Group 1.4.4 Congenital heart diseases 
A significant proportion of patients with congenital heart disease (CHD), in particular those with systemic-to-pulmonary shunts, will develop PAH if left untreated. Eisenmenger's syndrome is defined as CHD with an initial large systemic-to-pulmonary shunt that induces progressive pulmonary vascular disease and PAH, with resultant reversal of the shunt and central cyanosis [37,38]. It represents the most advanced form of PAH associated with CHD. It has been reported that a large proportion of patients with CHD develop some degree of PAH [39-41]. The prevalence of PAH associated with congenital systemic-to-pulmonary shunts in Europe and North America has been estimated between 1.6 and 12.5 cases per million adults, with 25-50% of this population affected by Eisenmenger’s syndrome.