Understanding Heart Disease in Dogs with MMVD

Mitral valve disease (MMVD) is the most common heart disease in dogs. In certain breeds - particularly Cavalier King Charles Spaniels - it affects the majority of dogs by middle age. Understanding what it is, how it progresses, and what the echocardiogram tells us helps owners make informed decisions about their dog's care.

What is MMVD?

The mitral valve sits between the left atrium and left ventricle of the heart and prevents blood from flowing backwards. In MMVD, the valve leaflets thicken and degenerate over time, causing them to close imperfectly. Blood leaks backwards (regurgitates) with each heartbeat, reducing the efficiency of the heart as a pump.

A heart murmur - the sound of turbulent blood flow through the leaking valve - is typically the first detectable sign, often heard by a vet during a routine health check before the dog shows any symptoms.

How does MMVD progress?

MMVD is staged using the ACVIM (American College of Veterinary Internal Medicine) classification. The stages are:

• Stage A: Breeds at high risk but no murmur yet detected
• Stage B1: Murmur present, heart not yet enlarged
• Stage B2: Murmur present, heart enlarged on X-ray or echocardiogram, no clinical signs yet
• Stage C: Heart failure - fluid accumulation, coughing, breathlessness
• Stage D: End-stage heart failure not responsive to standard treatment

Most dogs spend years at Stage B1 before progressing. The critical transition is from B1 to B2 - because this is when medication becomes indicated.

Why does Stage B2 matter?

A landmark clinical trial (the EPIC trial, 2016) showed that starting pimobendan - a cardiac medication - at Stage B2 delayed the onset of heart failure by an average of 15 months compared to placebo. This is significant because it means dogs that are identified at B2 and started on treatment live meaningfully longer before developing heart failure symptoms.

The only way to accurately determine whether a dog has reached B2 is with an echocardiogram. X-ray alone is insufficient for precise assessment.

When to call us

If your dog has been diagnosed with a heart murmur, the next step is to discuss staging. If your dog is a Cavalier King Charles Spaniel, regular cardiac screening from around 18 months of age is recommended. Early identification of Stage B2 opens the window for treatment that meaningfully delays disease progression.

Call us on (03) 9592 9811 or book online at middlebrightonvet.com.au/book-online.

Related services at Middle Brighton Vet: Echocardiography | Consultations and Health Checks