Heart problems or not, I think someone of Sander's age is not suitable to be POTUS due to the undoubted stress involved in holding such a high office.
There was a brief piece about this on Radio 4's 'More or Less' last week (maybe the week before?) talking about the average odds of someone of elevated age dying during a four year term - I think it looked at Trump, Sanders and Elizabeth Warren as the over-70's candidates and suggested there was between a 10 and 20% chance. Of course, that's an average for the nation, and their individual medical circumstances would alter those chances.
It's the visible impact of it happening on the news, for me, that scuppers Sanders chances. The Democrats probably can't afford to put him forward as their candidate and face the entire election campaign with Republicans and neutrals openly wondering if he'll survive the four years.
O.