The presence of a defect (opening) in the septum that separates the 2 atria of the heart with resultant flow. It can be congenital or acquired. Associated clinical issues may include flow of oxygenated blood from the left atrium to the right atrium, producing a volume overload on the right heart; flow of unoxygenated blood from the right atrium to the left atrium resulting in cyanosis; and/or traverse of emboli from the venous circulation to systemic circulation, causing a stroke, transient ischemic attack, or other embolic events.