Congestive Heart Failure is a condition that occurs when the heart is not able to pump oxygen rich blood to the rest of the body. It can be caused by:
1. weakness of the muscles of the heart,
2. a disorder caused by the stiffening of the heart muscles, or
3. an increase in the oxygen demand by the body beyond which the heart is able to satisfy and deliver adequate oxygen-rich blood (e.g like in some cases of anemia).
The pumping action of the ventricles can be impaired in several ways. For example, the ventricular muscles can be weakened by previous heart attacks or infections (myocarditis). This gives rise to systolic dysfunction (diminished pumping ability of the ventricles due to muscle weakening). After systole (each ventricular contraction) the ventricle muscles need to relax to allow blood from the atria to fill the ventricles. This relaxation of the ventricles is called diastole. Diastolic dysfunction, on the other hand, occurs when the ventricles are not able to relax and fill. This can be caused by the stiffening of the heart muscles due to iron overload (Hemochromatosis) or amyloidosis. However, longstanding high blood pressure is the most common cause of diastolic dysfunction which can result in a thickened (hypertrophied) heart.
This video below (from Biodigitalsystems) gives a visual explanation to congestive heart failure.