Steve opens his mouth to give it to him before coughing. Bucky's strangling him again. There's nothing he can do about it. He's going to die. Sam is going to die. He can't get this metal arm off his throat.
He claws at his throat desperately before jumping back to reality with a gasp. He died. Bucky killed him and Sam and now Natasha is alone effectively. His eyes are wild as he gasps for air and looks close to sobbing over what's happened. It doesn't even occur to him that the bed is different and Tony is there. All he can see is the darkness Bucky put him in.