Back In March My Mother Was Admitted To The Hospital

Back in march my mother was admitted to the hospital for chf. She was admitted on a wednesday but as the days progressed her breathing got worse and became labored. We knew something wasnt right bc she wasnt getting better. The nurse ended up giving my mother morphine bc she was having a "panic attack" even tho it wasnt a panic attack... she couldnt breathe bc her lungs were filling up with fluids from chf. 30 minutes later my mothers heart stopped after morphine and they had to recusistate. The nurse told me she got her back in a minute or two but reports show 12 minutes. They took my mother to ICU and an hour later or longer let us come in to see her and she began to come to. She was bucking her arms and legs bc the ventilator was in her mouth/throat so i grabbed her hand and asked her to squeeze my hand if she could hear me and she did immediately. They then sedated her but she was never the same after that. My mother later tested and showed she was severely brain damaged and the only part of her brain functioning was the lower stem of her brain. She stared off in space and blinked along with opening and closing her mouth. The first day she was able to follow me when her eyes opened but then as the day progressed she faintly could to she no longer could. How long does it take for brain damage to kick in? I understand every minute of no oxygen causes damage but my question is... how was she able to respond to me and able to squeeze my hand and move a little over an hour after being resuscitated, then sedated and never being the same from that point on?