Put very simply heart failure is the inability of the heart to meet the demands of the body. But more important than not meeting the demands of the body, the impairment and inefficiency of a heart in failure can lead to congestive heart failure which results in a decrease in forward blood flow and elevated pressures, similar to slow flowing or clogged plumbing systems. Consequently, similar to leaking pipes in a clogged system, fluid can weep out of blood vessels and onto lungs (pulmonary edema), around lungs (pleural effusion), and within the abdomen (peritoneal effusion).

Congestive heart failure is an emergency and can cause severe respiratory difficulties and eventually death if untreated. Treatment typically involves the mobilization or removal of this fluid by the use of diuretics or manual removal (thoracocentesis/abdominocentesis). Therapies are aimed at slowing the accumulation of fluid and improving the strength and efficiency of the pumping heart.

Congestive heart failure can occur from many different heart diseases in both dogs and cats.