When Oxygenated Blood Returns to the Wrong Side of the Heart
Introduction
Congenital cardiac defects can often be understood by asking three questions:
Where is the blood returning from?
Where is it going?
And where can oxygenated and deoxygenated blood mix?
Total Anomalous Pulmonary Venous Return — TAPVR — is a classic “blue baby” lesion because oxygenated blood returning from the lungs fails to enter the left atrium normally.
Instead, all of the pulmonary venous blood returns to the right side of the circulation.
Survival therefore depends upon an atrial communication that allows some of this mixed blood to cross to the left side of the heart and reach the systemic circulation. TAPVR is consequently a cyanotic congenital heart defect in which the problem is not failure to oxygenate blood in the lungs, but failure to deliver that oxygenated blood to the left atrium normally. (Merck Manuals)
Body
What is TAPVR?
Normally, oxygenated blood leaves the lungs through the pulmonary veins.
The normal pathway is:
Lungs → Pulmonary veins → Left atrium → Left ventricle → Aorta → Body
In Total Anomalous Pulmonary Venous Return, none of the pulmonary veins connect normally to the left atrium.
Instead, all pulmonary venous blood drains either directly or indirectly into the systemic venous circulation or right atrium. (Merck Manuals)
The oxygenated pulmonary venous blood therefore mixes with deoxygenated systemic venous blood on the right side of the heart.
Some of that mixed blood must then cross the atrial septum to reach the left atrium and left ventricle.
Without an adequate atrial communication, effective systemic circulation cannot be maintained.
Normal Pulmonary Venous Return
The lungs normally return oxygenated blood to the left atrium through the pulmonary veins.
Commonly there are four principal pulmonary veins:
- right upper pulmonary vein;
- right lower pulmonary vein;
- left upper pulmonary vein;
- left lower pulmonary vein.
These enter the posterior aspect of the left atrium.
This arrangement ensures that oxygenated pulmonary venous blood passes directly into the systemic side of the heart.
In TAPVR, that direct connection has failed to develop.
How Does TAPVR Develop?
During embryological development, a common pulmonary vein develops from the left atrium and establishes communication with the pulmonary venous plexus.
As development progresses, this common pulmonary venous connection becomes incorporated into the posterior wall of the left atrium.
Earlier embryonic connections between the pulmonary venous system and systemic veins normally regress.
If the normal pulmonary venous connection to the left atrium fails to develop or becomes interrupted, one or more of those primitive systemic venous pathways may persist.
The pulmonary veins then drain through an abnormal route into the systemic venous circulation. (NCBI)
The exact persistent connection determines the anatomical type of TAPVR.
Why is the Baby Blue?
The lungs may oxygenate blood perfectly normally.
The problem occurs after the blood leaves the lungs.
Instead of entering the left atrium, oxygenated pulmonary venous blood is returned to the right side of the circulation.
It mixes with deoxygenated systemic venous blood.
The right atrium therefore receives:
deoxygenated blood from the body
plus
oxygenated blood from the lungs.
This mixed blood then enters the right ventricle.
Most is pumped back towards the lungs.
For the child to survive, some must escape through an ASD or Patent Foramen Ovale into the left atrium.
Thus:
Right atrium → ASD/PFO → Left atrium → Left ventricle → Aorta
The systemic circulation therefore receives mixed blood rather than fully oxygenated pulmonary venous blood.
The child becomes cyanosed. (Merck Manuals)
The ASD is Essential
An atrial communication is not merely an associated defect in TAPVR.
It is essential to survival.
Because no pulmonary veins enter the left atrium normally, the left side of the heart depends upon blood crossing from the right atrium through:
- an ASD;
- a stretched Patent Foramen Ovale;
- or another atrial communication.
If this opening is restrictive, systemic output can become severely compromised.
This is another good example of why a “hole in the heart” is not always simply a problem to close.
In TAPVR, that hole is initially keeping the baby alive.
The Four Main Types of TAPVR
TAPVR is classified according to where the pulmonary veins ultimately drain.
There are four main types:
Supracardiac
Cardiac
Infracardiac
Mixed. (NCBI)
Supracardiac TAPVR
In supracardiac TAPVR, the pulmonary veins usually join behind the heart to form a pulmonary venous confluence.
Blood then travels upwards through an abnormal vessel known as the vertical vein.
A common route is:
Pulmonary veins → Confluence → Vertical vein → Innominate vein → SVC → Right atrium
The pulmonary venous blood therefore returns to the right atrium through the superior systemic veins.
Supracardiac drainage is one of the most common forms of TAPVR. (Merck Manuals)
The “Snowman” or “Figure-of-Eight” Sign
In an older child with unrepaired supracardiac TAPVR, enlargement of the vertical vein, innominate vein and SVC can create a characteristic radiological appearance.
The enlarged upper venous channels form the upper part of the image, while the cardiac silhouette forms the lower part.
This has historically been described as a:
“snowman”
or
“figure-of-eight” appearance.
It is a classic teaching image, although it generally develops over time and is not a typical neonatal finding.
Cardiac TAPVR
In cardiac TAPVR, pulmonary venous blood drains directly into the right side of the heart.
A common arrangement is:
Pulmonary veins → Pulmonary venous confluence → Coronary sinus → Right atrium
The coronary sinus can therefore become markedly enlarged.
Another less common cardiac pattern involves direct drainage into the right atrium.
Again, pulmonary venous blood mixes with systemic venous blood before some crosses the atrial septum to reach the systemic circulation. (Merck Manuals)
Infracardiac TAPVR
In infracardiac TAPVR, pulmonary venous blood drains downwards below the diaphragm.
The route may involve:
Pulmonary veins → Confluence → Descending vertical vein → Portal venous system / hepatic veins / IVC → Right atrium
The abnormal pulmonary venous pathway therefore travels below the diaphragm before eventually reaching the right heart.
This form is particularly important because pulmonary venous obstruction is very common and often severe.
Merck describes infradiaphragmatic drainage as typically severely obstructed, with affected neonates developing dramatic pulmonary oedema and cyanosis. (Merck Manuals)
Mixed TAPVR
In mixed TAPVR, the pulmonary veins do not all follow the same abnormal route.
Different pulmonary veins drain into different systemic venous pathways.
For example, some may drain supracardiac while others drain directly into the heart or below the diaphragm.
Mixed TAPVR can therefore be anatomically complex.
Detailed imaging is particularly important before surgery because the surgeon must identify and incorporate all pulmonary venous drainage pathways into the repair.
Obstructed Versus Unobstructed TAPVR
The single most important physiological distinction in TAPVR is often not the anatomical category itself.
It is:
Is the pulmonary venous pathway obstructed?
This dramatically changes the presentation.
Unobstructed TAPVR
If pulmonary venous blood can return freely to the right atrium, pulmonary blood flow may become very large.
Blood travels:
Lungs → Abnormal pulmonary venous pathway → Right heart → Pulmonary artery → Lungs
A proportion crosses the atrial septum to supply the body.
The right atrium and right ventricle therefore handle a large volume of blood.
The infant may have:
- mild cyanosis;
- tachypnoea;
- poor feeding;
- failure to thrive;
- signs of right-sided volume overload;
- and heart failure.
Cyanosis can be surprisingly mild because pulmonary blood flow may be very high and the mixed venous blood can contain a substantial quantity of oxygenated pulmonary venous return. (Merck Manuals)
Obstructed TAPVR
Obstructed TAPVR is very different.
If the anomalous pulmonary venous pathway becomes narrowed, blood cannot escape adequately from the lungs.
Pulmonary venous pressure rises.
This causes:
- pulmonary congestion;
- pulmonary oedema;
- increased pulmonary arterial pressure;
- reduced pulmonary blood flow;
- severe hypoxaemia;
- and cardiovascular instability.
The newborn may become profoundly cyanosed and critically unwell very rapidly. (Merck Manuals)
Obstructed TAPVR is therefore a neonatal surgical emergency.
Why Oxygen Alone May Not Work
A baby with obstructed TAPVR can remain severely cyanosed despite receiving high concentrations of oxygen.
This is because the main problem is not simply inadequate oxygen entering the lungs.
The problem is that oxygenated blood cannot leave the lungs effectively.
Pulmonary venous obstruction raises pressure within the pulmonary circulation and causes pulmonary oedema.
Giving more oxygen cannot remove the mechanical obstruction.
This is an important physiological clue.
Pulmonary Hypertension
Pulmonary venous obstruction causes pressure to build backwards through the pulmonary circulation.
Pulmonary arterial pressure may consequently become very high.
The right ventricle must pump against this increased resistance.
Severe pulmonary hypertension can therefore develop extremely quickly in obstructed TAPVR.
This creates a dangerous cycle:
Pulmonary venous obstruction → Pulmonary oedema → Increasing PVR → Reduced pulmonary flow → Worsening hypoxaemia
Urgent surgical correction is required to break that cycle.
Presentation
Unobstructed TAPVR
Presentation may include:
- mild cyanosis;
- tachypnoea;
- difficulty feeding;
- poor weight gain;
- recurrent respiratory symptoms;
- and signs of cardiac failure.
Some infants with unobstructed supracardiac or cardiac TAPVR may initially have relatively few symptoms. (Merck Manuals)
Obstructed TAPVR
The presentation is much more dramatic:
- profound cyanosis;
- severe respiratory distress;
- pulmonary oedema;
- tachypnoea;
- poor systemic perfusion;
- metabolic acidosis;
- pulmonary hypertension;
- and cardiovascular collapse.
The presentation can resemble severe neonatal lung disease or sepsis.
Diagnosis
Echocardiography
Echocardiography is the principal diagnostic investigation.
It can demonstrate:
- absence of normal pulmonary venous entry into the left atrium;
- the pulmonary venous confluence;
- the abnormal drainage route;
- the atrial communication;
- right atrial and ventricular enlargement;
- pulmonary hypertension;
- and evidence of venous obstruction. (Merck Manuals)
CT and MRI
CT angiography or cardiac MRI may be useful where the venous anatomy is complex or incompletely defined by echocardiography.
These investigations can be particularly valuable in:
- mixed TAPVR;
- unusual venous connections;
- recurrent pulmonary venous obstruction;
- and planning repeat intervention.
In a critically unstable neonate with clearly obstructed TAPVR, however, treatment should not be unnecessarily delayed simply to obtain elaborate imaging.
Cardiac Catheterisation
Cardiac catheterisation was historically important in defining TAPVR anatomy.
Modern echocardiography, CT and MRI mean diagnostic catheterisation is required much less frequently.
Catheter techniques can nevertheless play a role in selected complex or postoperative pulmonary venous stenosis cases.
Treatment
There is no definitive medical treatment for TAPVR.
Surgical repair is required. (Merck Manuals)
The urgency depends heavily upon whether the pulmonary venous pathway is obstructed.
Obstructed TAPVR requires emergency repair.
Unobstructed TAPVR is still surgically corrected, but there may be time to stabilise the infant before surgery.
Surgical Repair
The fundamental objective is beautifully simple:
Connect the pulmonary veins to the left atrium where they should have been connected in the first place.
In many forms of TAPVR, the pulmonary veins join behind the heart to form a venous confluence.
The surgeon creates a wide connection between this confluence and the posterior wall of the left atrium.
This is an anastomosis.
Thus:
Pulmonary veins → Confluence → Left atrium
The abnormal pathway into the systemic venous circulation is then usually ligated or divided so that pulmonary venous blood no longer returns to the right side. (Merck Manuals)
What Happens to the ASD?
Before surgery, the ASD is essential.
After normal pulmonary venous return has been established, its role changes.
The atrial communication can normally be closed or reduced as part of the repair because oxygenated pulmonary venous blood can now enter the left atrium directly.
This illustrates an important principle in congenital surgery:
A structure that is essential for survival before repair may become unnecessary once normal physiology has been restored.
Cardiac TAPVR and the Coronary Sinus
Cardiac TAPVR draining through the coronary sinus requires a slightly different repair.
The coronary sinus can be unroofed into the left atrium, allowing pulmonary venous blood to enter the left atrium.
The connection between the coronary sinus pathway and right atrium is then closed appropriately. (Merck Manuals)
The exact operation therefore depends upon the anatomical drainage pattern.
Cardiopulmonary Bypass
TAPVR repair requires cardiopulmonary bypass.
The surgeon must work on the posterior aspect of the heart and establish a wide, unobstructed pulmonary venous connection with the left atrium.
The important word is:
wide.
A narrow repair risks creating postoperative pulmonary venous obstruction.
Anaesthetic and Theatre Considerations
The physiology of TAPVR differs considerably depending upon whether obstruction is present.
In unobstructed TAPVR, the patient may have significant pulmonary over-circulation and right-sided volume loading.
In obstructed TAPVR, the major problem is pulmonary venous hypertension, pulmonary oedema and severe pulmonary hypertension.
Important considerations include:
- oxygenation;
- ventilation;
- pulmonary vascular resistance;
- acid-base balance;
- systemic perfusion;
- lactate;
- right ventricular function;
- pulmonary artery pressure;
- and the severity and location of pulmonary venous obstruction.
In severe obstruction, aggressive positive-pressure ventilation may be required for gas exchange, but high intrathoracic pressures can also have adverse haemodynamic effects.
These neonates can be extremely unstable.
After Surgical Repair
Following repair, particular attention is paid to:
- pulmonary venous flow;
- pulmonary artery pressure;
- left atrial pressure;
- right ventricular function;
- systemic cardiac output;
- and residual atrial shunting.
Pulmonary hypertension can remain significant immediately after surgery because the pulmonary vascular bed has been exposed to abnormal pressures before repair.
Pulmonary Hypertensive Crisis
One of the important postoperative concerns is a pulmonary hypertensive crisis.
A sudden rise in pulmonary vascular resistance can dramatically increase pulmonary artery pressure and right ventricular workload.
Potential triggers include:
- hypoxia;
- hypercarbia;
- acidosis;
- pain;
- agitation;
- hypothermia;
- and airway stimulation.
Careful postoperative intensive-care management is therefore essential.
Postoperative Pulmonary Vein Stenosis
One of the most important long-term complications is pulmonary vein stenosis.
Narrowing may develop at or near the surgical connection between the pulmonary venous confluence and left atrium.
In some patients the pulmonary veins themselves can develop progressive stenosis.
This can be a difficult complication because renewed obstruction once again raises pulmonary venous and pulmonary arterial pressures.
Patients therefore require continued congenital cardiac follow-up after repair.
TAPVR Versus PAPVR
TAPVR should be distinguished from Partial Anomalous Pulmonary Venous Return — PAPVR.
In TAPVR:
All pulmonary veins drain abnormally.
In PAPVR:
Only some pulmonary veins drain abnormally, while at least one retains a normal connection to the left atrium. (NCBI)
PAPVR therefore usually produces different physiology and is commonly an acyanotic left-to-right shunt rather than a classic neonatal cyanotic lesion.
Scimitar Syndrome
Scimitar syndrome is a particular form of partial anomalous pulmonary venous return.
It typically involves anomalous drainage of right-sided pulmonary veins towards the inferior vena cava and may be associated with:
- right lung hypoplasia;
- displacement of the heart towards the right;
- and abnormal systemic arterial supply to part of the right lung.
The name comes from the curved anomalous vein seen radiographically, which resembles a scimitar, a curved Middle Eastern sword. (NCBI)
Scimitar syndrome is therefore related to abnormal pulmonary venous development but is not TAPVR.
TAPVR in One Sentence
Total Anomalous Pulmonary Venous Return is a congenital abnormality in which all pulmonary veins drain into the systemic venous circulation or right atrium rather than the left atrium, making survival dependent upon atrial-level mixing.
The two most important questions are:
Where do the pulmonary veins drain?
and
Is that pathway obstructed?
The answer to the second question often determines whether the infant is relatively stable or critically ill.
Key Points
TAPVR is a cyanotic congenital heart defect.
All pulmonary veins fail to connect normally with the left atrium. (Merck Manuals)
Pulmonary venous blood instead returns to the right side of the circulation.
An ASD or PFO is essential for systemic blood flow.
There are four main anatomical forms:
Supracardiac
Cardiac
Infracardiac
Mixed
The most important physiological distinction is:
obstructed versus unobstructed pulmonary venous return.
Obstructed TAPVR can cause profound neonatal cyanosis, pulmonary oedema and pulmonary hypertension and requires emergency surgical repair. (Merck Manuals)
Surgery creates a wide connection between the pulmonary venous confluence and the left atrium.
Pulmonary vein stenosis is an important postoperative complication.
Glossary of Terms
Anastomosis
A surgically created connection between two structures.
In TAPVR repair, an anastomosis is commonly created between the pulmonary venous confluence and left atrium.
Atrial Septal Defect — ASD
An abnormal communication between the right and left atria.
An atrial communication is essential for survival in TAPVR because it allows mixed blood to reach the left ventricle and systemic circulation.
Cardiac TAPVR
A form of TAPVR in which pulmonary venous blood drains directly into the right heart, commonly through the coronary sinus.
Coronary Sinus
A large venous channel that normally returns blood from the myocardium to the right atrium.
In cardiac TAPVR it may receive the pulmonary venous confluence.
Cyanosis
Bluish discoloration caused by increased deoxygenated haemoglobin in systemic arterial blood.
Differential Diagnosis
Alternative conditions that can produce a similar clinical presentation and must be considered when determining the diagnosis.
Infracardiac TAPVR
A form of TAPVR in which pulmonary venous drainage passes below the diaphragm before reaching the systemic venous circulation.
It is particularly associated with pulmonary venous obstruction.
Mixed TAPVR
A form in which different pulmonary veins drain through more than one anomalous pathway.
Patent Foramen Ovale — PFO
Persistence of the foetal communication between the right and left atria.
In TAPVR it may provide the atrial-level pathway essential for systemic output.
Partial Anomalous Pulmonary Venous Return — PAPVR
A congenital abnormality in which some, but not all, pulmonary veins drain abnormally into the systemic venous circulation.
Pulmonary Hypertension
Abnormally elevated pressure within the pulmonary arterial circulation.
It can be particularly severe in obstructed TAPVR.
Pulmonary Vein Stenosis
Narrowing of one or more pulmonary veins or of the surgically reconstructed pulmonary venous pathway.
It is an important potential complication after TAPVR repair.
Pulmonary Venous Confluence
A chamber or common venous channel formed where the pulmonary veins join before draining through an anomalous pathway.
Pulmonary Venous Obstruction
Restriction to blood leaving the lungs through the pulmonary veins or anomalous pulmonary venous pathway.
Severe obstruction causes pulmonary oedema, pulmonary hypertension and profound hypoxaemia.
Scimitar Syndrome
A form of partial anomalous pulmonary venous drainage, usually involving right pulmonary veins draining towards the inferior vena cava and associated with characteristic right-sided pulmonary abnormalities.
Supracardiac TAPVR
A form in which pulmonary venous blood drains above the heart, commonly through a vertical vein into the innominate vein and SVC.
Systemic Venous Circulation
The venous system returning deoxygenated blood from the body to the right side of the heart.
TAPVC
Total Anomalous Pulmonary Venous Connection.
An alternative and often anatomically more precise term for TAPVR.
Total Anomalous Pulmonary Venous Return — TAPVR
A congenital cardiac abnormality in which all pulmonary veins drain abnormally into the systemic venous circulation or right atrium rather than the left atrium.
Vertical Vein
An anomalous venous channel frequently seen in supracardiac TAPVR, carrying pulmonary venous blood superiorly towards the innominate vein or SVC.
