Tony and Steve are stuck in a cell somewhere with a heavy door separating them to the corridor outside. When the Avengers arrive to rescue them, Natasha tells them (through the door) that they'll need to wait a bit since she can't open the door. Steve ("no need to wait") than gets the idea to use Tony's arc reactor ("I'll just borrow this") to power the electronic on the door to open it. The thing is, Steve doesn't know that the arc reactor is Tony's life support, so when the door open and Natasha sees Steve proudly holding the arc reactor up, she freaks out, takes the arc reactor from his hands and rushes to Tony, who is quietly having a heart attack in the back of the cell. Steve is of course remorseful on the plane ride back home, Natasha is protective and glaring at him, and Tony ends up forgiving him.
I think Tony might have been injured, freezing, or just not in good shape while in the cell, but I'm not sure.
Any help finding this fic would be greatly appreciated. Thank you.
EDIT:Thank you hungrydinosaurs for finding it: Don't Go Breaking My Heart