Yes, I have. I think it was 1989, but it was the Sunday after Thanksgiving. It was shortly after completing medic-first-aid certification required by my emergency-response-team memberhsip at my Intel site.
We were driving back north from Redding California, and about halfway between Woodward and Donald, a car going south on the other side of I-5.. the driver had a heart attack. He lost control, swerved across three lanes of traffic, hit the median strip (full of plants) and bounced up onto the oncoming lane. We were perhaps 3 minutes behind that, and I stopped, threw out a flare, and went up to help the people who were trying to see what had happened.
The man was in his 70s. The driver-side front windscreen was burst forward, and he was unresponsive. He wasn't breathing, his pulse wasn't there. I asked someone to cut off the seat belt which was holding him in, and we CAREFULLY moved him out so we could do mouth-to-mouth, and I cleared his mouth as much as I could.
It wasn't effective. He had apparently inhaled or swallowed a denture. After maybe 2 minutes, a national guard truck pulled alongside. The people in it were trainers for CPR, and I gladly gave over to them.
The man didn't make it. I checked on some of the other cars that had undergone a chain-reaction rear-end on the way, but nobody appeared to have been injured (and weren't willing to be checked).
When the cops finally made it through the clogged traffic, I described what I'd seen and that he wasn't breathing when in the car, etc. And showed them my cert card. I also pointed out the man's ID which had fallen to the ground just under the driver's side of his car. The officer cleared me to go.
Penny had insisted that I stop... I think I'm glad she did, but I knew when I was talking to him trying to get his attention that he was dying - he'd sort of 'death rattled' as I touched his shoulder.
Wow. It's been a while since I remembered that one.