Right pulmonary artery originating from the Aorta

Anomalous Origin of Right Pulmonary Artery from Aorta - Collaboration with chd_doodles (Yaeji Kim)

Anomalous origin of the pulmonary artery from the ascending aorta (AOPA) is an exceptionally rare congenital cardiac malformation. In a healthy heart, the pulmonary trunk normally divides into distinct right and left main pulmonary arteries. However, in AOPA, one pulmonary artery (right or left) originates directly from the ascending aorta, while the other maintains its normal connection to the right ventricular outflow tract. This condition is specifically characterized by the presence of separate aortic and pulmonary valves, which differentiates it from a common arterial trunk (truncus arteriosus), making the term "hemitruncus" an inaccurate and inappropriate description.

Clinically, this usually presents as severe neonatal pulmonary hypertension and respiratory distress. Since the right pulmonary artery receives blood at systemic pressure, there's significantly increased pulmonary blood flow into the right lung, and a reactive pulmonary hypertension develops in the left pulmonary artery as well. Infants may also show signs of right heart failure and even cyanosis due to a right-to-left shunt if the right ventricle is failing. Diagnosis is made via echocardiography showing the anomalous origin of the right pulmonary artery from the aorta and its absence from the main pulmonary artery. Early surgical reimplantation of the right pulmonary artery onto the bifurcation of the main pulmonary artery is the necessary treatment.

Graphic by chd_doodles (Yaeji Kim)

Epidemiology

AOPA accounts for a very small percentage of all congenital heart diseases, with incidence estimates generally around 0.1%. The anomaly is far more common in the right pulmonary artery (RPA) originating from the ascending aorta (AORPA) than in the left pulmonary artery (LPA). AORPA is observed 4 to 8 times more frequently than anomalous origin of the left pulmonary artery (AOLPA). The majority of AORPA cases, over 95%, are diagnosed during infancy, with only about 5% reported in adults. This late diagnosis in adults is uncommon due to the high mortality rate associated with the condition if left untreated in early life.

Associated Anomalies

AOPA is commonly associated with other congenital cardiac anomalies in the vast majority of cases, ranging from over 60% to 95%. The most frequently observed accompanying anomaly is patent ductus arteriosus (PDA), seen in two-thirds of AORPA cases. PDA is of particular importance, especially if there is pulmonary hypertension with significant diastolic steal from the aorta to the anomalous RPA and inadequate interatrial communication, as it can counteract this steal and decompress the right ventricle.

Other reported congenital heart defects associated with AORPA include:

• Aortopulmonary septal defect (APSD)

• Tetralogy of Fallot (TOF)

• Hypoplastic aortic arch or interrupted aortic arch

• Patent foramen ovale (PFO)

• Ventricular septal defect (VSD)

• Atrial septal defects (ASD)

• Aortic coarctation

• Anomalous subclavian arteries

AOLPA is more frequently associated with aortic arch anomalies, especially right-sided aortic arch, and with ventricular septal defects and Tetralogy of Fallot. Additionally, AOLPA has been associated with 22q11.2 deletion syndrome.

Clinical Presentation

Patients with AORPA often present with nonspecific clinical symptoms. Common manifestations, particularly in infants and newborns, include:

• Respiratory distress, tachypnea, or shortness of breath

• Congestive heart failure

• Recurrent respiratory tract infections

• Failure to thrive or poor feeding

• Cyanosis, which can be intermittent or persistent, and may include differential cyanosis

• Hemoptysis (red-stained sputum, recurrent nosebleeds)

• Cardiac murmurs, which are common findings upon auscultation

• Signs of pulmonary hypertension. Right ventricular systolic pressure (RVSP) can be significantly elevated, for example, 100-105 mmHg.

Diagnosis

A high index of clinical suspicion is crucial for diagnosing AOPA, especially in infants presenting with unexplained heart failure, recurrent hemoptysis, and pulmonary hypertension. Non-invasive diagnostic imaging studies play a vital role in early diagnosis and preventing high mortality rates.

Diagnostic methods include:

• Transthoracic Echocardiography (TTE): Often the initial diagnostic tool, TTE can define morphology and detect systemic or supra-systemic pressures in the right ventricle. It is helpful in evaluating the ascending aorta and pulmonary arteries, although PDA may not always be visualized unless large. Despite its importance, it can sometimes miss the diagnosis.

• Computed Tomography (CT) Angiography (CTA): Considered the gold standard of diagnosis and surgical planning, CTA provides noninvasive accurate diagnosis with 3-dimensional and cross-sectional images. It helps determine anatomical relations between the pulmonary artery and adjacent structures, and evaluate cardiac structures from various angles.

• Cardiac Magnetic Resonance Imaging (MRI/MRA): This non-invasive method can clarify anatomy using traditional black-blood sequences and MRA, providing additional information with 2D and 4D flow measurements.

• Angiography (Cardiac Catheterization/DSA): Can delineate the anomaly and measure pulmonary artery pressure and pulmonary vascular resistance.

• Electrocardiogram (ECG): May show right ventricular enlargement, right axis deviation, or biventricular hypertrophy.

• Chest Radiographs: May show mild volume loss of the right lung with mediastinal shift, ground-glass opacities, interstitial thickening, cardiomegaly, and increased pulmonary vascularity, especially in the affected lung.

• Fetal diagnosis has also been described in a few reports.

Pathophysiology and Pathology

Normally, the pulmonary trunk divides into right and left main pulmonary arteries. In AORPA, the right pulmonary artery (RPA) originates directly from the ascending aorta, often from its posterior or right lateral aspect. This creates a large high-flow, high-pressure left-to-right shunt from the systemic circulation (aorta) to the pulmonary circulation (right lung - if RPA; left lung - if LPA), and then to the left atrium. The contralateral lung (left lung if abnormal RPA origin; right lung if abnormal LPA origin) then receives the entirety of the right ventricular output. This condition precipitates pressure and/or volume overload in the pulmonary circuit, leading to accelerated progression of vascular disease in the lung circulation. The main complications of untreated AORPA are pulmonary hypertension and heart failure. The high pressure and systemic blood flow to the anomalous pulmonary artery from the ascending aorta make both lungs vulnerable to developing obstructive vascular disease. Microscopic features of pulmonary hypertension, such as intimal changes and medial hypertrophy, can be observed as early as the first month of life. Intimal fibrosis can progress from intimal hyperplasia with continued insult. The first reported case of AORPA with a large patent ductus arteriosus (PDA) associated with a small pulmonary artery aneurysm. The patient's hemoptysis was explained by the presence of a segmental pulmonary aneurysm. Further, patients may experience reactive pulmonary vascular constriction in the contralateral lung to the defect. As such, in abnormal RPA origin, the high flow and pressure transmission to the right lung leads to reactive pulmonary vasoconstriction in the right lung. This is thought to also lead to reactive pulmonary vasoconstriction in the left lung - connected to the right ventricle by the MPA. 

Impact on the Lung Connected to the Main Pulmonary Artery (Contralateral Lung) 

• Pressure and/or Volume Overload: Pulmonary arteries themselves are discontinuous. This arrangement creates a large left-to-right shunt from the aorta to the anomalous pulmonary artery, perfusing one lung with systemic pressure and fully oxygenated blood. The contralateral lung, which receives the full output from the right ventricle, is subjected to significant pressure and/or volume overload. This increased flow at high pressure can lead to the development of vascular disease in this lung's circulation. The abnormal condition initiates neurovascular reflexes and stimulates humoral vasoactive mediators, which further accelerate the progression of vascular disease in the lung circulation. This can lead to diffuse congestion and interstitial/intra-alveolar edema.

• Histologic examination reveals progressive structural changes in the pulmonary vascular bed of both lungs, including the contralateral lung, with advancing age. Medial hypertrophy is a common finding, seen as early as 1 month of age, affecting the muscular arteries in the pulmonary vascular tree. The thickness of the media in the contralateral lung can be significantly increased, indicating medial hypertrophy. Intimal changes, including intimal hyperplasia and proliferation, can occur as early as 1 month of age and progress to intimal fibrosis with continued insult. 

• While some experimental studies show that increased flow to one lung (e.g., after pneumonectomy) may not always cause pulmonary hypertension in the other lung, it is postulated that in AORPA, a "reflex vasoconstriction" or "neurogenic crossover" from the affected (anomalous origin) side contributes to elevated LPA pressure. Additionally, chronic left heart failure, resulting from the overloaded systemic circulation (due to the aortic perfusion of the anomalous lung) and leading to pulmonary venous congestion, can also contribute to pulmonary hypertension in the contralateral lung eventually by a post-capillary phenomenon.

Embryology

Several theories have been proposed to explain the anomalous origin of the pulmonary artery from the ascending aorta:

• Malseptation Hypothesis: This theory attributes AORPA to an asymmetric division of the truncus arteriosus, leaving the right sixth aortic arch on the aortic side. This is supported by the high incidence of aortopulmonary septal abnormalities found with this lesion. It suggests that an eccentric growth of the aortopulmonary septum from the dorsal wall of the aortic sac, which then fuses with the distal ends of the outflow cushions, provides a plausible explanation for the RPA's origin from the ascending aorta.

• Neural Crest Migration Defect: Some studies suggest a relationship with a defect in the migration of neural crest cells during the septation of the common arterial trunk into the aorta and pulmonary artery. Abnormal origin could be due to "subnormal" migration of the RPA, leaving its origin related to the aorta after normal truncal septation.

• Abnormal Development of the Sixth Aortic Arch: One perspective suggests that it is due to an abnormality in the evolution of the sixth right aortic arch, specifically the persistence of its distal portion rather than its proximal part.

• Faulty Timing: Another hypothesis points to faulty timing in the simultaneous events of migration of the RPA and truncal septation.

The term "hemitruncus" is considered inaccurate because, unlike true truncus arteriosus, separate aortic and pulmonary valves are present. The intrapericardial origin from the aorta also distinguishes AOPA from discontinuous pulmonary arteries with ductal origin. Experimental studies involving vitamin A deficient rats and studies with teratogens have produced similar aortic arch and cardiovascular defects, lending support to these developmental theories.

Treatment

Early diagnosis and prompt surgical intervention are mandatory and play a crucial role in improving the prognosis for patients with AOPA. Without surgical repair, the mortality rate can exceed 80% during the first year of life. The mortality rate is reported to be as high as 70% within the first year if untreated, with 30% of infants dying within 3 months.

Surgical techniques aim to reimplant the anomalous pulmonary artery branch to the main pulmonary artery. These include:

• Direct Implantation: This is the most common and often preferred method. It involves disconnecting the RPA from the aorta and directly anastomosing it to the side of the main pulmonary artery, with or without patch augmentation of the defect in the ascending aorta. This technique is generally reserved for cases where the RPA originates from the posterior aspect of the aorta, in close proximity to the MPA, allowing for a tension-free anastomosis. It is favoured for its growth potential and prevention of anastomotic obstruction.

• Interposition Graft Conduit: Used when direct implantation is not feasible, typically involving a Dacron woven conduit to bridge the gap between the RPA and MPA. A drawback is that the patient may outgrow the conduit, requiring reintervention.

• Aortic Ring/Flap Techniques: These methods involve using portions of the aorta or main pulmonary artery to elongate the anomalous RPA. This includes:

  • Using an aortic ring to elongate the RPA.

  • Using an aortic flap to simultaneously repair AORPA and an aortopulmonary window.

  • Double flap techniques using anterior or posterior main pulmonary artery flaps.

• LeCompte Maneuver: This technique involves bringing the right pulmonary artery anterior to the aorta to avoid compression, especially if the RPA originates laterally. It can substantially reduce right ventricular pressure.

• Palliative Treatments: Historically, pulmonary artery banding, PDA ligation alone, or PDA ligation with RPA ligation were performed, but these were largely unsuccessful and associated with high mortality rates (up to 82%).

Cardiopulmonary bypass (CPB) is typically necessary for these surgeries. PGE1 infusion may be used pre-surgically to maintain PDA patency, especially in cases with aortic arch flow reversal and pulmonary hypertension, as it can decompress the right ventricle and maintain systemic blood flow. PDA may also be maintained open with PGE1 if the pulmonary artery originates from the PDA (see here and here). Post-operative complications can include restenosis at the anastomotic site, pulmonary hypertension crises, and sometimes require reintervention. One case also reported renal and hepatic injury that improved after stabilizing the patient's condition.

Prognosis and Follow-up

Patients who undergo early diagnosis and surgical treatment have good short- and long-term outcomes. For example, surgical survival rates have been reported as high as 84% at 1 year, with some centers reporting 0% mortality and excellent short and mid-term results in their patient groups. Postoperative follow-up is vital due to the risk of pulmonary branch stenosis at the suture lines. The incidence of requiring reintervention (e.g., for anastomotic stenosis) has been reported to be between 12.5% and 36%. Some patients may require balloon angioplasty or further surgical repair for restenosis. Long-term follow-up can extend for many years, with some patients remaining asymptomatic and well-grown with normal pulmonary pressures even years after surgery.

Echocardiography Example

In the case below, the RPA has an anomalous origin from the Aorta: 

• The right pulmonary artery (RPA) is connected directly to the systemic circulation. By definition, pressure in the RPA = systemic (aortic) pressure due to pressure transmission. The right lung is therefore exposed to both systemic pressure and high volume (depending on distal resistance and compliance of the right lung pulmonary vasculature). 

• LPA from MPA: The left pulmonary artery (LPA) receives the entire right ventricular (RV) output via the main pulmonary artery (MPA). 

• PDA connection: A ductus arteriosus may connect the LPA (pulmonary artery side)/MPA to the aorta. The direction and restrictiveness of shunting reflect relative pressures between the LPA and the aorta.

• Restrictive PDA (Right-to-Left) 

  • If the PDA is restrictive and right-to-left, it means that LPA pressure > Ao pressure at least at the ductal connection point. This can happen if: The LPA is under very high pressure due to increased pulmonary vascular resistance (PVR) in the left lung, and/or the LPA is experiencing very high flow (since it carries the entire RV output) and cannot decompress efficiently, so pressure builds above systemic.

  • If the PDA is unrestrictive/large, the physiology simplifies to pressure equalization between connected structures. If the PDA is unrestrictive, the LPA and Ao must have the same mean pressure (ignoring small gradients due to flow). Therefore, if you still see right-to-left shunting in that scenario, it cannot be explained by “excess flow alone.” It implies that the driving pressure in the LPA (from RV → MPA → LPA → PDA → Aorta) is greater than systemic, which occurs if PVR ≥ SVR (at least transiently during period of shunting). In a nonrestrictive communication, pressures equilibrate. Flow differences affect volume load, but they cannot maintain a persistent pressure gradient against systemic unless there is increased resistance. Thus, a large unrestrictive PDA with right-to-left PDA shunting indicates that PVR > SVR.

Summary - In this condition (RPA origin from Aorta), if a PDA is open: 

• A restrictive right-to-left PDA from LPA → Ao suggests LPA pressure exceeds systemic, either from high flow or high PVR. 

• A nonrestrictive right-to-left PDA can only occur if LPA pressure = systemic or higher; if the shunt is persistently right-to-left, that means PVR is at least as high as, or higher than, SVR. Excess flow alone cannot explain it in the absence of elevated PVR.

• If there is a large steal from the Aorta to the RPA, there may be a decrease in systemic flow, which may contribute to the physiology and encourage a right to left PDA shunt. 

Prognosis is favorable. Require re-implantation of the right pulmonary artery to the main pulmonary artery under cardiopulmonary bypass. Occasional appearance of a stenosis at the origin of the re-implanted artery.

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