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A Direct Connection Between the Aorta and Pulmonary Artery

Introduction

Aortopulmonary Window — AP Window or APW — is a rare congenital cardiac defect in which there is an abnormal opening between the ascending aorta and the pulmonary artery.

Importantly, the aorta and pulmonary artery are otherwise separate vessels with separate semilunar valves.

This distinguishes AP Window from Truncus Arteriosus, in which there is a single common arterial trunk.

Because aortic pressure is normally higher than pulmonary artery pressure after birth, blood flows through the defect from the aorta into the pulmonary circulation.

The result is usually a large left-to-right shunt, making AP Window primarily a Pink Baby lesion. (Merck Manuals)

Body

Normal Anatomy

Normally:

Left ventricle → Aorta → Body

and

Right ventricle → Pulmonary artery → Lungs

The ascending aorta and pulmonary artery lie close together but remain completely separate.

In an Aortopulmonary Window, an abnormal communication exists between them.

Thus:

Aorta → AP Window → Pulmonary artery

This allows oxygenated systemic arterial blood to return directly to the lungs.

⸻

Why is the Baby Usually Pink?

The systemic circulation still receives oxygenated blood from the left ventricle.

The abnormality does not normally divert deoxygenated venous blood directly into the systemic circulation.

Instead, oxygenated blood is being recirculated back through the lungs.

Therefore systemic oxygen saturation is usually maintained.

The principal problem is:

too much pulmonary blood flow

rather than:

too little oxygen in systemic blood.

This makes AP Window a classic acyanotic or Pink Baby lesion. (Merck Manuals)

⸻

Embryological Development

During foetal development, the primitive arterial outflow tract must divide into:

Aorta

and

Pulmonary artery

This separation occurs through development and fusion of the conotruncal ridges.

In Aortopulmonary Window, separation is incomplete in part of the great arterial septum.

A defect therefore remains between the ascending aorta and pulmonary artery. (PubMed Central (PMC))

This differs from Truncus Arteriosus, where separation of the common outflow tract fails much more extensively.

⸻

Aortopulmonary Window Versus Truncus Arteriosus

These two defects can appear superficially similar because both allow direct communication between the systemic and pulmonary circulations.

The essential difference is:

Aortopulmonary Window

There are:

two separate great arteries

and

two separate semilunar valves

with an abnormal window between the vessels.

Truncus Arteriosus

There is:

one common arterial trunk

with a single truncal valve.

This distinction is anatomically and surgically fundamental. (PubMed Central (PMC))

⸻

What Determines the Physiology?

The physiological effect depends principally on:

  • size of the window;
  • pulmonary vascular resistance;
  • systemic vascular resistance;
  • associated cardiac defects;
  • and development of pulmonary hypertension.

Most AP Windows are sufficiently large to produce substantial shunting. (PubMed Central (PMC))

⸻

What Happens After Birth?

Before birth, pulmonary vascular resistance is high.

After birth, the lungs expand and pulmonary vascular resistance falls.

As the pulmonary circulation becomes lower resistance, blood increasingly flows from the higher-pressure aorta through the AP Window into the pulmonary arteries.

Thus:

Aorta → Pulmonary artery → Lungs

The amount of pulmonary blood flow can become enormous.

⸻

Pulmonary Over-Circulation

Excess blood passes through the lungs and returns through the pulmonary veins to the left atrium.

The left side of the heart then has to pump this extra volume again.

A cycle develops:

Left ventricle → Aorta → AP Window → Pulmonary artery → Lungs → Pulmonary veins → Left atrium → Left ventricle

This creates significant pulmonary over-circulation and left-sided volume overload.

⸻

Heart Failure

Large AP Windows commonly produce symptoms of heart failure during infancy.

These may include:

  • tachypnoea;
  • poor feeding;
  • sweating during feeds;
  • failure to gain weight;
  • recurrent respiratory symptoms;
  • hepatomegaly;
  • and increased work of breathing. (Merck Manuals)

The baby may therefore remain pink while becoming profoundly unwell.

Again:

Acyanotic does not mean mild.

⸻

Bounding Pulses

A large AP Window can produce a substantial diastolic run-off from the aorta into the pulmonary artery.

This may lead to:

  • wide pulse pressure;
  • bounding peripheral pulses;
  • and hyperdynamic circulation.

The physiology is similar in principle to a large PDA, although the anatomical connection is different.

⸻

Pulmonary Hypertension

A large AP Window exposes the pulmonary circulation to both:

high flow

and

systemic-level pressure.

Pulmonary vascular disease can therefore develop early.

Irreversible pulmonary hypertension may develop considerably earlier than in many smaller left-to-right shunt lesions. (NCBI)

This is one reason early repair is recommended.

⸻

Eisenmenger Physiology

If severe pulmonary vascular disease develops and pulmonary vascular resistance rises sufficiently, the pressure difference across the AP Window may decrease.

Eventually the shunt may become:

bidirectional

or

right-to-left.

Deoxygenated pulmonary arterial blood can then pass into the aorta.

The previously pink patient becomes cyanotic.

This represents development of Eisenmenger physiology.

At that stage, closure of the defect may no longer be safe because the communication can act as a pressure-relief route for the pulmonary circulation.

⸻

The Mori Classification

One commonly used classification was described by Mori and colleagues.

It divides Aortopulmonary Windows into three principal anatomical types. (PubMed Central (PMC))

Type I — Proximal

The defect lies proximally between the ascending aorta and main pulmonary artery, above the semilunar valves.

This is the most common form. (PubMed Central (PMC))

Type II — Distal

The defect is located more distally near the pulmonary artery bifurcation and right pulmonary artery.

This form can be associated with anomalous origin of the right pulmonary artery from the aorta. (PubMed Central (PMC))

Type III — Total

There is a large defect involving most or essentially all of the aortopulmonary septum.

This combines features of proximal and distal defects. (PubMed Central (PMC))

⸻

Associated Congenital Cardiac Abnormalities

AP Window may occur in isolation, but associated cardiovascular abnormalities are common.

These can include:

  • Atrial Septal Defect;
  • Ventricular Septal Defect;
  • Patent Ductus Arteriosus;
  • Coarctation of the Aorta;
  • Interrupted Aortic Arch;
  • Tetralogy of Fallot;
  • abnormal pulmonary artery origin;
  • and coronary artery abnormalities. (Merck Manuals)

The presence of associated lesions can significantly change both physiology and surgical planning.

⸻

Interrupted Aortic Arch

The association between AP Window and Interrupted Aortic Arch is particularly important.

In this combination, systemic blood flow to the descending aorta may be duct-dependent.

The neonate can therefore have:

a large left-to-right AP Window shunt

while simultaneously requiring the ductus to maintain systemic perfusion.

This creates complex physiology requiring careful preoperative management.

⸻

Clinical Presentation

Large defects usually become symptomatic early.

Presentation may include:

  • tachypnoea;
  • feeding difficulty;
  • sweating;
  • poor weight gain;
  • recurrent chest infections;
  • bounding pulses;
  • cardiac enlargement;
  • and signs of congestive heart failure. (Merck Manuals)

Small AP Windows are much less common and may present later.

⸻

Murmur

The murmur can resemble that of a large Patent Ductus Arteriosus.

Depending on the size of the defect and pulmonary pressures, the murmur may be:

  • continuous;
  • systolic;
  • or relatively unimpressive despite a large defect.

As pulmonary artery pressure rises, the pressure difference between the aorta and pulmonary artery decreases, and the murmur may become less obvious.

Therefore:

a quieter murmur does not necessarily mean less severe disease.

⸻

Diagnosis

Echocardiography

Echocardiography is the principal diagnostic investigation.

It can identify:

  • the communication between the aorta and pulmonary artery;
  • defect size and position;
  • direction of shunting;
  • pulmonary artery pressure;
  • ventricular function;
  • and associated intracardiac abnormalities.

The proximal great vessels must be examined carefully because the defect can sometimes be subtle on routine views. (Merck Manuals)

⸻

CT and MRI

CT angiography and cardiac MRI can clearly demonstrate:

  • the size and extent of the AP Window;
  • pulmonary artery anatomy;
  • aortic arch abnormalities;
  • anomalous pulmonary artery origins;
  • coronary anatomy;
  • and other associated vascular abnormalities. (Merck Manuals)

⸻

Cardiac Catheterisation

Cardiac catheterisation is no longer routinely required purely to establish the diagnosis.

It may still be valuable when there is uncertainty about:

  • pulmonary artery pressures;
  • pulmonary vascular resistance;
  • magnitude of shunting;
  • or suitability for closure in an older patient. (Merck Manuals)

⸻

Initial Medical Management

Medical treatment does not correct the defect.

Infants with heart failure may temporarily require:

  • diuretics;
  • nutritional support;
  • respiratory support;
  • and other measures to control pulmonary over-circulation and heart failure. (NCBI)

Definitive treatment is closure of the abnormal communication.

⸻

Surgical Treatment

Because significant pulmonary vascular disease can develop early, surgical repair is generally performed soon after diagnosis. (Merck Manuals)

The objective is simple:

separate the aorta from the pulmonary artery.

⸻

Patch Closure

Most significant AP Windows are repaired using cardiopulmonary bypass.

The defect can be approached through:

  • the aorta;
  • the pulmonary artery;
  • or by dividing the connection directly.

A patch is used to close the communication while preserving the normal diameter of both great vessels.

The surgeon must ensure that repair does not obstruct:

  • the ascending aorta;
  • main pulmonary artery;
  • right pulmonary artery;
  • left pulmonary artery;
  • or coronary arteries.

Patch closure is a widely used surgical technique. (NCBI)

⸻

Why Not Simply Tie It Off Like a PDA?

A PDA is a distinct tubular vessel connecting the aorta and pulmonary artery.

An AP Window is an actual defect in the wall separating two adjacent great arteries.

A large AP Window therefore cannot generally be treated simply by tying off a duct-like structure.

The defect itself must be closed while reconstructing two separate vessel walls.

⸻

Transcatheter Closure

Selected small AP Windows with suitable anatomical rims may occasionally be closed using catheter-delivered occlusion devices.

However, this is much less widely applicable than device closure of a PDA or secundum ASD.

Most significant AP Windows are treated surgically. (Merck Manuals)

⸻

Cardiopulmonary Bypass

Definitive repair generally requires cardiopulmonary bypass.

The operative team must understand:

  • exact location of the window;
  • relationship to the semilunar valves;
  • pulmonary artery bifurcation;
  • branch pulmonary arteries;
  • coronary origins;
  • and associated arch abnormalities.

Complex AP Window may require simultaneous reconstruction of several structures.

⸻

Anaesthetic and Theatre Considerations

Before repair, the major physiological problem is often massive pulmonary over-circulation.

The anaesthetic team must consider the relationship between:

  • pulmonary vascular resistance;
  • systemic vascular resistance;
  • pulmonary blood flow;
  • systemic perfusion;
  • ventricular function;
  • and pulmonary artery pressure.

A major fall in PVR can increase left-to-right shunting and worsen pulmonary over-circulation.

Conversely, an excessive increase in PVR can compromise right ventricular function and oxygenation.

The goal is therefore balanced circulation, not simply maximal oxygen delivery.

⸻

After Repair

Following closure, pulmonary blood flow suddenly falls towards normal.

The ventricles and pulmonary circulation must adapt to a major change in loading conditions.

Postoperative monitoring focuses upon:

  • ventricular function;
  • pulmonary artery pressure;
  • residual shunting;
  • aortic obstruction;
  • pulmonary artery obstruction;
  • and associated repaired lesions.

⸻

Pulmonary Hypertensive Crisis

Patients with significant preoperative pulmonary hypertension may remain vulnerable to pulmonary hypertensive crises after repair.

Triggers can include:

  • hypoxia;
  • hypercarbia;
  • acidosis;
  • pain;
  • agitation;
  • hypothermia;
  • and airway stimulation.

Careful postoperative intensive-care management is therefore particularly important in children repaired later or with established pulmonary vascular disease.

⸻

Long-Term Follow-Up

Early surgical repair generally has very good outcomes. (PubMed Central (PMC))

Long-term follow-up remains important to identify:

  • residual or recurrent shunting;
  • pulmonary artery stenosis;
  • aortic narrowing;
  • pulmonary hypertension;
  • ventricular dysfunction;
  • and problems associated with other congenital defects.

⸻

Aortopulmonary Window in One Sentence

Aortopulmonary Window is an abnormal communication between two otherwise separate great arteries that allows high-pressure aortic blood to pass directly into the pulmonary circulation, producing a large left-to-right shunt.

The key physiological question is:

How much systemic blood is being diverted back through the lungs?

⸻

Key Points

Aortopulmonary Window is a rare congenital cardiac defect accounting for less than 1% of congenital cardiac malformations. (Merck Manuals)

The aorta and pulmonary artery remain separate vessels with separate valves.

A communication exists between them.

Blood usually flows:

Aorta → Pulmonary artery

creating a left-to-right shunt.

The lesion is therefore normally classified as a Pink Baby defect.

Large AP Windows cause:

  • pulmonary over-circulation;
  • heart failure;
  • pulmonary hypertension;
  • and potentially early irreversible pulmonary vascular disease.

The main Mori anatomical types are:

Type I — proximal

Type II — distal

Type III — total. (PubMed Central (PMC))

Associated abnormalities can include Interrupted Aortic Arch, Coarctation, VSD, ASD and pulmonary artery abnormalities. (Merck Manuals)

Early surgical closure is generally recommended. (NCBI)

Glossary of Terms

Aortopulmonary Septum

The tissue separating the ascending aorta from the pulmonary arterial circulation during development.

Failure of complete formation can produce an Aortopulmonary Window.

Aortopulmonary Window — APW

A congenital communication between the ascending aorta and pulmonary artery despite the presence of two separate great vessels and separate semilunar valves.

Acyanotic

A condition in which systemic arterial oxygenation remains sufficient to prevent central cyanosis.

Cardiopulmonary Bypass — CPB

A technique in which a heart-lung machine temporarily maintains circulation and gas exchange during cardiac surgery.

Cyanosis

Bluish discoloration caused by increased amounts of deoxygenated haemoglobin in systemic arterial blood.

Eisenmenger Physiology

Advanced pulmonary vascular disease associated with reversal or bidirectional flow through a previously left-to-right congenital shunt, resulting in systemic cyanosis.

Left-to-Right Shunt

Abnormal movement of oxygenated systemic blood back towards the pulmonary circulation.

Mori Classification

A classification of Aortopulmonary Window into proximal, distal and total forms.

Patent Ductus Arteriosus — PDA

Persistence after birth of the foetal vascular connection between the pulmonary artery and aorta.

Unlike an AP Window, a PDA is a distinct vessel rather than a defect between adjacent great arterial walls.

Pink Baby

An informal teaching term for congenital cardiac lesions in which systemic arterial oxygenation is usually maintained despite abnormal blood flow or obstruction.

Pulmonary Hypertension

Abnormally increased pressure within the pulmonary arterial circulation.

Pulmonary Over-Circulation

Excessive blood flow through the pulmonary vascular bed.

A large Aortopulmonary Window can produce severe pulmonary over-circulation.

Pulmonary Vascular Disease

Structural remodelling of the pulmonary vasculature resulting from prolonged abnormal pressure or blood flow.

Pulmonary Vascular Resistance — PVR

Resistance encountered by blood flowing through the pulmonary circulation.

Qp:Qs

The ratio between pulmonary blood flow and systemic blood flow.

A significant left-to-right shunt produces a Qp:Qs greater than 1:1.

Semilunar Valves

The aortic and pulmonary valves controlling ventricular outflow into the great arteries.

Systemic Vascular Resistance — SVR

Resistance encountered by blood moving through the systemic circulation.

Truncus Arteriosus

A congenital abnormality in which a single arterial trunk supplies the systemic, pulmonary and coronary circulations.

It differs fundamentally from AP Window because AP Window has two separate great arteries.

Type I AP Window

A proximal defect between the ascending aorta and pulmonary trunk above the semilunar valves.

Type II AP Window

A distal defect located closer to the pulmonary artery bifurcation and right pulmonary artery.

Type III AP Window

A large or total defect involving most of the aortopulmonary septum.

Volume Overload

An increase in the volume of blood a cardiac chamber must handle.

Large AP Windows produce substantial pulmonary and left-sided volume loading.