Mr Richman told US media he didn't know whether the heart attack was brought on by a drug overdose, as reported by TMZ.
DMX has publicly battled substance abuse for years, and been to spend several periods in rehab.
He is seen as a hip-hop icon, and has collaborated with artists including JAY-Z, Ja Rule, Eve and LL Cool J as well as appearing in films.
The 50-year-old, whose real name is Earl Simmons, was taken to hospital in White Plains, New York late on Friday evening, Mr Richman said, where he is surrounded by his family.
