Late onset of cyanosis in heart defects not initially associated with cyanosis. First there is a left-right shunt, i.e. blood passes through a malformation from the aorta into the pulmonary artery or from the left ventricle into the right. Over time, a "counter-pressure" develops in the right ventricle and pulmonary arteries of the affected person and this gradually leads to the reversal of this abnormal blood flow connection - a right-left shunt with cyanosis develops (Eisenmenger reaction). Consequently, cyanosis results from the progressive increase in pulmonary vascular resistance and its consequence is shunt reversal. The existing left-right shunt is reversed into a right-left shunt over time. After a certain point, surgery for the heart defect is no longer possible.
