In the situation you describe, it is the easiest if you install "eyes" to the drone. Have at least 6 sensors to detect the closest obstacles (walls, ceiling, floor) on the 3 coordinates.
Additionally, you should have a "map" of the building, so the drone can navigate through it. GPS uses the same idea: some coordinates + some map.
However, you must be aware that inside a building there are many more things, than just walls. And a hospital is even more special. There are a lot of fixtures, lots of cables, conduits, lamps, perfusion "posts" (or however they are called). Even with installing good resolution cameras as "eyes", I still think that there is a significant safety risk for operating the drone within a building. What about the air flow from AC units? What about the drone accidentally crashing inside a patient under surgery? What about the drone running out of "juice" and just "crashing" wherever it happens to be?
While it is doable as a hobby / research, real-life use has limited applicability.
Did I say the sometimes ceilings are very close to the floors, and the drone will start navigating through the people's haircuts? :)