The rhythmic, regular beating of the heart is controlled by a natural pacemaker--a small patch of cells at the top of the right atrium called the sinus node which sends rhythmic electric impulses along specific conducting fibers to the heart muscles, stimulating them to contract and relax in a regular sequence. When the heart muscles fail to receive the pacemaker's signals, the heart ceases pumping blood ceases. Within a few minutes, the patient faints, and within a few more minutes, dies--unless the heart muscles can be stimulated to resume contracting. An artificial pacemaker is designed to help a damaged heart beat normally, programmed to send an electrical impulse to stimulate the heart muscle if it does not sense a normal heart beat within a specific amount of time. Dual-chamber pacemakers, the most common type, sense and pace activity in both the atrium (upper chamber) and ventricle (lower chamber...