An aortopulmonary (AP) or septal defect/AP window is one of the rarest congenital heart defects. This defect develops embryologically when there is incomplete septation of the great arteries. It can occur in isolation or as a portion of a larger group of cardiac defects. 
AP window defects account for significantly less than 0.5% of all congenital heart defects. It can be associated about 50% of the time with other congenital heart defects such as other conotruncal defects (e.g., tetralogy of Fallot, interrupted aortic arch, D-transposition of the great arteries), coarctation of the aorta, ventricular septal defect, and tricuspid atresia. Underlying identified genetic associations can be found such as 22q11 deletion (DiGeorge syndrome). 
An AP window is a rare congenital heart defect occurring in less than 0.5% of all congenital heart defects. Genetic causes have been identified; however, no specific maternal exposures have been found to be an associative cause.
An AP window occurs during embryonic life when there is incomplete septation of the common arterial trunk, allowing an abnormal connection between the ascending aorta and the pulmonary artery. The two semilunar valves (aortic and pulmonary) are typically formed normally. The location of the window defect is between the semilunar valves and the branch pulmonary arteries. Three types of AP window are type I (proximal), occurring between the posterior wall of the ascending aorta and lateral wall of the main pulmonary artery; type II (distal), occurring between the posterior wall of the ascending aorta and the anterior wall of the origin of the right pulmonary artery; and type III, which is a combination of type I and II. Type I is the most common type of AP window. The size of the connection is variable but is usually large, unrestrictive, and hemodynamically significant. In less than 10% of the cases, the AP window is small and pressure restrictive.
AP window defects present during the neonatal period or in early infancy. The typical signs and symptoms of an AP window are that of pulmonary over-circulation occurring as the pulmonary vascular resistance falls over the first few weeks of life. The symptoms would include diaphoresis (especially with feeds), tachypnea, tachycardia, poor weight gain, and increased respiratory symptoms with viral infections. The precordium is often hyperdynamic, and a mitral valve rumble can be appreciated. The pulses are often bounding as the systemic diastolic blood pressure is decreased secondary to aortic flow reversal in diastole. Rarely, there is a continuous murmur noted as the connection between the aorta, and pulmonary artery is usually large. AP window can be associated with other types of congenital heart defects. In tetralogy of Fallot, a pulmonary ejection murmur and pulmonary valve click can be noted. When AP window occurs with an interrupted aortic arch, the neonate can present with shock as the ductus arteriosus constricts. Occasionally the AP window can be restrictive and present with less significant symptoms of over circulation. In this scenario, a continuous heart murmur could be noted. Rarely, an AP window diagnosis is not made until late in infancy or during childhood. This could present with features of Eisenmenger syndrome, including cyanosis and clubbing.
In children with an AP window, a chest x-ray will show cardiomegaly and increased pulmonary vascular markings. Electrocardiogram will demonstrate tachycardia and increased right and left sided voltages. The diagnosis of an AP window is made by echocardiography after suspicion of a large left to right shunt. Because the connection is usually without significant restriction, color Doppler echocardiography will not detect a high-velocity jet (communication). When the suspicion is high, the 2D images are usually sufficient to measure the AP window communication. If echocardiogram imaging is insufficient, CT scan can potentially delineate the AP connection. Echocardiography should completely evaluate the remaining cardiac structures, including an evaluation of other cardiac diseases such as tetralogy of Fallot and interruption of the aortic arch. Unless the diagnosis is made late or not able to be made with less invasive means, cardiac catheterization adds little to the diagnostic management. When the diagnosis is made after a few months of life, catheterization could be utilized to evaluate the pulmonary vascular resistance as Eisenmenger syndrome can develop. If there is pulmonary vascular disease, reactivity testing should be performed during catheterization.
In general, the treatment for an AP window is surgery, although catheterization device closure has been described in case reports. Anticongestive medications such as diuretics (e.g., furosemide and chlorothiazide) and digoxin can provide temporary symptomatic improvement but should not significantly alter the course of the disease. Afterload reduction can be considered with ACE inhibition. Medical therapy should be approached with caution as there can be abnormal renal perfusion. Surgery should be considered at the time of diagnosis as there will likely be little growth with this physiology and there is a risk of developing irreversible pulmonary hypertension over time. Furthermore, the AP window does not restrict or get hemodynamically less significant over time. In general, surgery involves separation of the great arteries with either suture division or patch closure of the aorta and pulmonary artery, and this occurs with a low surgical mortality. Catheterization can be considered when a defect is small enough to allow for device closure without causing stenosis of the great arteries or interference with the semilunar valves. If other cardiac defects are present, repair of the other defects should occur at the same operation. Postoperative aorta, rarely, and, pulmonary artery stenosis in the main and branch pulmonary arteries, more commonly, can occur. If this stenosis is hemodynamically important, future cardiac catheterization with balloon angioplasty or stent implantation can be employed.
The diagnosis of an AP window is usually made by echocardiography. It cannot be emphasized enough that a high clinical suspicion of a large left to right shunt and bounding pulses should prompt an echocardiogram with a focus to include an evaluation for an AP window. Genetic testing for DiGeorge syndrome should be performed. Late repair of AP window is a risk for persistence of important pulmonary hypertension, and there should be a low threshold for postoperative cardiac catheterization for evaluating the pulmonary vascular resistance. In general, early repair of an AP window results in an excellent long-term prognosis.
The diagnosis and management of AP window requires a multidisciplinary team that includes a pediatrician, cardiologist, cardiac surgeon, geneticist, intensivist and a radiologist. The disorder is managed with surgery, but the key it ensure that the child does not have any other associated congenital heart defects. Once the surgery is performed, the infant is usually monitored by cardiac nurses. If this is an isolated defect, the outcomes are good. For children with multiple other defects, the outlook is guarded. A geneticist should be consulted to test for DiGeorge Syndrome.
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