Valvular heart disease, also known as endocardiosis, is the leading cause of acquired heart disease in dogs. Senior dogs, particularly small breeds, are prone to developing it; a third of all dogs that are affected by it are over the age of twelve.

Stated simply, the heart valves become inflamed, thickened, and scarred. This causes the valves to leak and wear out over time, preventing them from closing as tightly as they should. Instead of pumping blood forward, the blood leaks backward into the atria of the heart. A veterinarian can detect this in a routine examination. He can hear a heart murmur — the sound of the blood flow moving backward from the ventricle to the atrium — when he listens to the dog's chest with a stethoscope. When the blood backs up on the left side of the heart, it leads to lung congestion and coughing. When the blood backs up on the right side, fluid builds up in the abdomen, making breathing difficult.

The disturbance in forward blood flow causes compensatory changes to the structure of the heart. Because blood backs up into the atrium, the atrium will overfill. In response, it will become enlarged to accommodate the excess blood. Eventually the atrium will not be able to expand any further and pressure will build, causing blood to back up into the lungs or abdomen. This is called congestive heart failure.

No dietary changes have been found to reliably affect the outcome of valve disease, although salt restriction is recommended once congestive heart failure begins. Potassium supplementation must be recommended with caution because some heart medications cause potassium retention and hyperkalemia is potentially life threatening.