Atrial-Septal Defect (ASD). Congenital, acyanotic, heart defect in which the atrial septum does not fully close, and oxygenated blood from the left atrium is shunted to the right atrium. The majority of ASDs are well-tolerated, and as long as they remain left-to-right (thus acyanotic), they can be asymptomatic. A long-term increase in pulmonary blood flow can eventually lead to pulmonary hypertension, and a shunt reversal. Large or symptomatic ASDs are surgically corrected early in life.