I had one of these for my daughter. It had two diodes that stuck to her chest to track her heart beat and then something that went on her toe to track her oxygen. It would give a high pitch alarm when her breathing stopped or her heart beat dropped below a certain number. I guess the point of the high-pitch alarm was two fold:
A) to alarm us.
B) to startle my daughter out of her episode (which worked)
My insurance paid for this for the time she needed it so unfortunately I am unsure of where to get one or how much. I'm sure a Google search could find some. Definitely worth it knowing you can sleep without worrying about your daughter going into an episode and you not being aware. Kind of an inconvenience with the wires and battery and such but one I was more than willing to deal with considering the circumstances.