Yes, I have indeed heard of a myocardial infarction aka “heart attack.” If that is what Breitbart died from, then that should have been the cause of death, not “heart failure.”
There is also a little thing called "sudden arrhythmic death syndrome" and another called "sudden death syndrome" both due to a sudden cessation of the hearth rhythm. People just suddenly die. COT, better known as crib death, is thought to be a form of "sudden arrhythmic death syndrome". It is just not that unusual for people to fall over dead from the very young to very old.
It is usually referred to as Heart failure, because, quite simply, that is what happens, the heart just stops beating.
It is usually very difficult to determine the cause of death in such cases.