I talked to an ER doctor who was asked to attend a function where the President was going to attend. They put him in a room and told him if he opened the door without being called, they would shoot him. He would not see the pres unless there was an emergency.
The ER doctor was fibbing. In the bubble - the security apparatus that surrounds the president - there is always a military physician that accompanies the CIC, always. This physician is a member of the White House Medical Unit.