically well.

Stage C — Congestive heart failure, current or past. Cough, tachypnoea, exercise intolerance, pulmonary oedema.

Stage D — Advanced failure refractory to standard therapy.

B1 or B2 — the decision that matters

B1 cannot be separated from B2 with a stethoscope. Murmur intensity gives a rough impression but cannot serve as the criterion. The distinction is made by imaging:

• Echocardiography is the gold standard. Left atrium-to-aorta ratio and normalised left ventricular internal dimension are measured.
• Thoracic radiography assesses cardiac size by vertebral heart score and shows the lungs — which is its real value, because in a coughing dog the differential is not confined to the heart.

Why does the distinction matter so much? Because starting pimobendan in stage B2 delays the onset of congestive heart failure by more than a year on average. That is the finding of the EPIC study, and it changed cardiology practice. Starting the same drug in B1, by contrast, has no demonstrated benefit.