I'm no expert, but I know of a few instances where people have had cardiac arrest, and due to quick responses by great people, they have made a full recovery.
I also know someone who had their heart stop at work. Unfortunately, they were surrounded by people who only knew to call 911, there was no AED, and was in a town that had a volunteer EMS. It took a long time for any first response, and almost an hour before they got to a hospital. They did not make it.
Thankfully, Demar's happened where it did, and the resources to save his life immediately were all there.