A portable external defibrilator, courtesy Ernstl, via Wikipedia

A defibrilator is a piece of medical equipment designed to give a controlled electric shock to a patient's chest in order to either restore normal heart rhythm when a heart goes into fibrillation (usually from tachycardia), or to restore a heart beat after cardiac arrest. There are several types of defibrilators available on the market, ranging from sturdy models with hand paddles to portable units with hands-free electrodes and mini-models for use in open heart surgery.

The concept behind the defibrilator is that because the beating of the heart depends on a regular surge of nerve impulses from the brain stem, when those impulses are interrupted or become uncoordinated, a single electrical shock can bring the heart back into its proper rhythm, allowing the nerve impulses to take over. 

The design of the defibrilator is even simpler, consisting of two electrodes which are placed to the front upper right side of the patient's chest and lower back left side of the patient's abdomen, putting the heart directly in line of the electric current between the two electrodes.  The current is supplied by a battery, which stores a charge in a capacitor so that, like a flash bulb, it can be released all at once in a very short burst.  A defibrilator can supply 1000 volts of direct current for periods of about 5/1000ths of a second.  The voltage and amperage used depends on the likely distance between the electrodes and the heart.  For example, an open heart defibrilator can operate on fairly low voltage and amperage.  These are typically used to re-steart the heart after open heart surgery.

Lawrence Kutner was well known for his risky use of the defibrilator, using it once in a high oxygen environment (starting a fire and setting off the sprinklers) and once using it on a wet patient (where the electrical charge passed through the water to Kutner himself, resulting in him needing treatment for electric shock).     

Defibrillator is often used in the series to restore heartbeat to flatlining patient, which is in fact impossible in real life.

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