I am trying to find a story, that I think I may have read in a young adult compilation in the late 1980s.
The main character is a teenager, who, prior to the story, had had some kind of severe injury. At the time of the story, he is in the hospital, hooked up to some kind of machine that is monitoring a transdimensional portal. There is some conflict with his parents and his doctors, who want him to do something to open the portal wider. There seems to be some kind of being, which they describe as a guardian angel with a sword, waiting by the gate. Eventually, he opens the portal wider, and the "angel," which was actually on the far side, waiting for it to open wide enough, comes through and slaughters everyone in the hospital.