I feel for you Herc.
I've lost a few due to old age, but I did loose the best dog I've ever had due to unknown cause. One minute he was fine and the next minute he was hanging on to life. We rushed him to the vet's where he basically died in my arms. My hand was on his chest so I could feel his heart beat, he was gasping and trying his best to stay with us, then I felt his heart stop. The vet and her colleagues did everything they could. By the time it was over, there wasn't a dry eye in the room.