Aortopulmonary window (APW) is defined as a communication between the ascending aorta and pulmonary artery above the level of the two semilunar valves. It results from the failure of the fusion of the conotruncal ridges that septate the truncus arteriosus into the aorta and the pulmonary artery, a process which normally occurs during the fifth week of development. Therefore, the aortopulmonary foramen is not closed by the spiral growth of the conotruncal ridges and the common arterial trunk is not completely divided. By definition, this occurs distal to two normal semilunar valves, distinguishing it from common arterial trunk.
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