According to a study in the American Heart Association Journal the median heart attack survival rate for victims aged 75 years or older is 3.1 years. Bernie Sanders had a heart attack in October and is 78 years old.

In other words, if elected he is less than likely to survive even half a term. Plus, one should consider that the individuals used in the study were not enduring the rigors of a presidential campaign; nor were they being subjected to the stresses of the most difficult job in the world.

Also, as is made clear in the article, the greatest danger for him lies in the first year of recovery from his heart attack. That roughly coincides with the remainder of the presidential campaign.

Median survival [of a heart attack (myocardial infarction)] … for patients aged ≥75 years was 3.1 years.

https://www.ahajournals.org/doi/full/10.1161/jaha.117.007230?etoc=