The Classical “Blue Baby” Congenital Heart Defect
Introduction
Congenital cardiac defects can often be approached by asking three questions:
Where is the anatomical abnormality?
How does it alter the direction of blood flow?
Is sufficiently oxygenated blood reaching the systemic circulation?
This provides the basis for the traditional distinction between the “pink baby” and the “blue baby.”
Tetralogy of Fallot is one of the classic causes of the blue baby.
The characteristic cyanosis occurs because obstruction to blood leaving the right ventricle towards the lungs encourages deoxygenated blood to cross a ventricular septal defect and enter the aorta.
However, even this apparently straightforward classification has exceptions.
A child with relatively mild right ventricular outflow obstruction may initially remain pink, while severe obstruction may produce profound cyanosis.
Tetralogy of Fallot therefore provides an excellent demonstration of a fundamental principle of congenital cardiac physiology:
the anatomy creates the pathway, but pressure and resistance determine which way the blood actually flows.
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Who Was Fallot?
Tetralogy of Fallot is named after the French physician Étienne-Louis Arthur Fallot, who worked in Marseille.
Fallot published his detailed description of the condition in 1888, describing patients with what was then commonly called maladie bleue — “blue disease.” He recognised a recurring combination of abnormalities in patients with cyanosis and described their pathological anatomy in considerable detail. (STS Cardiothoracic Surgery)
Fallot was not actually the first person to observe the condition.
The Danish anatomist Niels Stensen — Nicolas Steno — had described a remarkably similar congenital cardiac abnormality more than two centuries earlier, in the seventeenth century. Other physicians subsequently reported comparable cases. (McGill University)
Fallot’s importance was that he recognised the abnormalities as a recurring anatomical and clinical entity and provided a detailed description linking the anatomy to cyanosis.
Interestingly, Fallot himself did not coin the name Tetralogy of Fallot.
The terminology became established later, and the name is particularly associated with the work of Canadian physician and congenital-heart pioneer Maude Abbott, who popularised the designation during the early twentieth century. (Wiley Online Library)
What Does “Tetralogy” Mean?
The word tetralogy simply means a group of four.
Classically, Tetralogy of Fallot consists of:
1. Ventricular Septal Defect — VSD
2. Right Ventricular Outflow Tract Obstruction — RVOTO, often described clinically as pulmonary stenosis
3. Overriding Aorta
4. Right Ventricular Hypertrophy — RVH
These remain the accepted four classical features of Tetralogy of Fallot. (www.heart.org)
However, this list can give a slightly misleading impression that four completely independent developmental abnormalities have happened simultaneously.
They have not.
Modern understanding sees TOF largely as the consequence of abnormal development and displacement of the outlet or conal septum.
That single developmental abnormality contributes directly to:
- the VSD;
- obstruction of the right ventricular outflow tract;
- and displacement of the aortic root so that it overrides the ventricular septum.
The fourth classical feature — right ventricular hypertrophy — develops largely as a consequence of the right ventricle pumping against the outflow obstruction.
In other words, three major structural abnormalities create the characteristic anatomy, while the fourth is principally a physiological response to them. (PubMed Central (PMC))
What About a “Fifth” Abnormality?
A number of additional abnormalities can accompany Tetralogy of Fallot.
One historically important combination is an associated atrial septal defect — ASD.
Tetralogy of Fallot with an ASD has sometimes been termed Pentalogy of Fallot.
The ASD is therefore an additional associated abnormality, rather than one of five interchangeable criteria from which three are selected to diagnose TOF. (PubMed Central (PMC))
This distinction is important.
It is sometimes tempting to teach congenital abnormalities as a checklist, but TOF is better understood as a spectrum of conotruncal anatomy centred upon VSD, aortic override and obstruction of the right ventricular outflow tract.
Development of the Abnormality
During normal cardiac development, the primitive ventricular outflow tract divides to create separate systemic and pulmonary pathways.
The developing outlet septum contributes to separating the aortic and pulmonary outflow tracts and aligning them correctly with their respective ventricles.
In Tetralogy of Fallot, the outlet septum is displaced anteriorly and superiorly.
This abnormal position produces two particularly important consequences.
First, the outlet septum fails to align normally with the ventricular septum.
A VSD is left beneath the aortic root.
Second, the abnormal septal position encroaches upon the pathway from the right ventricle to the pulmonary artery.
The right ventricular outflow tract becomes narrowed.
At the same time, the aortic root is displaced towards the right and sits partly above the VSD.
The anatomy therefore develops together rather than as four unrelated abnormalities.
The Four Classical Features
1. Ventricular Septal Defect
The VSD in Tetralogy of Fallot is usually large.
It frequently behaves as a non-restrictive VSD, meaning that the defect itself provides little resistance to blood moving between the ventricles.
This is important because right and left ventricular systolic pressures can therefore become similar.
The direction in which blood crosses the VSD is determined principally by the resistance to blood leaving each ventricle.
The left ventricle has access to the aorta.
The right ventricle should have access to the pulmonary artery.
But in TOF, that pulmonary route is obstructed.
Blood therefore takes the easier route.
2. Right Ventricular Outflow Tract Obstruction
The degree of RVOTO is arguably the most important determinant of the child’s physiology.
The obstruction may occur at several levels:
- beneath the pulmonary valve in the infundibulum;
- at the pulmonary valve;
- above the valve;
- within the pulmonary artery branches;
- or as a combination of these.
At the severe end of the spectrum, there may be pulmonary atresia, meaning there is effectively no normal forward pathway from the right ventricle into the pulmonary arteries.
The greater the obstruction, the harder it becomes for the right ventricle to eject blood towards the lungs.
Consequently, increasing amounts of blood are diverted across the VSD towards the aorta. (Long School of Medicine)
This is why two children who both have “Tetralogy of Fallot” can look physiologically very different.
3. Overriding Aorta
Normally, the aortic root is committed almost entirely to the left ventricle.
In Tetralogy of Fallot, the aorta is displaced towards the right and lies partly over the VSD.
The aorta therefore receives blood from both ventricles.
The term overriding describes this relationship.
Because deoxygenated right ventricular blood can cross the VSD and enter the overriding aorta, it can be delivered directly to the systemic circulation.
This creates the characteristic cyanotic physiology.
4. Right Ventricular Hypertrophy
The right ventricle must generate increased pressure to overcome the obstruction in its outflow tract.
The ventricular myocardium responds to this workload by becoming thicker.
This is right ventricular hypertrophy — RVH.
Unlike the VSD, aortic override and outflow obstruction, RV hypertrophy is largely a secondary consequence of the abnormal haemodynamics.
It is therefore useful to think of TOF not merely as four boxes to tick, but as three closely related structural abnormalities producing a fourth physiological adaptation.
Why Is the Baby Blue?
The key is the right ventricular outflow obstruction.
If the pathway to the lungs is severely restricted, right ventricular pressure rises.
Because the VSD is large, blood can cross from the right ventricle into the left ventricular outflow pathway and overriding aorta.
Instead of travelling:
Right ventricle → Pulmonary artery → Lungs
some blood travels:
Right ventricle → VSD → Aorta → Body
This is a right-to-left shunt.
Deoxygenated systemic venous blood consequently bypasses the lungs and enters the systemic circulation.
The result is arterial desaturation and cyanosis.
The “Pink Tet”
Not every child with Tetralogy of Fallot is markedly cyanosed.
If right ventricular outflow obstruction is relatively mild, much of the right ventricular output can still reach the lungs.
The pressure relationship across the VSD may then allow predominantly left-to-right rather than right-to-left flow.
The child may consequently appear relatively well oxygenated.
This is sometimes described informally as “pink Tet.”
As obstruction becomes more severe, the degree of right-to-left shunting increases and cyanosis becomes more pronounced. (Long School of Medicine)
This provides a particularly good example of why the labels “pink baby” and “blue baby” are useful starting points rather than absolute categories.
The same underlying anatomical diagnosis can produce different physiology.
Pulmonary Blood Flow Determines the Clinical Picture
The central question in Tetralogy of Fallot is:
How much blood can reach the lungs?
If pulmonary obstruction is mild, pulmonary blood flow may be relatively well preserved.
If obstruction is severe, pulmonary blood flow decreases.
Less blood reaches the lungs to become oxygenated.
More blood crosses the VSD and enters the systemic circulation.
The child becomes increasingly cyanosed.
At the extreme end of this spectrum is:
Tetralogy of Fallot with pulmonary atresia.
In these patients, pulmonary blood flow may depend upon a patent ductus arteriosus or major aortopulmonary collateral arteries.
Hypercyanotic or “Tet” Spells
One of the characteristic clinical features of Tetralogy of Fallot is the hypercyanotic spell, commonly called a Tet spell.
During a spell there is an acute increase in right-to-left shunting and a sudden reduction in pulmonary blood flow.
The child becomes profoundly cyanosed.
There may be:
- rapid breathing;
- irritability;
- increasing cyanosis;
- reduced oxygen saturation;
- weakness;
- syncope;
- seizures;
- and, in severe prolonged episodes, cardiovascular collapse.
Tet spells are particularly associated with dynamic narrowing of the infundibular portion of the right ventricular outflow tract and changes in systemic vascular resistance. (MSD Manuals)
Why Does Squatting Help?
An older child with unrepaired Tetralogy of Fallot may instinctively squat following exertion.
This classic observation provides an elegant demonstration of cardiovascular physiology.
Squatting increases systemic vascular resistance.
This makes it more difficult for right ventricular blood to escape across the VSD into the systemic circulation.
Relatively more blood is therefore encouraged towards the pulmonary circulation.
Pulmonary blood flow increases and arterial oxygenation can improve.
For an infant during a hypercyanotic spell, bringing the knees towards the chest works on a similar physiological principle.
It increases systemic vascular resistance and can reduce right-to-left shunting.
Management of a Hypercyanotic Spell
A significant Tet spell is a medical emergency.
Management is directed towards:
- calming the child and reducing oxygen demand;
- improving systemic vascular resistance;
- improving venous return;
- reducing infundibular spasm;
- and increasing pulmonary blood flow.
The knee-to-chest position is a classic immediate manoeuvre.
Oxygen may be administered.
Fluids, analgesia/sedation and medications affecting vascular tone or heart rate may be required according to clinical circumstances.
The important physiological objective is:
reduce the right-to-left shunt and encourage blood towards the lungs.
Diagnosis
Echocardiography
Echocardiography usually establishes the diagnosis and defines the principal anatomy.
The echocardiographer assesses:
- the VSD;
- degree of aortic override;
- level and severity of RV outflow obstruction;
- pulmonary valve annulus;
- main and branch pulmonary arteries;
- ventricular function;
- associated ASDs;
- aortic arch anatomy;
- and coronary anatomy where possible.
Understanding the pulmonary artery anatomy is particularly important when planning surgery.
Associated Abnormalities
Tetralogy of Fallot can occur with several additional cardiovascular abnormalities.
These include:
- right-sided aortic arch;
- abnormalities of the pulmonary arteries;
- pulmonary atresia;
- absent pulmonary valve;
- atrial septal defect;
- atrioventricular septal defect;
- and anomalous coronary arteries. (PubMed Central (PMC))
A right aortic arch is particularly well recognised in TOF.
Coronary anatomy is extremely important to the surgeon because an anomalous coronary artery may cross the right ventricular outflow tract.
A surgeon cannot safely make a ventricular incision through a coronary artery.
This can therefore materially alter the operative approach.
Association With 22q11.2 Deletion
Tetralogy of Fallot is one of the congenital cardiac lesions associated with 22q11.2 deletion syndrome.
The association is particularly important in more complex forms, including TOF with pulmonary atresia and aortic arch abnormalities. (NCBI)
Recognition is important because affected children may have additional problems involving:
- calcium regulation;
- immune function;
- palatal development;
- feeding;
- and other organ systems.
These issues can have direct implications for perioperative management.
Surgical Treatment
The objective of definitive repair is conceptually straightforward:
close the abnormal route between the ventricles and establish an adequate unobstructed pathway from the right ventricle to the lungs.
In practice, achieving this while preserving the pulmonary valve and right ventricular function can be considerably more complicated.
Closing the VSD
A patch is generally used to close the ventricular septal defect.
Importantly, the patch is positioned so that the left ventricle is directed towards the overriding aorta.
The repair therefore effectively restores the aorta to the left ventricular systemic circulation.
After successful VSD closure, right ventricular blood should no longer be able to escape directly into the systemic circulation through the defect.
Relieving the Right Ventricular Outflow Obstruction
The obstruction between the right ventricle and pulmonary arteries must then be relieved.
Depending upon the anatomy, this may involve:
- dividing obstructing muscle bundles;
- enlarging the right ventricular outflow tract;
- pulmonary valvotomy;
- enlargement of the pulmonary annulus;
- patch augmentation;
- or reconstruction of the pulmonary arteries.
One of the major considerations is whether the pulmonary valve annulus can be preserved.
The Transannular Patch
If the pulmonary valve annulus is too small to provide an adequate outflow tract, the incision may need to cross the annulus.
A patch is then used to enlarge the pathway.
This is known as a transannular patch.
It can provide excellent relief of obstruction.
However, cutting across the pulmonary valve annulus can result in significant pulmonary regurgitation.
Blood can therefore leak backwards from the pulmonary artery into the right ventricle during diastole.
This may be well tolerated initially but can produce chronic right ventricular volume loading over many years.
This helps explain why someone who underwent successful childhood repair of TOF may later require pulmonary valve replacement.
Palliative Surgery
Not every infant is suitable for immediate complete repair.
Historically, palliative procedures were extremely important in maintaining pulmonary blood flow until definitive correction could be performed.
The best-known is the Blalock–Taussig shunt, now more accurately referred to as the Blalock–Taussig–Thomas shunt.
The original operation connected the subclavian artery directly to the pulmonary artery.
Modern modified BTT shunts generally use a synthetic graft connecting a systemic artery to a pulmonary artery.
The aim is simple:
provide more blood to the lungs.
Other catheter or surgical strategies can also be used to improve pulmonary blood flow depending upon the anatomy.
A Historical Turning Point
The development of systemic-to-pulmonary shunting represents one of the major landmarks in congenital cardiac surgery.
Before effective surgical palliation, severely cyanotic children with Tetralogy of Fallot had few treatment options.
The development of the Blalock–Thomas–Taussig operation in the 1940s demonstrated that surgically increasing pulmonary blood flow could dramatically improve cyanosis.
It did not correct the underlying defect.
It bought time.
Later development of cardiopulmonary bypass allowed surgeons to open the heart, close the VSD and reconstruct the right ventricular outflow tract.
Tetralogy of Fallot therefore occupies an important place not only in congenital cardiology but in the history of cardiac surgery itself.
Anaesthetic and Theatre Considerations
From an anaesthetic perspective, the important concept is the balance between pulmonary vascular resistance and systemic vascular resistance.
Anything that substantially lowers systemic vascular resistance can encourage additional right-to-left shunting.
Anything that markedly increases pulmonary vascular resistance can further reduce pulmonary blood flow.
Factors capable of increasing pulmonary vascular resistance include:
- hypoxia;
- hypercarbia;
- acidosis;
- and excessive airway pressures.
Maintaining adequate circulating volume is also important.
A hypovolaemic child has reduced venous return and may be more susceptible to dynamic right ventricular outflow obstruction and hypercyanotic episodes.
The anaesthetic team therefore attempts to maintain a physiological balance that encourages blood towards rather than away from the pulmonary circulation.
Cardiopulmonary Bypass
Complete intracardiac repair is normally undertaken using cardiopulmonary bypass.
The heart can then be isolated from the circulation while the surgeon:
- closes the VSD;
- assesses the right ventricular outflow tract;
- removes obstructing muscle where necessary;
- repairs or enlarges the pulmonary valve/outflow pathway;
- and deals with associated abnormalities.
The adequacy of the repair must then be assessed.
Residual VSD, residual RVOT obstruction, right ventricular pressure and ventricular function are particularly important.
Transoesophageal or epicardial echocardiography may assist in assessing the repair before leaving theatre.
After Repair
The term “corrective surgery” is useful clinically, but should not be interpreted as meaning that the heart has become anatomically identical to a heart that never had TOF.
Patients require lifelong congenital cardiac surveillance.
Long-term issues can include:
- pulmonary regurgitation;
- residual pulmonary stenosis;
- right ventricular dilation;
- right ventricular dysfunction;
- residual VSD;
- aortic root dilation;
- aortic regurgitation;
- atrial or ventricular arrhythmias;
- and sudden cardiac death risk in selected patients.
The American Heart Association therefore recommends lifelong follow-up for people who have undergone TOF repair. (www.heart.org)
Tetralogy of Fallot in One Sentence
Tetralogy of Fallot is a conotruncal congenital cardiac abnormality in which a large VSD, overriding aorta and obstruction of the right ventricular outflow tract allow deoxygenated right ventricular blood to enter the systemic circulation, with secondary right ventricular hypertrophy developing from the pressure load.
The most useful question to ask is:
How difficult is it for blood to reach the lungs?
The answer largely determines whether the child is pink, blue, or profoundly cyanosed.
Key Points
Tetralogy of Fallot was comprehensively described by Étienne-Louis Arthur Fallot in 1888, although similar anatomy had been recognised much earlier. (STS Cardiothoracic Surgery)
Its four classical features are:
VSD + RV outflow obstruction + overriding aorta + RV hypertrophy.
The first three are closely related structural consequences of abnormal conotruncal development.
RV hypertrophy is predominantly secondary to the resulting pressure load.
An associated ASD produces the historical Pentalogy of Fallot; TOF is not currently defined by a “three from five” diagnostic rule. (PubMed Central (PMC))
The degree of RV outflow obstruction largely determines the severity of cyanosis.
Mild obstruction can produce a relatively “pink Tet.”
Severe obstruction produces increased right-to-left shunting and cyanosis.
Acute increases in right-to-left shunting can produce hypercyanotic or Tet spells.
Definitive repair generally involves VSD closure and relief of the RV outflow obstruction.
Patients require lifelong congenital cardiac follow-up after repair. (www.heart.org)
Glossary of Terms
Aortic Override
Displacement of the aortic root towards the right ventricle so that the aorta sits above the ventricular septal defect and can receive blood from both ventricles.
Atrial Septal Defect — ASD
An abnormal communication between the right and left atria.
An ASD associated with classical TOF has historically been described as part of Pentalogy of Fallot.
Blalock–Taussig–Thomas Shunt — BTT Shunt
A surgically created systemic-to-pulmonary arterial connection designed to increase pulmonary blood flow.
Modern versions generally use a synthetic graft.
Cardiopulmonary Bypass — CPB
A technique in which a heart-lung machine temporarily maintains circulation and gas exchange while cardiac surgery is undertaken.
Cyanosis
Bluish discoloration of the skin and mucous membranes caused by increased concentrations of deoxygenated haemoglobin in arterial blood.
Hypercyanotic Spell
An acute episode of increased right-to-left shunting and reduced pulmonary blood flow in Tetralogy of Fallot, producing sudden worsening of cyanosis.
Also known as a Tet spell.
Infundibulum
The muscular outflow portion of the right ventricle leading towards the pulmonary valve.
Narrowing of this area is an important component of RV outflow obstruction in TOF.
Non-Restrictive VSD
A ventricular septal defect large enough to offer little resistance to flow between the ventricles, allowing ventricular pressures to approach each other.
Overriding Aorta
See Aortic Override.
Pentalogy of Fallot
A historical term generally describing Tetralogy of Fallot accompanied by an atrial septal defect.
Pulmonary Atresia
Absence of an open connection through the pulmonary valve/outflow pathway.
Tetralogy of Fallot with pulmonary atresia represents the severe end of the TOF spectrum.
Pulmonary Regurgitation
Backward flow of blood from the pulmonary artery into the right ventricle during diastole.
It is an important potential long-term consequence of TOF repair involving a transannular patch.
Pulmonary Stenosis
Narrowing affecting blood flow from the right ventricle towards the pulmonary arteries.
Pulmonary Vascular Resistance — PVR
The resistance encountered by blood flowing through the pulmonary circulation.
Right-to-Left Shunt
Passage of deoxygenated blood from the right side of the circulation into the systemic arterial circulation without first passing through the lungs.
Right Ventricular Hypertrophy — RVH
Thickening of the right ventricular myocardium in response to increased pressure workload.
Right Ventricular Outflow Tract — RVOT
The pathway by which blood leaves the right ventricle and enters the pulmonary artery.
Right Ventricular Outflow Tract Obstruction — RVOTO
Restriction to blood flow between the right ventricle and pulmonary circulation.
The severity of RVOTO is one of the principal determinants of cyanosis in TOF.
Systemic Vascular Resistance — SVR
The resistance encountered by blood moving through the systemic circulation.
Changes in SVR can substantially alter right-to-left shunting in Tetralogy of Fallot.
Tet Spell
See Hypercyanotic Spell.
Tetralogy
A group or combination of four features.
Tetralogy of Fallot — TOF
A congenital cardiac malformation characterised classically by ventricular septal defect, right ventricular outflow obstruction, overriding aorta and right ventricular hypertrophy.
Transannular Patch
A patch used to enlarge the right ventricular outflow tract across the pulmonary valve annulus when the native annulus is too small to provide adequate pulmonary blood flow.
Ventricular Septal Defect — VSD
An abnormal communication between the right and left ventricles.
In TOF the VSD is typically large and forms a central part of the abnormal intracardiac circulation.
