Any heart with a reduced ability to contract is considered a failing heart. Almost any dog with heart disease that leads to chamber enlargement or increased wall thickness has a failing heart, but the body usually is able to compensate in other ways for these physical changes. As a result, the dog has no signs and is not in heart failure or congestive heart failure.

Heart failure and congestive heart failure are medical syndromes in which a dog exhibits signs related to a complex interaction between a failing heart and the blood vessels.

In heart failure, the blood flow is insufficient to supply organs with enough oxygenated blood for proper function. Depending on the degree of severity, signs of heart failure may appear while the dog is at rest, during mild exertion, or during moderate or extreme exercise.

In congestive heart failure, blood dams up in organs—usually the lungs but occasionally in the body’s other major organs—and causes the congested organs to function abnormally, become swollen with fluid, or both.