64-year-old Duggan was hospitalized last week after feeling "out of shape" and sensing a pain in his chest ... and says he had 6-8 people working on him in the ICU at one time.

Turns out ... Hacksaw's heart had gone into atrial fibrillation -- aka AFib -- which, according to medical sites, is an irregular heartbeat that can lead to strokes, blood clots and other heart problems

While it's not necessarily a life-threatening condition ... Hacksaw says it could've been much worse if he didn't take his wife's advice by going straight to the hospital.

"I kinda knew something was wrong, but I was scared to go to the doctor," Hacksaw says. "If there's something wrong, go to the doctor. They can save your life like they might've done mine."

Hacksaw says he's thankful to have WWE friends like Hulk Hogan and Jerry Lawler reach out to check in on him ... as well as his fans who have stuck with him for the past 40 years.