In *Iron Man*, Tony Stark uses a minified version of the Arc Reactor in his chest to keep shrapnel out of his heart and other organs.

When the Arc Reactor is removed (for changing palladium, stolen by Obadiah Stane, damaged, etc) Tony starts agonizing, but why he didn't die immediately?

I think the blood would carry the shrapnel in a really high speed so Tony would have his organs damaged almost immediately, and even after replacing the Arc Reactor, it would be too late and his organs were already damaged.