Correct me if I'm wrong (and I'm no expert), but all I've ever seen posted regarding the so-called pre-existing heart condition is some relative by marriage was quoted to that effect in the press.
I think someone close to him said he had had a previous heart attack. I would think any previous heart attack would also have shown up in the autopsy, but I don't know if it did.
People think Breitbart was too young to have died from heart disease. Actually, these days 40 is becoming the prime age for death from heart disease. Too many people are ticking time bombs and most aren't aware of it. Or they have symptoms which they ignore.