About 20 years ago my DB was in a medically induced coma, on ventilation with pneumonia caused by Leigonnaire's disease.
The doctors tried everything and pumped him full of so many different drugs and antibiotics he puffed up and went orange - remember DM saying he looked a little like Elvis. We were told several times he wasn't expected to make it and we should be prepared for the worst.
After 3 or 4 days the Dr said blood poisoning was causing his organs to shut down. Apparently the blood poisoning was caused by the carbon dioxide in his body, so in essence the ventilation was killing him. So they had to stop the ventilation, which would then kill him as he couldn't breathe. It was nightmarish.
When the life support was withdrawn he didn't die - he just kept going. In a short space of time - less than 6 hours - he woke up, complained about being hungry, tried to eat a sandwich - which i'd wandered around the hospital looking for as there's no food provision in ICU - projectile vomited over a nurse and apologised profusely.
Staff from other parts of the hospital came to visit and have a look at him as they couldn't believe what had happened.
It was like a miracle. We were lucky that he was young, strong, fit and didn't smoke, or he wouldn't have made it.
It took a while for his kidneys and sense of balance to recover but there's no lasting physical effects.
DM maintains while there's life there's hope. Good luck.