Aortopulmonary septal defect (APSD), an uncommon congenital cardiac defect, is a deficiency in the septum between the aorta and pulmonary artery, resulting in a communication between the two. This defect is present as an isolated lesion in about one half of patients and in conjunction with another defect or more complex heart disease in the other half of patients.
Developmentally, the defect results from incomplete separation of the common tube of the truncus arteriosus and the aorticopulmonary trunk.  During early embryonic development, the aorta and pulmonary arteries separate by growth of a spiral septum dividing the common trunk into the aorta and the pulmonary artery. The spiral septum is created by fusion of a truncal septum growing cephalad from the semilunar valves and the aorticopulmonary spiral septum growing caudally from the pulmonary bifurcation. Incomplete development of these septa results in aortopulmonary septal defect.
van Mierop subdivided aortopulmonary septal defect into 3 subtypes.  The first subtype is believed to result from nonfusion between the aorticopulmonary septum above and the truncal septum below, resulting in a small-to-moderate defect midway between the semilunar valves and the pulmonary bifurcation. The second type is also believed to arise from a failure of fusion of the aorticopulmonary septum above and the truncal septum below; however, this failure of fusion results in a large, nonrestrictive defect without a continuous posterior border, in which the defect describes more than one spiral turn. The third type is absence of the aorticopulmonary septum; the defect is large and without a posterior border, and the right pulmonary artery may arise directly from the aorta. Although this classification system may correlate with the various embryologic origins of aortopulmonary septal defect itself, it does not account for other anomalies encountered with aortopulmonary septal defect.
Patent ductus arteriosus (PDA) is encountered in almost three fourths of patients with aortopulmonary septal defect. [3, 4] An interrupted aortic arch type A or severe coarctation is present in 10-15% of patients with aortopulmonary septal defect.  Discontinuity of the aorta in interrupted aortic arch type A occurs distal to the left subclavian artery, as in a severe form of aortic coarctation. This is quite different developmentally from interrupted aortic arch type B, in which discontinuity occurs between the left carotid artery and left subclavian arteries. Interrupted aortic arch type B is frequently associated with DiGeorge/velocardiofacial/22q-chromosome arm deletion, unlike interrupted aortic arch type A. When interrupted aortic arch occurs without a ventricular septal defect (VSD), an aortopulmonary septal defect is usually present. 
Tetralogy of Fallot and anomalous coronary from pulmonary artery are each present in about 5% of cases. [7, 8] Other reported anomalies associated with aortopulmonary septal defect include VSD, aortic atresia, transposition of the great arteries, [9, 10] double aortic arch, and other more complex heart diseases.
Aortopulmonary septal defect has been described in other mammals including dogs, cats, and horses. 
The fetus is unaffected by this defect. Problems arise after birth with the fall in pulmonary vascular resistance (PVR) that typically takes place over the first days and weeks of life. As PVR falls, progressive shunting of blood from the systemic circuit to the pulmonary circuit results in pulmonary edema and signs and symptoms of congestive heart failure (CHF) similar to those seen with a large VSD or PDA. Left untreated, irreversible pulmonary vascular obstructive disease (PVOD) is likely to develop. In some cases, PVR does not fall significantly after birth and the phase of CHF is not apparent. In these instances, PVOD is a consequence nonetheless.
Aortopulmonary septal defect is a rare defect that comprises about 0.1-0.3% of congenital heart diseases in children. No attempt to assess regional or worldwide variation in incidence has been made.
A large case series from India reported an overall frequency of surgery for aortopulmonary septal defect of 0.6% of all surgeries performed for congenital heart disease. 
Left untreated, an aortopulmonary window results in irreversible pulmonary vascular changes and early mortality. With surgical treatment in the absence of PVOD, the prognosis for isolated aortopulmonary window is good. In the presence of more complex heart disease, prognosis depends more on the nature of other lesions.
No racial predilection is observed.
The male-to-female ratio is approximately 1.8:1.
As a congenital disease, all cases are present from birth. The diagnosis is typically made in infancy but may be delayed if persistently elevated PVR occurs. Because of improved fetal ultrasonography, prenatal diagnosis of aortopulmonary septal defect has also been reported. [13, 14]
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