In heart failure, the heart does not pump blood as well as it should and fluid builds up in the lungs. The symptoms of heart failure include difficulty breathing, decreased ability to exercise, and leg swelling. Although many drugs help patients with heart failure, there is no cure. The condition is disabling and even fatal for many patients. In patients with heart failure, conduction of electrical impulses through the heart is often abnormal. This abnormal conduction, in turn, can lead to uncoordinated contraction of the ventricles, the large pumping chambers of the heart. Cardiac resynchronization is a newer heart failure treatment that uses a special type of pacemaker to synchronize the contraction of the ventricles. Cardiac resynchronization improves quality of life and exercise ability, but its effect on survival has been less clear. Only one study has been large enough to show that this therapy may improve survival.