Continuing Education Activity

An aortopulmonary or septal defect/aortopulmonary 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 part of a greater syndrome involving multiple cardiac defects. Untreated aortopulmonary septal defects commonly cause morbidity and mortality from a large left to right shunt and pulmonary hypertension. This activity reviews the evaluation and management of aortopulmonary septal defects and highlights the role of the interprofessional team in caring for patients with this condition.

Objectives:

• Describe the pathophysiology of aortopulmonary septal defects as the cause of congestive heart failure in infants.
• Review the epidemiology of patients diagnosed with aortopulmonary septal defects.
• Outline the evaluation of a patient with a probable aortopulmonary septal defect.
• Explain the treatment and management necessary for patients with aortopulmonary septal defects.

Introduction

An aortopulmonary (AP) septal defect, also known as an AP window, is one of the rarest congenital heart defects accounting for less than 0.5% of forms of congenital heart disease.[1] This defect can occur in isolation or concurrently with other forms of congenital heart disease such as a ventricular septal defect, interruption of the aortic arch, tetralogy of Fallot, and very rarely, coronary artery anomalies.[2][3][4][5][6] By definition, the AP window is a direct "side to side" connection of the ascending aorta to the main pulmonary artery but with the formation of a normal aortic valve and a right ventricular outflow tract, differentiating it from truncus arteriosus.[7] Embryologically an AP window develops when there is incomplete septation of the great arteries. 

Etiology

Aortopulmonary window defects account for significantly less than 0.5% of all congenital heart defects.[1] They are associated approximately 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, coronary artery anomalies, and tricuspid atresia.[2][3][7][4][8][5][6][9] While this defect would seem to be similar to other conotruncal malformations embryologically, there is a surprisingly low association with DiGeorge syndrome.[10][11] This suggests this is not related to an abnormality in the cardiac neural crest.[11]

Epidemiology

As discussed above, an AP window is a rare congenital heart defect occurring in less than 0.5% of all congenital heart defects.[1] Genetic associations such as VACTERL and Bohring-Opitz have been identified; however, no specific maternal exposures have been found to be an associative cause.[8][12]

Pathophysiology

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 main pulmonary artery. In an isolated AP window, the two separate semilunar valves (aortic and pulmonary) are typically formed normally.[13] The location of the window defect is between the semilunar valves and the branch pulmonary arteries. Three types of AP window are identified: 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.[13] 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.[14]

History and Physical

While it is conceivable to diagnose an isolated AP window in utero by fetal echocardiography, the more typical presentation is in the neonatal period or early infancy.[15][16][17] The typical signs and symptoms associated with an AP window are pulmonary over-circulation occurring as the pulmonary vascular resistance falls over the first few weeks of life. This usually results in a large left to right shunt, and the symptoms would include diaphoresis (especially with feeding), 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 the pulmonary artery is usually large, not allowing for a significant pressure gradient (restriction) to develop.

As noted above, an AP window can be associated with other types of congenital heart defects, and the associated defect can alter the presentation.  In the tetralogy of Fallot, a pulmonary ejection murmur and pulmonary valve click can be noted. When the 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 later in childhood or even adult life. This could present with features of Eisenmenger syndrome, including cyanosis and clubbing.[18][19][20] However, a late presentation of the AP window still requires a detailed hemodynamic evaluation as some patients could still be candidates for repair if there is an acceptable pulmonary vascular resistance or an appropriate response to vasodilator testing.[21][22]

Evaluation

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 color jet. When the suspicion is high, the 2D images are usually sufficient to measure the AP window communication. If echocardiogram imaging is insufficient, a CT scan can potentially delineate the AP connection.

Echocardiography should completely evaluate the remaining cardiac structures, including evaluating other cardiac diseases such as tetralogy of Fallot, interruption of the aortic arch, and care should be taken to identify the coronary arteries. Unless the diagnosis is made late or cannot be made with less invasive means, cardiac catheterization adds little to the diagnostic management. When the diagnosis is made after infancy or even into adult life, especially if cyanosis is noted, 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 to determine the advisability of closure.[22]

Treatment / Management

In general, the treatment for a large AP window is surgical patch closure, and the results are generally good when the repair occurs early in infancy.[23][24] Smaller defects can be surgically closed primarily with double ligation or suture closure.[24][25] Logically, associated congenital heart disease makes surgical repair more difficult worsens outcomes.[23][26] Catheterization device closure with double disk closure devices (atrial septal defect and patent ductus arteriosus devices) have been described in case reports in defects that are typically restrictive, allowing for closure later in childhood.[27][28] 

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 separating the great arteries with either suture division or patch closure of the aorta and pulmonary artery, with 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 or palliation of the other defects should obviously 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.  Furthermore, in the setting of interrupted aortic arch repair, transcatheter aortic interventions for residual arch obstruction could be necessary.[26]

Differential Diagnosis

• Truncus arteriosus is the most common lesion confused with an AP window but has a truncal valve instead of two semilunar valves. The early pathophysiology is similar.
• Patent ductus arteriosus (PDA) has similar physiology. A "window" type PDA is a connection from the proximal descending aorta and left pulmonary artery near the bifurcation. The AP window is a connection of the ascending aorta and main pulmonary artery.  
• A large ventricular septal defect has similar physiology but should be easily distinguished from an AP window.

Prognosis

The prognosis for an isolated AP window is good when treated early with surgical closure and avoidance of the development of pulmonary vascular disease.[24] Late diagnosis can be associated with the development of pulmonary hypertension/Eisenmenger syndrome, which carries a poor prognosis.  An associated AP window with other congenital heart lesions increases the complexity of the surgery.[23][26]

Complications

Complications of missed diagnosis include the development of Eisenmenger syndrome.[29] The repair can be associated with residual defects, including branch pulmonary artery stenosis and residual AP window that might need to be addressed in the future.[30] Caution should be used in balloon angioplasty as an AP window can theoretically recur with aggressive angioplasty. Recurrent laryngeal nerve injury should be suspected if hoarseness or choking with feeds is noted post-repair.[31]

Consultations

Treatment of AP window requires an interprofessional team approach. Generally, consultation should occur with cardiology, cardiac surgery, and cardiac intensive care to optimize outcomes.

Pearls and Other Issues

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 focusing on an evaluation for an AP window. Late repair of AP window is a risk for persistence of substantial pulmonary hypertension, and preoperative catheterization is recommended in advance of late AP window repair. There should be a low threshold for postoperative cardiac catheterization for evaluating pulmonary vascular resistance. In general, early repair of an AP window results in an excellent long-term prognosis.

Enhancing Healthcare Team Outcomes

The diagnosis and management of AP window require an interprofessional team that includes a pediatrician, cardiologist, cardiac surgeon, intensivist, and radiologist. The disorder is typically managed with surgery, but the key is that the child does not have any other associated congenital heart defects. Once the surgery is performed, the infant is usually monitored by cardiac or critical care nurses. If this is an isolated defect, the outcomes are good and are typically good, even when associated with other congenital heart defects. Pharmacists should review the dosing of medications and provide teaching to families of patients. Specialty care nurses help coordinate care and update the team on changes in patient status. Indefinite long-term follow-up with cardiology is advised after successful repair to screen for late abnormalities such as branch pulmonary artery stenosis.