Heart disease is an important cause of illness in cats but can be difficult to diagnose, especially in its early stages. As with humans, early detection could make treatment more successful. Diagnosis frequently requires specialized equipment that is expensive and is frequently available only at specialists. In humans, neurohormones that are normally produced by the heart and blood vessels are released into the blood in increased amounts during heart failure. Sensitive screening tests that measure concentrations of these neurohormones have been developed for people to indicate the early stages of heart disease. These investigators will determine if the same neurohormones are useful for diagnosis and treatment of heart disease in the cat.