Two Routes to the Blue Baby
Introduction
Congenital cardiac defects are often easiest to understand by asking:
Where is the obstruction?
How is blood reaching the lungs?
And where is the blood able to mix?
Pulmonary Atresia and Tricuspid Atresia are both classic causes of the “blue baby”, but they produce cyanosis for different anatomical reasons.
In Pulmonary Atresia, blood cannot leave the right ventricle normally and enter the pulmonary artery.
In Tricuspid Atresia, blood cannot enter the right ventricle normally from the right atrium.
Both therefore depend upon alternative pathways to maintain pulmonary blood flow and systemic oxygenation.
Body
Pulmonary Atresia
What is Pulmonary Atresia?
Pulmonary Atresia — PA — is a congenital abnormality in which there is no patent opening between the right ventricle and the pulmonary arterial circulation.
In the common form described as Pulmonary Atresia with Intact Ventricular Septum — PA/IVS, the pulmonary valve fails to form as a functional opening.
Blood therefore cannot travel normally:
Right ventricle → Pulmonary valve → Pulmonary artery → Lungs
Instead, another route to the lungs is required. The right ventricle and tricuspid valve may also be small or underdeveloped. (www.heart.org)
Pulmonary atresia can also occur with a Ventricular Septal Defect — VSD. This represents a different anatomical group and is closely related to the severe end of the Tetralogy of Fallot spectrum. (NCBI)
Why is the Baby Blue?
The problem is fundamentally one of reduced pulmonary blood flow.
Systemic venous blood returns normally to the right atrium.
It may pass through the tricuspid valve into the right ventricle.
But if the pulmonary outlet is completely atretic, it cannot travel forwards into the pulmonary artery.
For the baby to survive, blood must therefore find another route to the lungs.
In the newborn this is commonly through the Patent Ductus Arteriosus — PDA.
Thus:
Aorta → PDA → Pulmonary artery → Lungs
The ductus therefore becomes an essential route for pulmonary blood flow. (www.heart.org)
As the ductus begins to close after birth, pulmonary blood flow can fall dramatically and cyanosis can rapidly worsen.
Atrial-Level Mixing
In PA/IVS, venous blood returning to the right atrium also needs somewhere to go.
Because the route through the right ventricle is ineffective, blood crosses an atrial communication such as a:
- Patent Foramen Ovale;
- or Atrial Septal Defect.
It enters the left atrium and mixes with oxygenated pulmonary venous blood.
The left ventricle then pumps this mixed blood into the systemic circulation.
This explains the cyanosis:
deoxygenated venous blood is entering the systemic circulation because it cannot follow its normal route through the right ventricle and lungs.
The Right Ventricle
The anatomy of the right ventricle is critical in PA/IVS.
In some babies, the right ventricle is reasonably well developed.
In others it is markedly hypoplastic.
This difference has major consequences for treatment.
If the right ventricle is sufficiently developed, attempts may be made to establish forward blood flow from the right ventricle into the pulmonary artery and ultimately preserve a two-ventricle circulation.
If the right ventricle is too small to provide useful pulmonary output, the child may require a single-ventricle pathway, involving Glenn and Fontan palliation. (www.heart.org)
Coronary Sinusoids
One particularly important feature of PA/IVS is the possible presence of abnormal communications between the right ventricle and the coronary circulation.
These are often called coronary sinusoids.
Because the right ventricle may be exposed to very high pressure when its outlet is completely obstructed, abnormal communications can develop between the ventricular cavity and the coronary arteries.
In severe cases, parts of the coronary circulation may become dependent upon pressure within the right ventricle.
This is termed right-ventricle-dependent coronary circulation.
This is critically important because simply decompressing the right ventricle could then compromise coronary perfusion.
The coronary anatomy therefore plays a major role in determining whether a two-ventricle repair is safe. (www.heart.org)
Initial Management
A newborn with pulmonary atresia and inadequate pulmonary blood flow requires urgent treatment.
The immediate priority is usually:
Keep the ductus arteriosus open.
An intravenous Prostaglandin E1 — PGE1 infusion is commonly started to maintain ductal patency and preserve pulmonary blood flow. (www.heart.org)
The child may also require:
- respiratory support;
- correction of acidosis;
- management of systemic perfusion;
- treatment of hypoglycaemia;
- and urgent congenital cardiac assessment.
Establishing Pulmonary Blood Flow
There are several possible approaches.
Ductal Stenting
A catheter-delivered stent can be placed within the ductus arteriosus to maintain blood flow from the aorta to the pulmonary arteries.
Systemic-to-Pulmonary Shunt
A surgical shunt can be created between the systemic and pulmonary arterial circulation.
A modified Blalock–Thomas–Taussig shunt is one example.
Opening the Pulmonary Valve or Outflow
If the right ventricle is suitable, catheter or surgical intervention may be used to establish a direct route from the right ventricle into the pulmonary artery.
This can include:
- perforation of the atretic pulmonary valve;
- balloon dilation;
- or surgical reconstruction.
The aim is to encourage growth and function of the right ventricle where possible. (www.heart.org)
Pulmonary Atresia With VSD
Pulmonary Atresia with VSD is physiologically and anatomically different from PA/IVS.
Here there is no effective route from the right ventricle to the pulmonary artery, but a large VSD allows ventricular mixing.
Pulmonary blood flow may arise from:
- a PDA;
- native pulmonary arteries;
- or Major Aortopulmonary Collateral Arteries — MAPCAs.
MAPCAs are systemic arteries that supply portions of the pulmonary vascular bed directly from the aorta or its branches. (NCBI)
These patients can have extremely variable pulmonary artery anatomy.
Some have well-developed central pulmonary arteries.
Others may have small or discontinuous pulmonary arteries supplied largely through multiple collateral vessels.
Unifocalisation
Where MAPCAs provide substantial portions of pulmonary blood flow, surgical treatment may involve unifocalisation.
The concept is to gather separate collateral sources of pulmonary blood flow and reconstruct them into a more unified pulmonary arterial circulation.
Where possible, the reconstructed pulmonary arteries are then connected to the right ventricle using a conduit or other outflow reconstruction.
This is highly specialised surgery and varies greatly according to individual anatomy.
Tricuspid Atresia
What is Tricuspid Atresia?
Tricuspid Atresia — TA — is a congenital abnormality in which the tricuspid valve fails to form, leaving no direct connection between the right atrium and right ventricle.
Normally:
Right atrium → Tricuspid valve → Right ventricle
In Tricuspid Atresia, that pathway does not exist.
The right ventricle therefore receives little or no normal inflow and is usually underdeveloped. (www.heart.org)
Tricuspid Atresia is consequently a form of functionally single-ventricle congenital heart disease.
Where Does the Blood Go?
Systemic venous blood still returns normally to the right atrium.
But it cannot pass through the tricuspid valve.
It must cross the atrial septum.
Thus:
Right atrium → ASD/PFO → Left atrium
The systemic venous blood mixes with oxygenated blood returning from the lungs.
The mixed blood then enters the left ventricle.
The left ventricle therefore becomes the principal pumping chamber.
How Does Blood Reach the Lungs?
That depends upon the associated anatomy.
A VSD often allows some blood to pass:
Left ventricle → VSD → Small right ventricle → Pulmonary artery
If pulmonary blood flow through this route is inadequate, the child becomes cyanosed.
If the pulmonary outflow is severely obstructed or atretic, pulmonary blood flow may depend upon the PDA.
Conversely, some anatomical forms can produce excessive pulmonary blood flow rather than too little. (www.heart.org)
This makes Tricuspid Atresia another useful reminder that the name of a congenital defect alone does not describe the whole physiology.
Why is the Right Ventricle Small?
The developing right ventricle depends upon normal blood flow entering through the tricuspid valve.
If the tricuspid valve does not form, that inflow is absent.
The right ventricle therefore fails to develop normally and remains hypoplastic.
This is fundamentally different from pulmonary atresia, where the right ventricle may receive blood normally through the tricuspid valve but cannot eject it through the pulmonary outflow.
This difference can be summarised simply:
Pulmonary Atresia: blood gets into the right ventricle but cannot get out normally.
Tricuspid Atresia: blood cannot get into the right ventricle normally.
The Importance of the ASD
An atrial communication is essential in Tricuspid Atresia.
Without it, systemic venous blood arriving in the right atrium would have nowhere to go.
The atrial septal defect or patent foramen ovale therefore provides the route by which venous blood reaches the left side of the heart.
If the atrial communication is restrictive, intervention may be required to enlarge it.
The Importance of the VSD
The VSD often provides the route by which blood reaches the pulmonary circulation.
Mixed blood from the left ventricle can cross the VSD into the small right ventricle and pulmonary artery.
The amount of pulmonary blood flow therefore depends upon:
- VSD size;
- pulmonary valve and artery anatomy;
- pulmonary vascular resistance;
- and any associated pulmonary stenosis.
A small or restrictive VSD can reduce pulmonary flow.
A large VSD with little pulmonary obstruction can produce excessive pulmonary flow.
Initial Management of Tricuspid Atresia
Treatment depends upon how much blood is reaching the lungs.
If pulmonary blood flow is inadequate, the ductus may need to be kept open with Prostaglandin E1, followed by a ductal stent or systemic-to-pulmonary shunt where appropriate. (www.heart.org)
If pulmonary blood flow is excessive, the opposite problem exists.
The lungs can become over-circulated and pulmonary artery pressure may rise.
In selected cases, pulmonary artery banding may be used to restrict excessive pulmonary blood flow. (www.heart.org)
Again, treatment follows physiology rather than simply the diagnostic label.
The Single-Ventricle Pathway
Because the right ventricle in Tricuspid Atresia is generally too small to support a normal two-ventricle circulation, most patients proceed towards staged single-ventricle palliation.
This commonly includes:
Bidirectional Glenn
followed later by
Fontan completion. (www.heart.org)
The aim is ultimately to direct systemic venous blood directly into the pulmonary arteries while leaving the functional left ventricle to support the systemic circulation.
Glenn
During the Bidirectional Glenn, the Superior Vena Cava is connected directly to the pulmonary arteries.
Thus:
Upper-body venous blood → SVC → Pulmonary arteries → Lungs
This reduces the volume workload on the single functional ventricle.
Fontan
During Fontan completion, Inferior Vena Caval blood is also directed into the pulmonary arterial circulation.
The final pathway becomes:
Systemic veins → Pulmonary arteries → Lungs
followed by:
Pulmonary veins → Left atrium → Left ventricle → Body
There is no dedicated ventricle pumping blood through the lungs.
This is classic Fontan physiology.
Comparing Pulmonary and Tricuspid Atresia
Although the terms sound similar, the lesions are fundamentally different.
Pulmonary Atresia
The main obstruction is after the right ventricle.
Blood cannot travel normally:
Right ventricle → Pulmonary artery
Pulmonary blood flow therefore needs an alternative route.
The right ventricle may be usable, hypoplastic, or severely abnormal depending upon the anatomy.
Tricuspid Atresia
The main obstruction is before the right ventricle.
Blood cannot travel:
Right atrium → Right ventricle
The right ventricle therefore becomes hypoplastic.
Blood must first cross the atrial septum, and long-term treatment is usually based upon single-ventricle palliation.
Why are Both Babies Blue?
In both conditions, insufficient blood may reach the lungs to become oxygenated.
At the same time, deoxygenated systemic venous blood must mix with oxygenated pulmonary venous blood before entering the systemic circulation.
The systemic arterial blood therefore contains less oxygen than normal.
The result is cyanosis.
But the anatomical reason is different.
In Pulmonary Atresia:
the exit from the right ventricle is blocked.
In Tricuspid Atresia:
the entrance to the right ventricle is blocked.
Anaesthetic and Theatre Considerations
Both lesions require careful understanding of how pulmonary blood flow is being maintained.
A seemingly small physiological change can profoundly alter oxygenation.
Important factors include:
- ductal patency;
- pulmonary vascular resistance;
- systemic vascular resistance;
- preload;
- atrial-level mixing;
- haemoglobin concentration;
- acid-base balance;
- and ventricular function.
In duct-dependent pulmonary circulation, sudden ductal constriction can produce catastrophic loss of pulmonary blood flow.
Anything that markedly increases pulmonary vascular resistance can also worsen cyanosis.
This includes:
- hypoxia;
- hypercarbia;
- acidosis;
- hypothermia;
- and excessive airway pressure.
The anaesthetic goal is therefore not simply to increase oxygen concentration.
It is to preserve the pathways through which blood can actually reach the lungs.
Long-Term Follow-Up
Neither condition can be regarded as simply “fixed” after surgery.
Patients may have undergone:
- catheter interventions;
- shunts;
- RV outflow reconstruction;
- Glenn;
- Fontan;
- conduits;
- or combinations of procedures.
Potential long-term issues include:
- arrhythmias;
- ventricular dysfunction;
- residual or recurrent obstruction;
- cyanosis;
- thromboembolic complications;
- Fontan-related complications;
- and need for repeat intervention.
Lifelong follow-up with congenital cardiac specialists is therefore required for both pulmonary and tricuspid atresia. (www.heart.org)
Pulmonary and Tricuspid Atresia in One Sentence
Pulmonary Atresia prevents blood from leaving the right ventricle normally towards the lungs, while Tricuspid Atresia prevents blood from entering the right ventricle normally from the right atrium.
Both can cause profound cyanosis because normal pulmonary blood flow cannot be maintained without alternative pathways.
Key Points
Pulmonary Atresia is an obstruction of the right ventricular outflow.
Tricuspid Atresia is an obstruction of right ventricular inflow.
Both are cyanotic congenital heart defects.
Pulmonary Atresia may occur:
with an intact ventricular septum or with a VSD.
PA/IVS may be associated with a hypoplastic right ventricle and abnormal coronary sinusoids.
Pulmonary blood flow in the newborn may be duct-dependent.
Tricuspid Atresia requires an atrial communication for survival.
A VSD commonly provides part of the pathway towards the pulmonary circulation.
Tricuspid Atresia usually follows a single-ventricle pathway culminating in Glenn and Fontan surgery.
The anatomy — not simply the diagnostic label — determines treatment.
Glossary of Terms
Atrial Septal Defect — ASD
An abnormal communication between the right and left atria.
It provides an essential pathway for systemic venous blood in Tricuspid Atresia and PA/IVS.
Atresia
Congenital absence or complete closure of a normal opening, valve or vascular connection.
Bidirectional Glenn
A surgical connection between the Superior Vena Cava and pulmonary arteries.
It forms an important stage of single-ventricle palliation.
Blalock–Thomas–Taussig Shunt
A surgically created connection between systemic and pulmonary arteries used to increase pulmonary blood flow.
Coronary Sinusoids
Abnormal vascular communications between the right ventricle and coronary arterial circulation, particularly important in PA/IVS.
Cyanosis
Bluish discoloration caused by increased quantities of deoxygenated haemoglobin in systemic arterial blood.
Duct-Dependent Pulmonary Circulation
A circulation in which adequate pulmonary blood flow depends upon continued patency of the ductus arteriosus.
Fontan Procedure
A stage of single-ventricle palliation in which inferior as well as superior systemic venous blood is directed towards the pulmonary arteries without passing through a dedicated pumping ventricle.
Hypoplastic
Abnormally small or incompletely developed.
MAPCAs
Major Aortopulmonary Collateral Arteries.
Systemic arterial vessels supplying parts of the pulmonary circulation, particularly associated with Pulmonary Atresia with VSD.
Patent Ductus Arteriosus — PDA
Persistence of the foetal vascular connection between the pulmonary artery and aorta.
In pulmonary atresia it may provide the principal source of pulmonary blood flow.
Patent Foramen Ovale — PFO
Persistence of the foetal opening between the atria.
It can provide essential atrial-level mixing.
Prostaglandin E1 — PGE1
A medication used to maintain ductal patency in duct-dependent congenital heart disease.
Pulmonary Atresia — PA
Absence of an effective connection between the right ventricle and pulmonary arterial circulation.
PA/IVS
Pulmonary Atresia with Intact Ventricular Septum.
Pulmonary Artery Banding
A surgical technique that intentionally restricts pulmonary arterial flow when pulmonary blood flow is excessive.
Pulmonary Vascular Resistance — PVR
Resistance to blood flow through the pulmonary circulation.
Right-Ventricle-Dependent Coronary Circulation
A situation in which coronary perfusion depends partly upon high right ventricular pressure through abnormal ventricular-coronary connections.
Single-Ventricle Physiology
A circulation in which one functional ventricle ultimately supports the systemic circulation because a sustainable two-ventricle repair cannot be established.
Tricuspid Atresia — TA
Congenital absence of a functional tricuspid valve connection between the right atrium and right ventricle.
Unifocalisation
Surgical reconstruction in which multiple systemic-to-pulmonary collateral vessels are incorporated into a more unified pulmonary arterial circulation.
Ventricular Septal Defect — VSD
An abnormal communication between the right and left ventricles.
It is an important part of the physiology of many patients with Tricuspid Atresia and Pulmonary Atresia with VSD.
