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I know that heart attack is when some of cells are dead. That's why heart beats abnormally and needs AED to stop it and restart. After that it beats again normally and my question is - how come since cells are dead?

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    Welcome to Health.SE. Where have you read that heart attacks only occur when some of the cells are dead? – Chris Rogers May 26 '18 at 16:09
  • Haven't said thats it's the only case – Adrian Dąbek May 27 '18 at 16:21
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That's right Adrian, it restarts because the damage wasn't sufficient to turn it incapable of functioning. But it may develop cardiac insufficiency as the time passes by.

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  • Thanks, that's exactly what I wanted to know. Marking as the answer and +1 for everyone's effort. – Adrian Dąbek May 27 '18 at 18:11
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TL;DR

An AED stops the heart from beating entirely (beneficial in some situations), and has nothing to do with the death of cells during a heart attack.


The rhythm of the heart is regulated by electrical signals which are controlled by the sinoatrial node and the atrioventricular node. An adequate simulation of this can be seen in this gif:

enter image description here

Source for animation: Battlinbill07 on imgur.com

This cycle is fairly stable and beats throughout your whole life (although some beats are skipped and some people have heart arrhythmias from birth onwards).

Sometimes, the cycle messes up and heart arrhythmia ensues. Some types prevent proper blood circulation and result in the death of a patient. Some are also problematic because CPR is not efficient in those scenarios.

The AED sends a strong electric signal through the heart. For some shockable rhythms, this is recommended and will result in an asystole (a flatline). In some TV dramas, this causes the patient to wake up, but in fact a de-fibrillator only stops the heart activity entirely, thus also eliminating fibrillations. Sometimes, this will synchronise the nodes and the heart starts to beat again normally, and sometimes even the synchronisation doesn’t help and an asystole is the result. This is why defibrillating a flatline is totally useless, despite being often done in TV shows. After the AED created an asystole, CPR is most effective and will result in better survival chances than CPR with fibrillation.


A heart attack is the result of a blocked artery and leads to the slow death of the heart muscle due to low oxygen supply. The heart stops beating long before the entire muscle died, and some of the process is reversible. Usually, a heart attack leads to an asystole when the heart stops beating, which is not a shockable rhythm and an AED will not recommend a shock.

A cardiac arrest (wrongly used interchangeably for heart attack) is the general failure of the heart to beat and has many causes (including a heart attack), some of them arrhythmias where a shock may improve survival chances.

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    Where's the part about automaticity and the heart restarting in a normal rhythm? – Carey Gregory May 26 '18 at 21:02
  • @CareyGregory Thanks for pointing that out, I’ve added that part. Anything else I omitted/forgot/am ignorant about? – Narusan May 26 '18 at 22:23
  • Thanks for thorough response. Yet if some part of muscle is dead and there is the same electric signal sent as always, how can heart maintain its work? It has less power because muscle is partially down yet it can start working again. How is that possible is intriguing me – Adrian Dąbek May 27 '18 at 16:07
  • Not 100% heart power is needed, and cells die slowly. Also, they can partly be replaced. Worst part is the blocked artery, cells also don’t function without oxygen prior to being irreversible dead. – Narusan May 27 '18 at 16:12
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    @AdrianDąbek Not necessarily. Just because you managed to restore a normal rhythm doesn't mean it will remain that way, and it doesn't mean the heart hasn't been injured severely. Every case is different. – Carey Gregory May 27 '18 at 21:11

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