i saw a tv show on near death experiences not too long ago actually. as long as you arnt killed instantly or unconcious i think that youd understand whats happening and feel your heart stop for a split second. blood is still circulating so as long as its oxyginated i think youd know. one of the reasons they stopped decapitation is because the head was sometimes still alive for a short time. they even used to pick the head up and show it the body before it died. it was only a few seconds but you can remain concious with no heartbeat.
in some of the cases it was very convincing that after the person had died something still occured. a woman had her brain completly drained of blood for an operation and she was able to talk about what happened and what the instruments looked like during the procedure. they dont uncover the instruments until the patient is completly unconcious so she didnt see them before hand, and she didnt regain conciousness until after she had been taked from the room. there was no brain activity at all becuase their was no blood, but she could remember the conversations and what everyone was doing. she said she was in the top corner of the room watching it happen. there were a bunch of cases similar with people ariving unconcious and remembering the nurses faces and what doctors said during the proceedure, but none of them had a patient that was completly braindead like the woman whos brain was drained.
they also had someone explaining why it is that you see a tunnel and all that from a scientific point of view and how certian parts of the eye black out first giving the illusion of a dark tunnel with light at the end. ultimatly the number of cases and some of the details made me side against her pretty strongly. she couldnt explain the woman with no blood and some other things like that.
most of them, reguardless of the trauma, said it was peaceful and painless. the woman who didnt believe in it said that the brain releases alot of hormones that would give you a euphoria just before you died, so while you might feel your heart stop very briefly i dont think it would bother you much. if you were burning alive or something like that you might still feel alot of pain and stuff but if you were shot or suffered extreme injuries and were still concious, it seems that from a scientific and spiritual point of view that your last moments wouldnt involve much pain at all. leading up would but when your heart was about to stop beating and your brain was dying you would feel peaceful and happy.