In general the settings and themes are established in medical dramas by using convincing life like props and costumes that you would expect to see in a hospital.
The setting is represented in the opening scene of ER , in short little clips that show you the surrounding areas of one of the characters. It will normally include iconography of hospitals i.e wards, corridors, reception and operating theatres. Which suggests that the show is taking place in a hospital. The quick editing with short sequences emphasises the idea that it is a fast paced hospital with too much going on for one person, which suggests that the characters all work as part of a team that have to work well together to keep people alive.
The mise en scene of the opening sequence suggests that hospitals are fast paced, hard working and very busy. The doctors costumes bring across the factual ideas of the show with large white coats and stethoscopes around their necks. Having the stethoscope around their neck at all times suggests that they are always needed to do something and don't have time to put down anything. The surgeons are seen in their green scrubs and nurses pink scrubs - this fortifies the idea that you are in a hospital and makes the show more believable. The sets vary from operating theatres to wards to reception desks. This creates boundaries for the importance of different characters in which the surgeons and doctors are more important than the receptionists and so have more screen time. The make up is very simplistic even on female characters as when working in a hospital it is unusual to be in full make up for health and safety reasons. The props are very iconic for what you would see in a hospital and therefore add realism to the show.
In a typical medical drama themes that are explored include death, murder, suicide, illness, family issues etc. As an audience we expect the show to give us the most ridiculous of situations (i.e swallowing a coat hanger) that we would not expect to see in the real world and yet it is happening and so it is introducing us to real world problems that we don't realise are happening. This touches the audience and so they feel more connected to the characters and so they feel the pain of the doctors when they are unable to help someone.
The music on the opening scene has a sense of the sounds that medical equipment make along with techno sounds that just enhance the feel of being in a hospital.