system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why can doctors restart a transplanted heart with presumably high success rates, while a flat-lined heart is usually fatal?

For the sake of my question, let’s assume that cardiac arrest was due to factors that were then corrected (such as massive blood loss).

Why is not having a shockable rhythm in cardiac arrest different than having a heart that was taken out of the donor’s body and at that point doesn’t have any rhythm at all?

I understand that during a transplant doctors are literally inside someone’s chest and shocking the donor heart to restart it that way. But why do those shocks have a different effect than the shocks from a defibrillator, and wouldn’t doctors be able to save someone in cardiac arrest if they took them to the OR and cracked their chest open and shocked their heart in the same manner transplant surgeons do?
gpt
A heart transplant surgery takes 4-10 hours. 

The only reason the recipient doesn’t die  is because they have a bypass machine pumping their blood for them, and a ventilator breathing for them. That keeps their brain function going. 

If a person has a 4 hour long severe cardiac event that requires correction, the odds are that they’ve endured brain damage if not brain death. If there’s no brain to power the nervous system, there’s nothing to make the heart beat.

So a transplant heart can be “jump started” and then the brain takes over, whereas a brain dead patient may or may not be able to sustain a heartbeat depending on the type/severity of the damage. 

It all depends on the brain.