The Los Angeles County coroner revealed Breitbart's cause of death in a news release published by his namesake Breitbart.com.
Breitbart's heart failure was caused by an enlarged heart and heart disease, CNN said.
A final autopsy is expected to be released within the next couple of weeks.
The coroner's initial report said no prescription or illicit drugs were found in Breitbart's system and that his blood alcohol level was at .04 percent — well below the legal limit for California drivers.
Breitbart died March 1 after collapsing while walking his dog near his Southern California home.
More from GlobalPost: Andrew Breitbart dead at 43
More from GlobalPost: How Israel dodged the economic crisis