Michael Clarke Duncan -- Lack of Oxygen Proved Fatal

The dramatic minutes Michael Clarke Duncan was without oxygen during his heart attack damaged his organs so badly ... he could never recover -- TMZ has learned.

Sources involved in Duncan's treatment tell us ... on July 13 the actor went without oxygen for more than 5 minutes before Omarosa was able to re-start his heart with CPR. We're told 5 minutes is the outer limit before permanent damage is likely.

Our sources say Duncan's decline was slow but steady. First it was a kidney problem, then the pancreas, then extremely high blood pressure, and so on, until his body just shut down.

It was especially hard on Michael's fiancee, Omarosa ... who was told time and time again by doctors that things were looking up and he would be released soon. But every time Omarosa got assurance, something else went wrong.

Omarosa is now at Forest Lawn, making arrangements for the funeral ... and we're told she is inconsolable.