> you could seriously hurt yourself with this thing as well,
Well, yes, that is the whole idea. They give a really good kick.
> If its strong enough to deter a bear, its strong enough to stop your heart.
Sorry, but no way.
Look, tens (hundreds?) of thousands of farmers use electric fences to manage animals. And every one of them has leant on the fence a few times at least. (Speaks from experience here!)
Afaik, there have been no recorded instances of someone's heart being affected like that. Too much muscle length in the way. You just get a good jolt.
Can a battery-powered unit give a big enough jolt? Of course. They are very common in some places, up in the mountains.