system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes some heart attacks to be instantly fatal while others you might not even notice?
gpt
The main issue is whether the clot causes an arrhythmia or not.  People often confuse the terms "heart attack" and "cardiac arrest", but they are not the same thing.  A heart attack is when blood flow is blocked to the heart muscle, usually via clot (though the arteries that supply the heart can sometimes spasm and cause a heart attack as well).  As a result of the heart attack, the muscle starts dying (slowly, so there is time to stop it and save it) and can respond erratically to the heart's normal electrical rhythm.  At this point, the heart can go into "cardiac arrest", which means that the heart stops beating.  Sometimes it means the heart still beats, but too weakly to generate an effective pulse (this would be called pulseless electrical activity or PEA), or it may cease all electrical activity altogether (called asystole).

The heart attacks that kill quickly are those that induce a fatal cardiac arrest, through the disruption of the heart's normal electrical pattern.  A heart attack can also kill relatively quickly by making the heart so weak (a big clot in the left main coronary for example) that it can't pump blood around the body anymore.  The brain will quickly die without a pulse and oxygenated blood.