Who is the lady in white? a ghost that appears before something tragic is going to happen?

Who is this ghost dressed in white, I ume a lady becuase she is wearing a long gown and looks like she has long blond curly hair. I see her only for a second just before something bad happens like a heart attack or respiratory arrest.I'm in the medical field and whenever I see her its just for a second and she always stands in the doorway or stairway.....I've only seen her about 5 times but when i have, I always check my patient and everytime, my patient goes into cardiac or respiratory arrest like 15 seconds later.......its that 15 second warning that helps me to save my patient...............is this some type of angel or am I seeing the angel of death and intervening......because of these warnings, 5 people are alive today........I just wanted to know what it is??