Eisenmenger syndrome (ES) is a form of pulmonary arterial hypertension (PAH) associated with unoperated congenital heart disease and is characterized by congenital heart malformations with reversed or bi-directional shunting through an intra-cardiac or intervascular (usually aorto-pulmonary) communication with the development of PAH.

A congenital anomaly with an abnormal connection between the aorta and the main pulmonary artery resulting in an aortopulmonary shunt.