What is Eisenmenger’s physiology?

Eisenmenger's physiology develops when longstanding left-to-right shunting causes pulmonary hypertension and right ventricular hypertrophy.
The right ventricular pressure exceeds the left ventricular pressure and the shunt reverses, becoming a right-to-left shunt and resulting in a decreased systemic PaO2 (i.e., cyanosis).