It isn't natural for a 43 year old man's heart to just stop. It would be attributed to an underlying condition as the cause. Here's how the AMA classifies these things - it is subjective and it would not surprise me in the least if they kept Breitbart's cause of death this way, but that's not to say it's correct:
http://archinte.ama-assn.org/cgi/con...tract/156/1/25