I like your idea of the PIR sensor and I think with some logic and a IR tripwire sensor and a magnetic sensor on the door you may be able to have an autonomous system.
The order in which the tripwire and the door magnetic sensors are triggered could determine whether a person entered the room or left and the PIR sensor could tell you if there was movement in the room. If there was no movement for a period of time and there was a person in the room an alarm would be triggered. The benefit of the no movement indicator causing an alarm would give you a better response time if measured between the time someone had an external electrical caused heart attack and when someone brought in some basic life support. We installed some sensitive PIR in a secure room that would get triggered by mice running on the floor so they can pickup minor motion from a human.
Just some thoughts and I believe there are some holes in the above that can be filled in to create a functional system, but then aagain I may well have missed the boat entirely.