Friend of mine did the same stunt, i think.
He had a two-headed bong that was trying to kill him.
After the doublehit, he placed the bong on the table, got down on the floor, and looked like he was choking. Which he was.
He was gasping for air, and it pretty much looked like he was dying.
Apparently the smoke found a way into his chest cavity, where it would apply preasure to heart and lungs.
After 20-40 seconds, he was fine again.
A few months later or so, he decided to try again and failed again, almost died again.
Good times.