Hi, My Stepfather Wet To The Hospital Thursday Walked In

Hi,
My stepfather wet to the hospital Thursday walked in they found he had the start of pneumonia,atrial fibrillation they admitted him he was fine whenmy mither left then she got a call 4hours later saying he took a turn for the worst the doctor told her he was dead but brought him back twice he isnow inicu hooked up to a ventilator doctors are worried abot brain damage what signs if any can we watch for that there is brain activity