They're small, they're expensive, and they are a marvel of modern medicine and technology.
Pacemakers are electrical devices that monitor the heart rate and helps get it up to speed when it starts pumping too slowly. The small battery powered disk is inserted just beneath the skin above the heart. A couple of attached wires monitor the heart and carry signals and impulses to and from the heart.
The disk is the actual pacemaker, which creates electrical pulses when it senses disruptions in the heart's rhythm. For example, the wires let the pacemaker know when the heart gets too slow, carrying electrical pulses to set the heart back on its normal track.
The heart has a natural pacemaker called the SA, or sino-atrial, node. The SA node is responsible for regulating heart rate by sending out electrical signals. When you exercise, the SA node increases your heart rate. But the SA node can malfunction by improperly speeding up or slowing down its signals. When the signals get too slow, for instance, your heart doesn't pump fast enough to supply the body with enough blood. That's when the pacemaker kicks in.