I heard something about him having asymptomatic heart trouble. Then, read this:

NBC said Dr. Michael Newman, Russert's physician, determined that cholesterol plaque ruptured in an artery, causing blood flow to the heart to be blocked by a clot. The network said an autopsy shortly after his death showed he had an enlarged heart and significant coronary artery disease.

and, this:

"This was one of the most important years in Tim's life," Brokaw told viewers. "He loved this political campaign. He worked to the point of exhaustion so many weeks."

Just sad...