a congenital defect of the heart in which there is a hole in the partition (septum) separating the two atria (see septal defect). There are two kinds of ASD – ostium primum and ostium secundum . Ostium primum defects are rarer but more serious as the defect lies low down near the valves of the heart. Affected children often have heart failure, although in some a heart murmur detected at routine medical examinations is the only indication of the defect. Ostium secundum defects lie away from the valves and most children have no symptoms; the defect is most commonly indicated by the detection of a heart murmur, and may not be apparent until adulthood. Small defects may close spontaneously. Moderate and large ostium secundum and all ostium primum defects require surgery.
Intrauterine surgical techniques now enable a fetus in which an ASD has been detected to proceed to full term by using the placental circulation as a substitute for the extracorporeal circulation that would otherwise be required.