Yes a zapper can mess the heart beat and it can probably kill old people or too sick too weakened people.
I have recovered with no significant side effects but the several times it happened to me (i was holding electrodes while sleeping) it was really scary, you feel as if you fall and you have no strength to move or to sit/stand up but you have to, and you can notice that the heart beat is very weak and disturbed.
Personally my reflex was to sit and to breath deeply during a few minutes and then you can feel that the heart beat follow the inspirations expirations.
But the day after i felt weak and tired.
That's why i recommend to not use a zapper while sleeping. You don't know how the body electric resistance can change while sleeping.