After I had my heart-attack last year[1], I was asked to participate in a clinical trial of a device called a "Life Vest". I did so briefly, although I ditched it pretty quickly because it was too uncomfortable to sleep in. But this thing was basically a wearable EKG/defibrillator. It monitored your heart rhythm and, if you went into a bad rhythm, would actually give you a shock from the defibrillator. Handy if you were asleep or had already fallen unconscious and couldn't call for help.
Of course it gave you a warning buzz first and had a "cancel" button so you could avoid being shocked if you were actually still OK. And it wouldn't shock you, AFAIK, unless you were in a "shockable rhythm"[2].
Anyway, it's still a bit big, bulky and uncomfortable and I doubt you'll see millions of people wearing something like this anytime soon, but as things get lighter and cheaper and better, it might just become mainstream.