How does a defibrillator work?


The sinus node of the heart is its natural pacemaker. It sends an electrical impulse to make the heart muscles beat in rhythm. Cardiac arrest is usually caused by a life-threatening change in the rhythm of the heart (known as dysrhythmia).

When the rhythm of the heart goes awry in cardiac arrest, the defibrillator delivers a dose of electric current to the heart. The process is not fully understood, but this current depolarises a large amount of the heart muscle, which ends the dysrhythmia. Once this happens, the pacemaker of the heart can re-establish a normal rhythm.

How does a defibrillator work? © Dan Bright

In defibrillators designed for untrained people to use, two self-adhesive electrodes are attached to the body, to deliver the electric current to the heart

If the shock delivered by the defibrillator isn’t strong enough, the heart might not completely repolarise, and the abnormal rhythm continues. Defibrillators monitor the new heartbeat and might advise the user to deliver another shock.

A common misconception is that defibrillators will restart a heart that is flatlining (known as asystole). But this isn’t true; once the heart is unable to create its own electrical pulse, a defibrillator will not work. Defibrillators do not jump-start the heart like jump-starting a car; they reset the natural pacemaker, like rebooting a computer.

They are straightforward for anyone to use, and it’s worth remembering that they don’t typically allow the user to administer a shock if a ‘shockable’ rhythm is not detected – so you can’t go far wrong, and speed is of the essence.

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Asked by: Felicity Brown, via email

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