For 6 months, King had been seeing doctors while suffering from breathing issues.
He has had heart issues for 30 years and has previously suffered two heart attacks.
King has been a leading advocate for heart health and check-ups ever since his first heart attack.
He revealed what his lifestyle was like before his heart attack in 2004, when he released his book Taking on Heart Disease.
"In early 1987 I was smoking three packs of Nat Sherman cigarettes a day, eating fried this-and-that, enjoying lamb chops with lots of fat because that always improves the taste, ordering banana cream or lemon meringue pie for dessert — and feeling absolutely fine," wrote King.
That incident changed King's life, and the way he thought about heart health.
"I had never spent much time thinking about my heart. But when I did, it was always the other heart — the romantic one," wrote King.
"So let's get to it right here in the first paragraph: Not paying attention to the heart [the one inside your chest] and the warnings it will send is a theme in the pages to come."