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Defibrillation is a technique used in emergency medicine to save lives
when the heart is in an abnormal or damaging rhythm. The most common
indications for emergency defibrillation is for
ventricular fibrillation and ventricular tachycardia.
is a common cause of
cardiac arrest. The
equipment used in this process is called a defibrillator.
Defibrillation is part of advanced cardiac life support.
Often, ventricular fibrillation is preceded by ventricular
tachycardia. While these are both fast rhythms in the lower chambers
of the heart (the left and right ventricles), ventricular tachycardia
is a more organized rhythm, while ventricular fibrillation is
described as chaotic activity of the ventricles. While ventricular
tachycardia can lead to ventricular fibrillation, it may break on its
own, allowing the person's normal rhythm to resume. Ventricular
fibrillation never breaks on its own, and will always lead to death if
it is not terminated quickly. Ventricular tachycardia may be
associated with an adequate blood pressure to perfuse the brain.
Often, however, the heart is beating too fast for blood to enter the
heart, causing a low blood pressure and decreased perfusion of the
brain and other vital organs. In ventricular fibrillation, since the
electrical activity in the ventricle is chaotic, there's no organized
pumping of blood, and therefore there is no blood perfusion of the
The purpose of defibrillation of ventricular arrhythmias is to apply a
controlled electrical shock to the heart, which leads to
depolarization of the entire electrical conduction system of the
heart. When the heart repolarizes, the normal electrical conduction
may assert itself, in which case the ventricular arrhythmia is
terminated. However, if enough energy is not used for defibrillation,
the heart may not be completely depolarized, in which case the
ventricular tachycardia or fibrillation may not be terminated. Also,
if the heart itself is not getting enough oxygen or if there is an
instability of the electrolytes in the cardiac cells, the ventricular
arrhythmia may recur.
If there is no electrical activity in the heart (electrical asystole,
or standstill), then the heart is already depolarized, and
defibrillation will have no effect.
Although the process of defibrillation can be repeated, the number of
attempts is, in practice, limited, to a series of three or four
attempts at increasing voltages, as the likelihood of restoring normal
heart rhythm is much less in successive attempts.
Increasingly, automatic external defibrillators (AEDs) are being made
available at airports, shopping centres, etc.
Defibrillators can also be implanted (implantable cardioverter-defibrillators
or ICDs) in people at high risk of developing an arrhythmia. The ICD
is rather like an artificial pacemaker, and indeed performs all the
functions of a pacemaker, but it can also deliver a defibrillation
shock if a dangerous arrhythmia is detected.
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