The way the heart works is different to most muscles. Basically only one area gets the brain signal and each subsequent nerve pulses off the back of that. So if that first part doesn't get a decent signal, ventricular fibrillation, each of those nerves starts to twitch randomly.
So rather than all of them going one after the other so your ECG reading looks like this (although I can't put the things below the line, but you've seen enough medical shows to know what a proper one looks like!):
____/\/\_____
They end up looking like this (loads of smaller peaks and no real beat):
^^^^^^^^^
I expect you twitch because you're awake for it and panicking like a mofo. And of course as the brain is starved of oxygen the rest of your body starts going a bit mental too.
Various other types of heart attacks have different effects, but that's the most common. Brachycardia is just the heart slowing so it's likely you'd just fall asleep. Tachycardia is the opposite, so if you get high and your pulse quickens, this is what you're experiencing. Too much of that and the heart can't handle it.
I think that's the only ones I know.. (wait for it)... off by heart.
Thank ya, I'm here all week.