How a pacemaker works

A pacemaker is a small titanium box under the skin, below the collarbone, with a battery and a tiny computer that listens, keeps time, and fires. Its lead runs through a vein into the lower right chamber, where the tip senses each beat and the timer starts over. If a beat doesn't arrive in time, it fires a pulse of a few volts for less than a thousandth of a second, and the heart muscle cells at the tip pass an electrical wave from cell to cell until the muscle contracts. When the heart beats on its own, it stays quiet.

1. 0:00 Hook: knows when not
2. 0:05 The heart's own wiring
3. 0:27 Device exploded
4. 0:41 The lead
5. 0:49 Listening
6. 1:05 A missed beat
7. 1:19 Pulse to heartbeat
8. 1:34 Stays quiet