Eisenmenger Syndrome is a condition that occurs in some people who were born with heart defects. Sometimes those defects will cause a hole in the heart to be formed which causes oxygenated blood to go back into the lungs and not get distributed where it needs to be.

Erica Zahn describes this very well in her article.

“Simply put, the heart pumps blood left to right within the chambers, and that causes abnormal blood flow to the lungs leading to severe damage. The arteries become stiff and it becomes more and more difficult to circulate oxygenated blood throughout the body. It leads to cyanosis (the skin takes on a blue tint) and clubbing of the fingers.”

Here are links for more info:
U.S. National Library of Medicine
Web M.D.
Patient Worthy