Cardiac Pacemaker
The heart has a special group of cells called the sinoatrial (SA) node that emits pulses of electricity that make the heart beat. If this natural pacemaker is defective, or if the electrical impulses are blocked, the heart can beat too quickly, too slowly, or irregularly.
Using tiny batteries and electrodes, an artificial cardiac pacemaker regulates the heartbeat and overcomes any blocked pathways. Many pacemakers have built-in sensors that switch the pacemaker on when the natural heartbeat gets too slow and switch it off when it is too fast.