"Wrap columnist Michael Adams recalled that Murphy told him in 2001 about a heart condition.
"it was in an interview about 'Sidewalks Of New York' but I asked her about other projects and she mentioned 'Spun,'" he wrote in an email on Sunday. "I asked her - as a joke - if she'd taken meth for research and she laughed and said even if she wanted to she couldn't take drugs because
she'd had a heart condition since she was a little girl."
http://www.thewrap.com/article/brittany-murphy-collapses-set-12032
The article also states someone said that during filming of "Something Wicked" , "“She’d go in and out of consciousness in the middle of takes.”
I wonder if she might have had, all this time, a heart condition that most people didn't know about.