No TV medical drama show is complete without a doctor at some point shouting, “Clear!” as they apply a defibrillator on a dying patient. More often than not, with a massive convulsion, the patient is shocked with hundreds of joules of electricity and miraculously brought back to life.

In the real world, defibrillators are a little less dramatic but they do save countless lives.A defibrillator is a device that gives a high energy electric shock to the heart through the chest wall to someone who is in cardiac arrest. This high energy shock is called defibrillation, and it's an essential life saving step in the chain of survival.

The high energy shock or the high electric current – about 300 joules of electrical energy is passed through the heart. This has the effect of stopping the uncontrolled trembling and resetting the beating to normal.Rather than restart the heart, the device works to reset the natural pacemaker in our bodies to return the heart to normal function and rhythm. If the heart has stopped, a defibrillator will do little to restart it so other techniques such as CPR are applied. They differ from a pacemaker, however, as they do not assist the heart in maintaining a pace. Rather they deliver an electrical shock when they sense that the heart is approaching a dangerous heartbeat.

The shock produced by the defibrillator is generated via a built-in battery, which releases a massive pulse of energy. This electrical energy is directed down two wires, each ending at a pad, known as a paddle.

Here is a clip on how they work

https://www.youtube.com/watch?v=BuUUm2waYtw