A rare heart malformation involving the presence of an abnormal opening between the two atrial chambers of the heart which allows abnormal mixing of oxygenated and deoxygenated blood. The severity of the symptoms depends on the size and location of the defect with mild cases being asymptomatic until adulthood. Atrial septal defect 2 is caused by a mutation on chromosome 8p23.1-p22. See also Atrial septal defect 2