A congenital cardiac flow abnormality caused by a septal defect which reverses left-to-right shunting, allowing backflow and mixing of oxygenated blood from the lungs with deoxygenated returning circulatory blood. Atrial septal defect, ventricular septal defect, or patent ductus arteriosus may cause the flow reversal. Eisenmenger complex is characterized by pulmonary hypertension, cyanosis, high red blood cell count, and arterial oxygen desaturation. Left untreated, heart damage from high pulmonary pressure may be irreversible. Surgical repair may be successful if performed prior to onset of severe pulmonary hypertension and subsequent heart damage.

With a poor prognosis in pregnancy, the condition is life-threatening to both mother and fetus and requires careful monitoring of mother and baby if pregnancy is not discontinued.