Situations like those, while rare, are why I always keep the triage desk drawers stocked. As soon as I saw the pulsing red rainbow arcing over the desk, I grabbed a wad of 4x4s from the drawer and clamped down on the arm. A nurse ran over a wheelchair, and we rushed him to the trauma room with my hands still locked around his arm. It wasn't until a large BP cuff was over-inflated above my hands that my brief career as a human tourniquet ended.
Word to the wise: when playing poker with an unsavory crowd, try to avoid cheating the dude with the knife.