Myxomatous Mitral Valve Disease (MMVD) in Australian Shepherds

The mitral valve sits between the heart's left atrium and left ventricle, and its job is to close firmly enough that blood only flows one direction through the heart. In MMVD, the valve's leaflets gradually thicken and become floppy — the 'myxomatous' change refers to this structural degeneration of the valve tissue itself. A floppy valve doesn't seal completely, so with each heartbeat some blood leaks backward instead of moving forward, which is the sound a stethoscope picks up as a murmur.

Over time, as more blood leaks backward, the heart's left side has to work harder and gradually enlarges to compensate. This is a slow process, typically unfolding over years rather than weeks, which is exactly why staging matters more here than in almost any other cardiac condition.

Veterinary cardiologists commonly use a staging system (often referenced as ACVIM stages) that separates dogs with a murmur and mild valve changes but no heart enlargement from dogs whose heart has started to enlarge in response, and further separates those with heart enlargement but no symptoms from dogs who have actually developed a cough or exercise intolerance from it. A dog can sit in an early stage for years, sometimes for its entire life, with nothing more than a murmur and periodic monitoring.

That's the most important honest point about MMVD: a murmur alone doesn't tell you the stage, and the stage is what actually determines the monitoring schedule and any treatment approach — which is why an echocardiogram, not just a stethoscope, is what defines where a given dog actually sits.