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When Some Pulmonary Veins Return to the Wrong Side of the Heart

Introduction

Congenital cardiac defects can often be understood by asking:

Where is the blood returning from?

Where is it actually draining?

And how much extra blood is being sent back through the lungs?

Partial Anomalous Pulmonary Venous Return — PAPVR — is usually a “pink baby” lesion.

In PAPVR, one or more pulmonary veins connect abnormally to the systemic venous circulation instead of the left atrium.

Unlike Total Anomalous Pulmonary Venous Return, at least some pulmonary veins still connect normally to the left atrium.

The result is usually a left-to-right shunt rather than cyanosis: oxygenated blood returning from part of the lung is diverted back to the right side of the heart and sent through the lungs again. (PubMed Central (PMC))

Body

Normal Pulmonary Venous Return

Normally, oxygenated blood returns from the lungs through the pulmonary veins.

The normal pathway is:

Lungs → Pulmonary veins → Left atrium → Left ventricle → Aorta → Body

Typically there are four principal pulmonary veins:

  • right upper pulmonary vein;
  • right lower pulmonary vein;
  • left upper pulmonary vein;
  • left lower pulmonary vein.

All normally enter the left atrium.

What Happens in PAPVR?

In Partial Anomalous Pulmonary Venous Return, one or more pulmonary veins fail to connect normally to the left atrium.

Instead, the anomalous vein may drain into structures such as:

  • Superior Vena Cava — SVC;
  • Inferior Vena Cava — IVC;
  • right atrium;
  • innominate vein;
  • azygos system;
  • or other systemic venous pathways.

The oxygenated blood from that portion of lung is therefore returned to the right side of the circulation. (PubMed Central (PMC))

Instead of:

Lung → Pulmonary vein → Left atrium

the pathway becomes something like:

Lung → Anomalous pulmonary vein → SVC → Right atrium

The blood then travels:

Right atrium → Right ventricle → Pulmonary artery → Lungs

again.

Why is the Baby Usually Pink?

The anomalously draining pulmonary venous blood has already passed through the lungs and is oxygenated.

It is simply returning to the wrong side of the heart.

Therefore, systemic venous blood is not normally being diverted directly into the systemic arterial circulation.

Systemic oxygen saturation usually remains normal.

The problem is instead excess pulmonary blood flow and right-sided volume loading.

This makes PAPVR principally an acyanotic left-to-right shunt lesion. (PubMed Central (PMC))

How Big is the Shunt?

The physiological effect depends largely upon how much lung drains abnormally.

If only one small pulmonary vein is involved, the shunt may be minor.

The patient may remain asymptomatic for decades.

If an entire lung or several pulmonary veins drain anomalously, a much larger amount of oxygenated blood returns to the right heart.

This can produce:

  • right atrial dilation;
  • right ventricular dilation;
  • increased pulmonary blood flow;
  • pulmonary artery enlargement;
  • and eventually pulmonary hypertension.

The severity therefore depends on the proportion of pulmonary venous return that is anomalous. (PubMed Central (PMC))

The Relationship with ASD

PAPVR is frequently associated with an Atrial Septal Defect — ASD, particularly a sinus venosus ASD. (PubMed Central (PMC))

A classic pattern is:

Right upper pulmonary vein → SVC

associated with:

Superior sinus venosus ASD

The two abnormalities together can significantly increase the left-to-right shunt.

Blood from the anomalous pulmonary vein enters the systemic venous circulation, while blood from the left atrium may also cross the ASD into the right atrium.

The right side of the heart therefore receives volume from two sources.

Sinus Venosus ASD and PAPVR

A sinus venosus defect usually lies near the junction between the SVC and right atrium.

It is therefore anatomically quite different from a conventional secundum ASD.

The close developmental relationship between the SVC, atrial septum and right pulmonary veins explains why anomalous pulmonary venous connections are so commonly associated with this lesion.

For theatre practitioners, this matters because surgical repair must address both:

the atrial communication

and

the anomalous pulmonary venous drainage.

Simply closing the ASD without redirecting the pulmonary vein would not correct the physiology.

Right Heart Volume Loading

Because oxygenated blood is being returned to the right side, the right atrium and right ventricle handle more blood than normal.

Over time this can result in:

  • right atrial enlargement;
  • right ventricular enlargement;
  • increased stroke volume through the pulmonary artery;
  • and dilation of the pulmonary circulation.

This is similar in principle to the physiology of a significant ASD.

PAPVR therefore behaves as a pre-tricuspid volume-loading lesion. (PubMed Central (PMC))

Pulmonary Hypertension

Longstanding increased pulmonary blood flow can eventually contribute to pulmonary hypertension.

Most small PAPVR lesions never progress this far.

However, larger shunts can expose the pulmonary vascular bed to chronically increased flow.

Over many years, pulmonary vascular remodelling can occur.

PAPVR is therefore a recognised, potentially treatable cause of pulmonary hypertension. (PubMed Central (PMC))

In advanced disease, significant pulmonary vascular resistance may eventually limit the safety or benefit of surgical correction.

Clinical Presentation

Many patients are asymptomatic.

PAPVR may be discovered incidentally during:

  • echocardiography;
  • CT;
  • MRI;
  • investigation of a murmur;
  • or imaging performed for an unrelated reason. (PubMed Central (PMC))

Larger shunts may present with:

  • breathlessness;
  • reduced exercise tolerance;
  • fatigue;
  • palpitations;
  • recurrent respiratory symptoms;
  • right-sided chamber enlargement;
  • pulmonary hypertension;
  • or right heart failure.

Children with significant associated ASD may present similarly to patients with a large isolated ASD.

Cyanosis

Cyanosis is not typical of uncomplicated PAPVR.

If cyanosis develops, another physiological explanation should be sought.

Possible causes include:

  • severe pulmonary hypertension with shunt reversal;
  • additional congenital heart disease;
  • abnormal systemic venous connections;
  • or advanced right-to-left shunting.

Published reviews describe cyanosis as uncommon in PAPVR and generally a late feature when pulmonary vascular disease has become significant. (PubMed Central (PMC))

This again demonstrates that the Pink Baby / Blue Baby distinction describes physiology rather than an immutable diagnostic category.

Diagnosis

Echocardiography

Echocardiography may identify:

  • anomalous pulmonary venous drainage;
  • associated ASD;
  • right atrial enlargement;
  • right ventricular enlargement;
  • elevated pulmonary artery pressure;
  • and abnormal flow patterns.

However, individual pulmonary veins can sometimes be difficult to visualise completely using transthoracic echocardiography.

CT Angiography

CT angiography is particularly useful for defining pulmonary venous anatomy.

It can demonstrate:

  • exactly which pulmonary veins drain abnormally;
  • the systemic vein into which they connect;
  • associated pulmonary artery anatomy;
  • lung abnormalities;
  • and complex extracardiac vascular relationships.

CT is especially useful in Scimitar Syndrome. (PubMed)

Cardiac MRI

MRI can define both anatomy and physiology.

It can provide:

  • pulmonary venous anatomy;
  • right ventricular volumes;
  • right ventricular function;
  • pulmonary artery flow;
  • systemic flow;
  • and calculation of the pulmonary-to-systemic flow ratio.

This can be useful when deciding whether the shunt is haemodynamically significant.

Qp:Qs

The relationship between pulmonary and systemic blood flow can be expressed as:

Qp:Qs

where:

Qp = pulmonary blood flow

and

Qs = systemic blood flow.

A normal circulation has approximately:

1:1

In a significant left-to-right shunt:

Qp > Qs

For example:

Qp:Qs 1.5:1

means that pulmonary blood flow is approximately 50% greater than systemic blood flow.

This is one of the measurements used when assessing whether PAPVR is physiologically significant.

Treatment

Not every PAPVR requires surgery.

A small anomalous connection producing little right-sided volume overload may simply be monitored.

Intervention becomes more likely when there is:

  • a substantial left-to-right shunt;
  • significant right ventricular enlargement;
  • symptoms;
  • associated ASD requiring closure;
  • or pulmonary hypertension that remains potentially reversible.

The objective of treatment is straightforward:

redirect anomalous pulmonary venous blood into the left atrium.

Surgical Repair

The exact operation depends upon where the anomalous pulmonary veins enter the systemic circulation.

Some lesions can be repaired by creating an intracardiac baffle directing pulmonary venous blood through an ASD into the left atrium.

Other anatomical arrangements require reconstruction of the SVC or direct reimplantation of the pulmonary vein.

Modern surgical options include intracardiac baffling, patch techniques and procedures such as the Warden operation for suitable superior sinus venosus/PAPVR anatomy. (PubMed)

The Warden Procedure

The Warden procedure is particularly associated with PAPVR in which right pulmonary veins enter the SVC near a superior sinus venosus defect.

The basic principle is to separate systemic venous return from pulmonary venous return.

An intracardiac patch redirects pulmonary venous blood through the atrial communication towards the left atrium.

The SVC is divided above the anomalous pulmonary venous connection.

The upper SVC is then connected to the right atrial appendage.

Thus:

SVC blood → Right atrium

while:

Anomalous pulmonary venous blood → Left atrium

This avoids forcing both flows through the same reconstructed pathway.

Potential Surgical Complications

Repair of PAPVR can create new areas of obstruction if reconstruction is too narrow.

Potential complications include:

  • pulmonary venous obstruction;
  • SVC obstruction;
  • sinus node dysfunction;
  • residual shunting;
  • and arrhythmias.

The exact risks depend upon the anatomy and operative technique.

Scimitar Syndrome

What is Scimitar Syndrome?

Scimitar Syndrome is a specialised form of PAPVR involving anomalous pulmonary venous drainage from the right lung, usually into the Inferior Vena Cava.

However, it is more than simply an abnormal pulmonary vein.

It is a broader congenital cardiopulmonary syndrome.

Typical associated features include:

  • anomalous right pulmonary venous drainage to the IVC;
  • hypoplasia of the right lung;
  • hypoplasia or abnormality of the right pulmonary artery;
  • displacement of the heart towards the right;
  • and abnormal systemic arterial supply to part of the right lung. (PubMed Central (PMC))

Why is it Called “Scimitar” Syndrome?

A scimitar is a curved sword.

On a chest radiograph, the anomalous pulmonary vein may appear as a curved vascular shadow running down the right side of the heart towards the diaphragm.

Its shape resembles the blade of a scimitar.

The term was introduced because of this characteristic radiographic appearance. (PubMed Central (PMC))

The Scimitar Vein

The abnormal pulmonary vein commonly runs from the right lung inferiorly towards the diaphragm and drains into the IVC.

A typical pathway is:

Right lung → Scimitar vein → IVC → Right atrium

Oxygenated blood from the right lung is therefore returned to the right side of the heart rather than the left atrium.

This creates a left-to-right shunt.

Right Lung Hypoplasia

The right lung is frequently smaller than normal.

This is an important component of Scimitar Syndrome.

The reduced right lung volume can cause the mediastinum and heart to shift towards the right.

This is referred to as dextroposition.

Dextroposition should not necessarily be confused with true dextrocardia.

The heart has been displaced towards the right because the right hemithorax is smaller rather than because the heart developed as a mirror image. (PubMed Central (PMC))

Right Pulmonary Artery Hypoplasia

The right pulmonary artery may also be hypoplastic.

This further alters pulmonary blood flow.

The affected lung may therefore have:

  • reduced normal pulmonary arterial supply;
  • anomalous systemic arterial supply;
  • and abnormal pulmonary venous drainage.

This combination makes Scimitar Syndrome a much more complex cardiopulmonary abnormality than a simple isolated PAPVR.

Abnormal Systemic Arterial Supply

A striking feature of Scimitar Syndrome is the possibility that part of the right lung receives blood directly from the systemic arterial circulation.

The anomalous vessel may arise from:

  • the descending thoracic aorta;
  • abdominal aorta;
  • coeliac region;
  • or other systemic branches. (PubMed Central (PMC))

This creates additional haemodynamic complexity.

Systemic arterial pressure is much higher than normal pulmonary arterial pressure.

A substantial systemic collateral can therefore contribute to pulmonary over-circulation and heart failure.

Two Broad Clinical Patterns

Scimitar Syndrome has historically been described as having two broad clinical presentations.

Infantile Form

The infantile presentation tends to be more severe.

The baby may develop:

  • tachypnoea;
  • respiratory distress;
  • heart failure;
  • pulmonary hypertension;
  • recurrent respiratory problems;
  • and significant haemodynamic compromise.

Associated abnormalities are often more extensive in this group. (PubMed)

Childhood or Adult Form

Older children and adults may be much less symptomatic.

Presentation may include:

  • exercise intolerance;
  • recurrent right-sided chest infections;
  • breathlessness;
  • incidental abnormal chest radiograph;
  • or discovery during cardiac imaging.

Some patients remain essentially asymptomatic. (PubMed Central (PMC))

Pulmonary Hypertension in Scimitar Syndrome

Pulmonary hypertension can arise through several mechanisms.

These include:

  • increased pulmonary blood flow from the left-to-right shunt;
  • anomalous systemic arterial supply to the lung;
  • reduced pulmonary vascular bed because of lung hypoplasia;
  • pulmonary venous obstruction;
  • and associated congenital heart lesions.

Infants with severe Scimitar Syndrome can therefore develop significant pulmonary hypertension much earlier than would normally be expected in a simple PAPVR.

Pulmonary Venous Obstruction

The anomalous scimitar vein itself can become narrowed or obstructed.

This increases pulmonary venous pressure in the affected lung.

Pulmonary venous obstruction may worsen:

  • pulmonary hypertension;
  • respiratory distress;
  • pulmonary congestion;
  • and clinical instability.

Rarely, progressive stenosis can lead to complete atresia of the anomalous pulmonary venous pathway. (PubMed Central (PMC))

Diagnosis of Scimitar Syndrome

Diagnosis may involve several imaging modalities.

Chest X-ray

The classic finding is the curved scimitar shadow running along the right cardiac border towards the diaphragm.

Other findings may include:

  • small right lung;
  • rightward displacement of the heart;
  • and abnormal right-sided pulmonary vascular markings.

Echocardiography

Echo assesses:

  • cardiac anatomy;
  • associated ASD;
  • right heart size;
  • pulmonary pressures;
  • and anomalous pulmonary venous flow where visible.

CT Angiography

CT can demonstrate the entire vascular anatomy particularly clearly.

It can identify:

  • the scimitar vein;
  • its connection to the IVC;
  • right lung hypoplasia;
  • right pulmonary artery hypoplasia;
  • systemic arterial collaterals;
  • and associated thoracic abnormalities. (PubMed Central (PMC))

MRI

MRI provides similar anatomical information while also allowing quantitative assessment of shunt flow and ventricular function.

Treatment of Scimitar Syndrome

Management depends upon severity.

An asymptomatic patient with a small shunt and no significant pulmonary hypertension may require only surveillance.

Treatment becomes more likely when there is:

  • significant left-to-right shunting;
  • symptoms;
  • pulmonary hypertension;
  • right heart enlargement;
  • recurrent respiratory problems;
  • large systemic arterial collaterals;
  • or obstruction of the anomalous venous pathway. (PubMed Central (PMC))

Embolisation of Systemic Collaterals

If a significant systemic artery is supplying the right lung, catheter-based embolisation may be used.

The abnormal systemic artery can be occluded using:

  • coils;
  • vascular plugs;
  • or other embolisation devices.

This reduces excessive high-pressure systemic blood flow into the lung.

For some infants, this can significantly improve heart failure and pulmonary hypertension.

Surgical Redirection of the Scimitar Vein

If the anomalous venous drainage itself requires correction, the objective is to redirect right pulmonary venous blood into the left atrium.

Techniques include:

  • intracardiac baffling;
  • reimplantation of the anomalous vein;
  • or other forms of venous reconstruction.

The precise operation depends heavily upon the individual anatomy.

Theatre and Anaesthetic Considerations

PAPVR alone may produce relatively straightforward left-to-right shunt physiology.

Scimitar Syndrome can be substantially more complex.

Important considerations include:

  • magnitude of the left-to-right shunt;
  • right ventricular function;
  • pulmonary artery pressure;
  • pulmonary vascular resistance;
  • associated ASD;
  • systemic arterial collaterals;
  • right lung hypoplasia;
  • pulmonary venous obstruction;
  • and the planned surgical reconstruction.

In severe infantile Scimitar Syndrome, pulmonary hypertension and respiratory compromise may dominate the clinical picture.

After Repair

Following PAPVR or Scimitar repair, particular attention is paid to:

  • pulmonary venous flow;
  • SVC flow where reconstructed;
  • right ventricular size and function;
  • pulmonary artery pressure;
  • residual shunts;
  • and rhythm.

Pulmonary venous stenosis is an important postoperative concern.

Long-Term Follow-Up

Patients with repaired PAPVR generally require congenital cardiac follow-up.

Important long-term issues include:

  • residual or recurrent pulmonary venous obstruction;
  • right ventricular enlargement;
  • arrhythmias;
  • pulmonary hypertension;
  • and residual shunting.

Scimitar Syndrome additionally requires consideration of the affected lung, systemic collateral circulation and pulmonary vascular development.

PAPVR and Scimitar Syndrome in One Sentence

PAPVR is a congenital abnormality in which some pulmonary veins return oxygenated blood to the systemic venous circulation rather than the left atrium, creating a left-to-right shunt; Scimitar Syndrome is a specialised right-sided PAPVR associated with abnormal pulmonary venous drainage, right lung hypoplasia and other pulmonary vascular abnormalities.

The key questions are:

How much pulmonary venous blood is returning abnormally?

Where is it draining?

And what effect is that having on the right heart and pulmonary circulation?

Key Points

PAPVR is usually a Pink Baby lesion.

At least one pulmonary vein drains abnormally while other pulmonary veins retain a normal connection to the left atrium.

The abnormal pulmonary venous blood is already oxygenated.

PAPVR therefore usually produces a left-to-right shunt rather than cyanosis. (PubMed Central (PMC))

PAPVR is commonly associated with sinus venosus ASD. (PubMed Central (PMC))

Large shunts may cause:

  • right atrial enlargement;
  • right ventricular enlargement;
  • pulmonary over-circulation;
  • and pulmonary hypertension.

Surgical repair redirects anomalous pulmonary venous blood towards the left atrium.

Scimitar Syndrome is a specialised form of PAPVR involving the right lung.

Typical features include:

  • anomalous right pulmonary venous drainage to the IVC;
  • right lung hypoplasia;
  • right pulmonary artery hypoplasia;
  • rightward displacement of the heart;
  • and anomalous systemic arterial supply to the right lung. (PubMed Central (PMC))

The classic scimitar sign is produced by the curved anomalous pulmonary vein seen on chest radiography.

Glossary of Terms

Anomalous Pulmonary Venous Connection

An abnormal connection in which one or more pulmonary veins drain somewhere other than the left atrium.

Atrial Septal Defect — ASD

An abnormal communication between the right and left atria.

Sinus venosus ASD is particularly associated with PAPVR.

Baffle

A surgically constructed internal pathway used to redirect blood.

In PAPVR repair, a baffle can redirect anomalous pulmonary venous blood towards the left atrium.

Cyanosis

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

Cyanosis is not typical of uncomplicated PAPVR.

Dextrocardia

A congenital condition in which the heart itself is positioned predominantly on the right side of the chest, usually with altered cardiac orientation.

This differs from dextroposition.

Dextroposition

Rightward displacement of an otherwise normally oriented heart.

In Scimitar Syndrome this commonly occurs because the right lung is hypoplastic.

Embolisation

A catheter-based procedure in which a blood vessel is intentionally occluded using devices such as coils or vascular plugs.

It can be used to close abnormal systemic arterial collaterals in Scimitar Syndrome.

Inferior Vena Cava — IVC

The major vein returning blood from the lower body to the right atrium.

The scimitar vein most commonly drains into the IVC.

Left-to-Right Shunt

Abnormal movement of blood from the systemic side of the circulation back towards the pulmonary circulation.

PAPVR produces a left-to-right shunt because oxygenated pulmonary venous blood returns to the right side of the heart.

PAPVR

Partial Anomalous Pulmonary Venous Return.

A congenital abnormality in which some, but not all, pulmonary veins fail to connect normally with the left atrium.

PAPVC

Partial Anomalous Pulmonary Venous Connection.

An alternative anatomical term for PAPVR.

Pulmonary Hypertension

Abnormally elevated pressure within the pulmonary arterial circulation.

Large longstanding PAPVR shunts or severe Scimitar Syndrome can contribute to pulmonary hypertension.

Pulmonary Over-Circulation

Excessive blood flow through the lungs.

It can occur when a significant quantity of pulmonary venous blood is returned to the right side of the heart.

Pulmonary Vein

A vessel carrying oxygenated blood from the lungs to the left atrium.

Pulmonary Venous Obstruction

Narrowing or obstruction of pulmonary venous drainage.

It can complicate Scimitar Syndrome and may also occur following surgical repair.

Qp

Pulmonary blood flow.

Qs

Systemic blood flow.

Qp:Qs Ratio

The ratio of pulmonary to systemic blood flow.

A ratio greater than 1:1 indicates increased pulmonary blood flow caused by a left-to-right shunt.

Right Heart Volume Overload

An increase in the amount of blood handled by the right atrium and right ventricle.

This is a major physiological consequence of significant PAPVR.

Scimitar Sign

The characteristic curved vascular shadow seen along the right cardiac border on chest radiography, produced by the anomalous pulmonary vein in Scimitar Syndrome.

Scimitar Syndrome

A congenital cardiopulmonary syndrome characterised principally by anomalous right pulmonary venous drainage towards the IVC, commonly associated with right lung hypoplasia, right pulmonary artery abnormalities, dextroposition and anomalous systemic arterial supply.

Scimitar Vein

The anomalous curved pulmonary venous channel draining the right lung, usually into the IVC.

Sinus Venosus ASD

An atrial septal defect located near the entry of the SVC or IVC into the right atrium.

The superior form is strongly associated with PAPVR of the right pulmonary veins.

Superior Vena Cava — SVC

The major vein returning systemic venous blood from the upper body to the right atrium.

Right upper pulmonary veins may drain anomalously into the SVC in PAPVR.

Systemic Arterial Collateral

An abnormal artery arising from the systemic circulation and supplying lung tissue.

Such vessels commonly occur in Scimitar Syndrome.

TAPVR

Total Anomalous Pulmonary Venous Return.

A congenital defect in which all pulmonary veins drain abnormally and none connect normally with the left atrium.

This distinguishes TAPVR from PAPVR.

Warden Procedure

A surgical technique used for selected cases of superior sinus venosus ASD with PAPVR.

It separates systemic venous return from anomalous pulmonary venous return and redirects the pulmonary veins towards the left atrium.