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Defibrillators apply brief high-voltage electric to the heart, to restore a proper rhythm. The shock may be under the control of a individual qualified to interpret the electrical activity of the heart, or by a computer programmed to recognize and react to specific arrythmias. Such a computer may be external or internal to the body. Manual defibrillators most commonly are external, but some low-power models are designed to be used during surgery, directly against the heart.

They are lifesaving when the heart goes into purposeless ventricular fibrillation (VF) or ventricular tachycardia (VT), conditions that do not move enough blood to create a pulse. Essentially, they stop and restart the heart, with the its muscles contracting in the proper rhythm. In VT or VF, the heart muscles move fairly randomly.

In certain other arrythmias, the heart has a rhythmic pattern, but is pacing too quickly.

Not all arrythmias can be corrected by electric shock. Some will respond only to drugs, and some can be overridden with an artificial pacemaker, which, through the chest wall or threaded through blood vessels to the heart, make contact with the heart muscle.

Automated external defibrillator

Typical automated electrical defibrillators (AED) are designed to be used by an untrained rescuer. A computer generates spoken instructions to place the electrodes, Only if the machine senses a rhythm that will respond to the shock the device can deliver, it warns everyone except the operator not to touch the body, and then with the operator pressing two buttons so his or her hand position is known to be safe, delivers the shock.

Automated internal defibrillator

External cardioverter

Some defibrillators may have a mode in which they synchronize to a point on the patient's electrocardiogram waveform, and apply a lower-power shock than used for VF or VT. These restore the proper rate and shape of the heart rhythm in arrythmias such as atrial fibrillation (AF).