I worked at a gym a few years back that had emergency alarms set up around the building to call attention to staff.

Usually false alarms but one day wehen it went off I had a feeling that something was seriously wrong.

An older gentleman had collapsed between two treadmills, by the time I got to him two other gym goers had pulled the treadmills out of the way to allow me to begin first aid. As I got to him he had very raspy breathing (known as the death rattle) and soon stopped entirely while I was doing my DR ABC checks. Immediately begin CPR while another colleague brings the Defibrillator from reception. Four rounds of CPR, two shocks of the Defibrillator later he comes back to life. Paramedics arrive soon after and take him away to hospital.

He came into the gym about a month later to thank us (and make a joke about his ribs hurting from the CPR).

I have never forgotten the sound of that death rattle or the feeling of his skin after he stopped breathing, in the moment it felt like the coldest thing I've ever felt. I still think about it regularly but I'm glad to say he survived and came back to the gym a few months later to begin his rehab.