EDIT: I'm not lying, I was literally told this by the person performing the certifications. She said only 1 in 20 people survive without severe brain damage. Which is better than death to some people I guess.
Recently, an acquaintance of mine was saved by two of his employees who performed CPR on him for 30 mins until the ambulance arrived. He was very lucky to have collapsed in front of them and that they had both been in the army and had good first aid medical. He was hospitalised for 3 weeks and he has memory problems now, but he's out of bed and there for his kids.
"Each year nationwide, more than 350,000 people suffer a cardiac arrest outside of a hospital; only about 10 percent survive.
When the heart stops beating, a lack of oxygenated blood can cause death within minutes. If CPR is performed immediately, it can keep blood flowing to the brain and other vital organs, doubling or tripling a person’s chance of survival."
[0] https://www.heart.org/en/news/2018/08/22/cpr-training-at-sch...
[0] https://news.sky.com/story/euro-2020-denmark-captain-simon-k...