One Arterial Trunk Supplying Both the Lungs and the Body
Introduction
Congenital cardiac defects can often be understood by asking three questions:
How many effective outflow pathways leave the heart?
Where does blood mix?
And how is pulmonary blood flow being controlled?
Truncus Arteriosus is a classic cyanotic “blue baby” lesion.
Instead of developing separate aortic and pulmonary outflow tracts, the heart retains one common arterial trunk.
That single vessel supplies the systemic, pulmonary and coronary circulations.
A large Ventricular Septal Defect is usually present beneath the common trunk, allowing blood from both ventricles to enter it.
The result is a circulation in which oxygenated and deoxygenated blood mix before being distributed to both the lungs and the body. (NCBI)
Body
What is Truncus Arteriosus?
In the normal heart:
Right ventricle → Pulmonary artery → Lungs
and
Left ventricle → Aorta → Body
In Truncus Arteriosus, the embryological outflow tract fails to divide completely into the pulmonary artery and aorta.
Instead, there is:
One common arterial trunk
arising from the heart.
This common vessel gives rise to:
- the systemic circulation;
- the pulmonary arteries;
- and the coronary arteries.
A large Ventricular Septal Defect — VSD is normally present immediately beneath the truncal valve. (NCBI)
This means both ventricles eject into the same common outflow tract.
Development of the Abnormality
During normal embryological development, the primitive outflow tract is divided into two separate vessels:
the aorta
and
the pulmonary artery.
This separation involves development and spiralling of the outflow septum.
In persistent Truncus Arteriosus, this division fails.
The result is retention of a single arterial trunk.
Because septation of the ventricular outflow tract is also incomplete, a large malalignment VSD usually remains beneath the common vessel. (NCBI)
The abnormality is therefore best understood as a failure of conotruncal septation rather than simply “the aorta and pulmonary artery being joined together.”
They were never fully separated in the first place.
The Truncal Valve
The common arterial trunk has a single semilunar valve known as the truncal valve.
This valve may have one, two, three or four cusps, although three cusps are common. (NCBI)
The truncal valve can itself be abnormal.
It may be:
- stenotic;
- regurgitant;
- dysplastic;
- or a combination of these.
Truncal valve dysfunction is important because the valve controls the only major arterial outlet from the ventricles.
Significant regurgitation increases ventricular volume loading.
Significant stenosis increases ventricular pressure workload.
The Ventricular Septal Defect
The VSD in Truncus Arteriosus is usually large.
Because both ventricles communicate freely with the common arterial trunk, ventricular pressures tend to become similar.
Blood therefore mixes at ventricular level before entering the truncus.
The common trunk then distributes that mixed blood to:
- the body;
- the lungs;
- and the coronary arteries.
This explains the characteristic physiology.
The baby is cyanosed because the systemic circulation receives mixed rather than fully oxygenated blood.
At the same time, the lungs may receive far too much blood. (Merck Manuals)
Why is the Baby Blue?
Deoxygenated blood returning from the body enters the right side of the heart.
Oxygenated pulmonary venous blood enters the left side.
Because of the large VSD and common arterial outlet, the two blood streams mix.
The common trunk then ejects this mixed blood to both systemic and pulmonary circulation.
Thus:
Systemic venous blood + Pulmonary venous blood → Common ventricle-level mixing → Truncus → Lungs and Body
The systemic arterial oxygen saturation is therefore lower than normal.
The child becomes cyanosed.
However, the degree of cyanosis may not be as dramatic as in lesions with extremely restricted pulmonary blood flow, because pulmonary blood flow can actually be excessive.
Pulmonary Blood Flow
One of the most important physiological principles in Truncus Arteriosus is that the pulmonary arteries arise directly from the common trunk.
There is therefore no normal right ventricular-pulmonary arterial separation.
Pulmonary blood flow depends heavily upon the relationship between:
Pulmonary Vascular Resistance — PVR
and
Systemic Vascular Resistance — SVR.
Shortly after birth, pulmonary vascular resistance falls.
As PVR falls, increasing amounts of blood can flow from the truncus into the pulmonary arteries.
The lungs may therefore receive a very large proportion of cardiac output. (NCBI)
This produces pulmonary over-circulation.
Cyanosis and Heart Failure at the Same Time
Truncus Arteriosus is a good example of why congenital heart disease does not always fit neatly into either:
cyanosis
or
heart failure.
The child may have both.
The systemic circulation receives mixed blood, causing cyanosis.
At the same time, large quantities of blood may be recirculated through the lungs, causing pulmonary congestion and ventricular volume overload.
Typical features can therefore include:
- cyanosis;
- tachypnoea;
- poor feeding;
- sweating during feeds;
- poor weight gain;
- respiratory distress;
- bounding pulses;
- and signs of heart failure. (Merck Manuals)
Why are the Pulses Bounding?
When pulmonary vascular resistance falls, blood runs off from the common trunk into the pulmonary circulation.
This can produce a wide pulse pressure.
The arterial pulse may therefore feel unusually strong or bounding.
This is similar in principle to the run-off seen in a large Patent Ductus Arteriosus.
The important difference is that in Truncus Arteriosus, the pulmonary arteries arise directly from the common trunk itself.
Pulmonary Hypertension
If excessive pulmonary blood flow continues, the pulmonary vascular bed is exposed to both:
high flow
and
high pressure.
This can rapidly produce pulmonary vascular remodelling and pulmonary hypertension.
One of the reasons early surgical repair is so important is to prevent irreversible pulmonary vascular disease. (www.heart.org)
This is very different from some congenital lesions in which surgery can be safely delayed for years.
Truncus Arteriosus is generally repaired in early infancy.
Classification of Truncus Arteriosus
Several classification systems have been used.
The two best known are:
Collett and Edwards
and
Van Praagh.
The classifications describe mainly how the pulmonary arteries arise from the common trunk and whether significant associated aortic arch abnormalities are present. (NCBI)
Collett and Edwards Classification
Type I
A short main pulmonary artery arises from the truncus and then divides into right and left pulmonary arteries.
Type II
The right and left pulmonary arteries arise separately but close together from the posterior aspect of the truncus.
Type III
The branch pulmonary arteries arise separately and more widely apart from the common trunk.
Historical Type IV
The original Collett and Edwards Type IV described pulmonary arteries supplied from the descending aorta.
This is no longer generally considered true Truncus Arteriosus and is now regarded as a form of Pulmonary Atresia with VSD and collateral pulmonary blood supply. (NCBI)
This is an important example of how congenital cardiac classification has evolved as anatomical understanding has improved.
Van Praagh Classification
The Van Praagh system incorporates not only pulmonary artery anatomy but also associated arch abnormalities.
Broadly:
A1 — a main pulmonary artery arises from the common trunk.
A2 — right and left pulmonary arteries arise separately.
A3 — one pulmonary artery is absent or disconnected from the central pulmonary circulation.
A4 — Truncus Arteriosus is associated with significant aortic arch interruption or related arch abnormality. (NCBI)
Modern surgical practice often concentrates less on memorising a classification number and more on defining the exact anatomy that has to be reconstructed.
Interrupted Aortic Arch and Truncus
Truncus Arteriosus can occur together with Interrupted Aortic Arch.
This creates particularly complex physiology.
The common trunk must supply both pulmonary and systemic circulation, while the descending aorta may depend upon ductal flow because of the interrupted arch.
In this situation, prostaglandin may be necessary to maintain ductal patency before surgery. (Merck Manuals)
The operation must then address both:
the common arterial trunk
and
the interrupted systemic outflow pathway.
Association With 22q11.2 Deletion
Truncus Arteriosus has an important association with 22q11.2 deletion syndrome.
This has historically included conditions such as DiGeorge syndrome and velocardiofacial syndrome.
Merck notes that a substantial proportion of patients with Truncus Arteriosus have 22q11 deletion. (Merck Manuals)
Associated clinical issues may include:
- hypocalcaemia;
- immune abnormalities;
- palatal abnormalities;
- feeding difficulties;
- and other developmental problems.
This has clear implications for perioperative assessment.
Coronary Artery Anatomy
Because the coronary arteries arise from the truncal root, their anatomy can be variable.
This matters during surgical reconstruction.
The surgeon needs to understand where the coronary arteries arise before dividing or reconstructing the great vessels.
Unusual coronary origins can significantly influence the operative technique. (NCBI)
Diagnosis
Echocardiography
Echocardiography normally establishes the diagnosis.
It can demonstrate:
- the common arterial trunk;
- the VSD;
- truncal valve anatomy;
- origin of the pulmonary arteries;
- ventricular function;
- truncal valve stenosis or regurgitation;
- associated arch abnormalities;
- and sometimes coronary anatomy.
CT and MRI
CT angiography or cardiac MRI may be particularly useful where there is:
- complex pulmonary artery anatomy;
- interrupted or hypoplastic aortic arch;
- uncertain coronary anatomy;
- or previous surgical reconstruction.
These techniques can provide detailed three-dimensional information before intervention.
Initial Medical Management
The definitive treatment is surgical.
Before surgery, medical treatment may be needed to manage heart failure and pulmonary over-circulation.
This can include:
- diuretics;
- nutritional support;
- respiratory support where required;
- and careful management of systemic and pulmonary vascular resistance.
The objective is not to “treat” the anatomical defect medically.
It is to stabilise the child until surgical repair can be performed. (Merck Manuals)
Surgical Repair
The objective of surgery is to convert the single mixed circulation into two separate pathways.
The surgeon effectively needs to create:
Left ventricle → Aorta/Systemic circulation
and
Right ventricle → Pulmonary arteries/Lungs
The operation therefore has two major components:
close the VSD so the left ventricle is committed to the truncal root
and
create a new connection between the right ventricle and pulmonary arteries. (www.heart.org)
Closing the VSD
A patch is used to close the VSD.
The patch is positioned so that left ventricular blood is directed into the common truncal root.
The truncal root therefore becomes functionally the aorta.
This establishes:
Left ventricle → Truncal valve → Systemic circulation
The right ventricle is now separated from that pathway.
Separating the Pulmonary Arteries
The pulmonary arteries are detached from the common trunk.
The resulting opening in the truncal vessel is repaired.
The former common trunk now functions as the systemic arterial outflow.
The pulmonary arteries, however, still need a source of blood from the right ventricle.
Right Ventricle to Pulmonary Artery Conduit
A valved tube or conduit is placed between the right ventricle and pulmonary arteries.
This creates the new pulmonary pathway:
Right ventricle → Conduit → Pulmonary arteries → Lungs
The circulation is now separated into systemic and pulmonary components. (www.heart.org)
The Conduit Problem
A conduit inserted into a small infant does not grow as the child grows.
Over time it may:
- become relatively too small;
- develop stenosis;
- calcify;
- or develop valve dysfunction.
Many patients therefore require further catheter or surgical intervention as they grow. (www.heart.org)
This is an important point when discussing congenital heart surgery.
A successful neonatal repair may be the first operation, not necessarily the last.
Truncal Valve Repair
If the truncal valve is significantly regurgitant or stenotic, it may require intervention during the initial operation.
Repair can be technically difficult because the valve anatomy is variable.
Some patients ultimately require truncal valve replacement later in life. (www.heart.org)
Cardiopulmonary Bypass
Definitive repair requires cardiopulmonary bypass.
The operation involves:
- VSD closure;
- separation of the pulmonary arteries from the truncus;
- reconstruction of the systemic outflow;
- creation of the RV-to-PA conduit;
- and treatment of any associated truncal valve or aortic arch abnormality.
The physiology changes dramatically following repair.
Before surgery, both ventricles contribute to a single common circulation.
After repair, the ventricles once again have separate jobs.
Anaesthetic and Theatre Considerations
Before repair, the main challenge is the balance between pulmonary and systemic blood flow.
As pulmonary vascular resistance falls, excessive pulmonary blood flow can steal cardiac output from the systemic circulation.
The anaesthetic team therefore needs to understand the interaction between:
- PVR;
- SVR;
- pulmonary blood flow;
- systemic perfusion;
- ventricular function;
- and truncal valve function.
Excessive reduction in PVR may worsen pulmonary over-circulation.
Conversely, severe elevation in PVR can reduce pulmonary blood flow and worsen cyanosis.
The goal is therefore physiological balance rather than simply achieving the highest possible oxygen saturation.
After Repair
Postoperative management focuses on:
- right ventricular function;
- left ventricular function;
- pulmonary artery pressure;
- conduit function;
- truncal valve competence;
- systemic perfusion;
- and residual VSD or outflow obstruction.
Because the pulmonary vascular bed may have been exposed to high pressure and flow before repair, pulmonary hypertension can remain an important postoperative concern.
Long-Term Follow-Up
Patients require lifelong congenital cardiac follow-up.
Potential long-term problems include:
- RV-to-PA conduit stenosis;
- conduit regurgitation;
- branch pulmonary artery stenosis;
- truncal valve regurgitation;
- truncal valve stenosis;
- aortic root dilation;
- arrhythmias;
- ventricular dysfunction;
- and need for repeated catheter or surgical procedures. (www.heart.org)
Many patients will require conduit replacement as they grow.
This is expected within the long-term management pathway rather than necessarily representing failure of the original repair.
Truncus Arteriosus in One Sentence
Truncus Arteriosus is a congenital failure of separation of the embryonic outflow tract, leaving a single arterial trunk above a large VSD that supplies the systemic, pulmonary and coronary circulations.
The two key physiological questions are:
How much blood is flowing to the lungs?
and
How well is the truncal valve functioning?
Key Points
Truncus Arteriosus is a cyanotic congenital cardiac defect.
There is one common arterial trunk instead of separate aorta and pulmonary artery. (Merck Manuals)
A large VSD is usually present beneath the truncal valve.
Oxygenated and deoxygenated blood mix before entering the common trunk.
The child may therefore have cyanosis and heart failure simultaneously.
As PVR falls after birth, pulmonary over-circulation can become severe.
Early surgical repair is required to prevent pulmonary vascular damage. (www.heart.org)
Repair involves:
VSD closure + separation of the pulmonary arteries + RV-to-PA conduit.
Associated abnormalities can include:
- truncal valve disease;
- interrupted or hypoplastic aortic arch;
- abnormal coronary anatomy;
- and 22q11.2 deletion syndrome. (NCBI)
Patients require lifelong follow-up and commonly need further intervention on the RV-to-PA conduit as they grow. (www.heart.org)
Glossary of Terms
22q11.2 Deletion Syndrome
A chromosomal deletion syndrome associated with several conotruncal congenital heart defects, including Truncus Arteriosus.
It may also be associated with hypocalcaemia, immune dysfunction and palatal abnormalities.
Aortic Arch
The curved portion of the aorta connecting the ascending and descending aorta and giving rise to major vessels supplying the head and upper limbs.
Cardiopulmonary Bypass — CPB
A technique in which a heart-lung machine temporarily maintains circulation and gas exchange during cardiac surgery.
Conduit
A surgically implanted tube used to connect two cardiovascular structures.
In Truncus Arteriosus repair, a valved conduit commonly connects the right ventricle to the pulmonary arteries.
Cyanosis
Bluish discoloration caused by increased quantities of deoxygenated haemoglobin in systemic arterial blood.
Interrupted Aortic Arch — IAA
Complete loss of continuity between portions of the aortic arch.
It can occur in association with Truncus Arteriosus.
Pulmonary Hypertension
Abnormally elevated pressure within the pulmonary arterial circulation.
In Truncus Arteriosus it can develop rapidly because of excessive pulmonary blood flow and pressure.
Pulmonary Over-Circulation
Excessive blood flow through the pulmonary circulation.
It can occur in Truncus Arteriosus as pulmonary vascular resistance falls after birth.
Pulmonary Vascular Resistance — PVR
The resistance encountered by blood moving through the pulmonary circulation.
Right Ventricle-to-Pulmonary Artery Conduit — RV-PA Conduit
A surgically created valved pathway connecting the right ventricle to the pulmonary arteries after repair of Truncus Arteriosus.
Systemic Vascular Resistance — SVR
The resistance encountered by blood moving through the systemic circulation.
Truncal Valve
The single semilunar valve controlling flow from the ventricles into the common arterial trunk.
It may be stenotic, regurgitant or structurally abnormal.
Truncal Valve Regurgitation
Backward leakage of blood through the truncal valve when the valve fails to close effectively.
Truncus Arteriosus
A congenital heart defect in which the embryonic outflow tract fails to divide into separate aortic and pulmonary vessels, leaving a single common arterial trunk.
Ventricular Septal Defect — VSD
An abnormal communication between the right and left ventricles.
A large VSD is a characteristic feature of Truncus Arteriosus.
