Cardiac arrhythmias are common; some are life-threatening, others merely a nuisance. They are caused by abnormalities in impulse formation or conduction that lead to slow or fast, regular or irregular heart rhythms. It is not difficult to treat patients with slow rhythms: currently available pacemakers are able to adapt their function to the needs of the body. The situation is different, however, for patients with rapid rhythms. Such rhythms may originate anywhere in the heart and result from a variety of mechanisms. They may be focal, meaning that the abnormal impulse formation is confined to a small area, or they . . .