I know, this thread is old and the problem has probably already been solved in one way or another, but I just wanted to remark that there are devices called "dead man switches" on the market for ages. We use them in the lab I'm working at in case someone needs to work alone or out of sight of his colleagues for a longer time.
They are little devices which detect the position of the wearer. You clip them to your belt and if you fall over and lay on the ground for a certain time, they sound a so-called pre-alarm which you need to acknowledge. If you don't, the main alarm is forwarded to the security guys and they come looking for you.