Former President Clinton is undergoing decortication following his bypass surgery last year:

Former President Clinton will undergo a medical procedure this week to remove fluid and scar tissue from his left chest, six months after he underwent quadruple bypass surgery, his office said Tuesday.

The procedure, known as a decortication, will remove scar tissue that has developed as a result of fluid buildup and inflammation, causing compression and collapse of the lower lobe of the left lung, doctors said. The surgery will be done either through a small incision or with a video-assisted thoracoscope inserted between ribs.

Clinton’s problem is a relatively rare complication of his surgery, where inflammation of the lining of the heart develops and fluid builds around it or in the lungs, said Dr. John LaRosa, president of the State University of New York Health Science Center in Brooklyn. It doesn’t signal anything ominous about the former president’s outlook, he said.

Pleural effusions are common after cardiac surgery, occurring in up to 90 percent of patients following CABG (bypass surgery). Causes range can include congestive heart failure, postcardiac injury syndrome, pericarditis, surgical tissue trauma, or atelectasis. Effusions can occur early (less than 30 days) or late (greater than 30 days). Up to 60 percent of patients have effusions at 30 days after CABG. Effusions large enough for decortication are uncommon, as mentioned above. From UptoDate:

The majority of postoperative pleural effusions are small and are not associated with increased mortality or prolonged hospital stay . . . Large effusions that require decortication for control are rare.

Update:
ABC News with a good summary of what’s going on.

Update 2:
CodeBlueBlog agrees that the severity of this complication is rare, and speculates on other possible causes.