Heart failure develops when the heart malfunctions as a pump. Systolic heart failure is the inability of the heart to squeeze enough blood from the ventricles (heart chambers) to supply the body's needs. Diastolic heart failure results from the inability of the heart muscle to relax in between heartbeats, causing a backup of blood in the heart's chambers and in the blood vessels. Both systolic and diastolic heart failure can cause edema (fluid) to build up in the lungs and the rest of the body. The heart tries to make up for this malfunction by dilating (enlarging the heart chambers) or becoming hypertrophic (thickening of the heart walls). For individuals older than 65 years, heart failure is the most common cause of hospitalization. The November 16, 2011, issue of JAMA is a theme issue on cardiovascular disease. This Patient Page is based on one previously published in the May 13, 2009, issue of JAMA.