While I agree that it is something to consider before sleeping with a zapper, I will say that from what I have seen, it is far less of a problem that you describe.
My mother used a zapper for years and she did die of heart related problems that existed before she ever used a zapper.
Since she outlived the average by 15 years, did the zapper help or hurt?
My dad also used the zapper but only outlived the average by 8 years. His death came from developing pneumonia while in the hospital where he was not able to use the zapper.
I can assure you that if someone died during the use of a zapper or right after, it would be reported and there would be front page stories about it, even if the zapper was not the cause.