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, the movement of electrical impulses through the heart is often abnormal. This, 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 device to synchronize the contraction of the ventricles. Cardiac resynchronization improves outcomes for selected patients with heart failure. When heart failure worsens in patients with a resynchronization device, it might be because the device is not sending electrical signals to both ventricles. This problem is called “loss of capture” and often involves the left ventricle. Unfortunately, detection of loss of left ventricular capture has required that physicians use a special programming machine that is not available in most doctors' offices.