If your blood flow stopped almost instantly I think you might loose conciousness pretty damn quick. I think it might have more to do with a sudden drop in blood pressure rather than an instant lack of oxygen. You might not be actually DEAD when you hit the floor, but you might not know what hit you and never will.
Something I read about “dying in your sleep” was that a number of heart attacks are assocated with waking up. First thing in the morning, when you start to wake up, your body usually gets a blast of adrenalin to start it moving; as a result your heart beats faster, blood flow increases etc. If you were about to have a heart attack soon, this extra load and heart activity can trigger the attack.
“When I die, I want to go like my grandfather, peacefully in my sleep - not yelling and screaming like the passengers in his car…”
When I had my heart attack (obviously I survived), it started out feeling like a burning sensation on my chest - like when you use Vicks VaporRub and your skin burns. After a while it got worse, then began to feel like someone was squeezing my chest in a vise. I had heard someone once describe a heart attack as “an elephant sitting on my chest” and that was pretty accurate; it was pressure more than actual pain. All this time I was getting the kids ready for school, packing lunches, etc. so although it was painful, it was not debilitating; but somehow I knew something felt really wrong.
After I dropped the kids off at school I drove to the emergency room (stupid, I know now but at the time I didn’t know what was happening) When I walked into the ER, I was talking to the receptionist and I began to get a little woozy and started slurring my words, and they put me in a wheelchair within seconds.
I can’t tell you if a fatal heart attack gets progressively worse than that, as my symptoms abated after they started pumping me full of drugs; but I can tell you that the actual onset and initial symptoms were uncomfortable but not excruciating. Labor was much worse!
Most, if not all, cases where somebody “suddenly drops dead” are due to cardiac arrest as opposed to a myocardial infarction (the medical term for a blockage in the coronary arteries). It is also very common, enough to be disturbing when you ponder your chances of getting it, even if you think you are young and healthy (note how many children die from it):
I drove a neighbor to the hospital while he was having a heart attack. He described the pain as a mule in his chest trying to kick its way out. I don’t know it that’s typical, but his experience sounds quite painful.
IANAMD, but I think you could make a reasonable guess from posture, state of the sheets and blankets, etc. Not saying you could know for certain, of course.
Blocked coronary arteries can cause MI, but MI is dead heart tissue.
The STEMI program we are involved in is set to have a pt. into a cath lab within 90 min of onset of symptoms. Obviously EMS has to be activated with this onset, but wonderful results are seen with this great program.
I am a BLS EMT and we are doing much much more in pre-hospital care that just 2 years ago much less the 32 years I have been involved.
I lost, My daughter lost her husband 6 months ago to sudden cardiac arrest and he was in extreme agony for hours and hours prior. He had been through a full Cardiac workup prior and autopsy results revealed previous MI and our 12 lead recorded abnormal S-T elevation. His sudden cardiac arrest happened before he could be transferred to a cath lab.
He had been to several different MD’s with symptoms and his blockages and MI was not diagnosed.
Tomorrow there is a memorial motorcycle ride in his memory.
Everyone is different in symptoms and signs and no matter how remarkable people will still die. 
Very true. As a paramedic, I’ve seen quite a few people lose consciousness mid-sentence due to a lethal arryhthmia, and I’ve worked a number of cardiac arrests with skull fractures secondary to the event. A serious cardiac event can and sometimes will drop you like the proverbial ton of bricks.
In my grandmother’s heart atack mentioned above, that’s how it seemed. It wasn’t just the doctor’s opinion after the fact, but also a lady who had been sitting by her on the train told us she just got up, took a step or two like she was going somewhere, and then Bam! Down and dead.
The day I had a heart attack, I had an appointment with a doctor in the local hospital. So I was in the best place to have a heart attack. Certainly I had pain, but not much, nothing, say, like a toothache. At least that is what I recall, although it happened 47 years ago.
About 15 years ago, a colleague was diagnosed as having had a heart attack. He says he knows when it must have happened, a half a year earlier, so he obviously felt something, but not much. So I guess the answer is that it varies. A lot.