When the Two Circulations Run in Parallel
Introduction
Congenital cardiac defects can often be approached by asking three questions:
Where is the anatomical abnormality?
How does it change 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.”
Transposition of the Great Arteries is one of the classic blue baby lesions.
Unlike many congenital defects, the fundamental problem is not primarily an obstruction or a simple hole between chambers.
The problem is that the two great arteries are connected to the wrong ventricles.
The result is conceptually simple but physiologically profound:
the pulmonary and systemic circulations run in parallel rather than in series.
Survival after birth therefore depends upon blood being able to mix between those two circulations.
Body
What is Transposition of the Great Arteries?
In normal anatomy:
Right ventricle → Pulmonary artery → Lungs
and
Left ventricle → Aorta → Body
In dextro-Transposition of the Great Arteries — d-TGA:
Right ventricle → Aorta → Body
and
Left ventricle → Pulmonary artery → Lungs
The aorta therefore arises from the morphological right ventricle and the pulmonary artery arises from the morphological left ventricle.
This creates two circulations running alongside each other.
Normal Circulation: In Series
Normally, deoxygenated blood returns from the body to the right atrium.
It passes through:
Right atrium → Right ventricle → Pulmonary artery → Lungs
The blood becomes oxygenated and returns through the pulmonary veins:
Pulmonary veins → Left atrium → Left ventricle → Aorta → Body
The two circulations therefore form one continuous loop.
They operate in series.
TGA: Circulation in Parallel
In TGA, systemic venous blood returns normally to the right atrium and passes into the right ventricle.
But the right ventricle is connected to the aorta.
The deoxygenated blood is therefore pumped straight back to the body without first passing through the lungs.
At the same time, oxygenated pulmonary venous blood returns to the left atrium and enters the left ventricle.
But the left ventricle is connected to the pulmonary artery.
That oxygenated blood is therefore pumped back into the lungs.
The result is:
Body → Right heart → Aorta → Body
and separately:
Lungs → Left heart → Pulmonary artery → Lungs
Without a connection between these circuits, the condition is incompatible with sustained life.
Why is the Baby Blue?
The systemic circulation repeatedly receives poorly oxygenated venous blood.
The oxygenated blood is effectively trapped within the pulmonary circuit.
The baby therefore becomes cyanosed soon after birth.
The immediate solution, before definitive repair, is mixing.
There must be somewhere for oxygenated and deoxygenated blood to meet.
This may occur through:
- a Patent Foramen Ovale or Atrial Septal Defect;
- a Ventricular Septal Defect;
- or a Patent Ductus Arteriosus.
The amount of mixing largely determines how well the infant tolerates the lesion before surgery.
Mixing: The Immediate Priority
A newborn with good mixing may initially appear more stable than one with an almost intact atrial septum.
If communication between the two circulations is inadequate, severe hypoxaemia develops.
A prostaglandin E1 infusion may be used to maintain ductal patency and increase mixing while definitive treatment is planned.
However, an open ductus alone may not always provide adequate mixing.
In these circumstances the atrial communication may need to be enlarged.
Balloon Atrial Septostomy
The Rashkind balloon atrial septostomy transformed the early management of TGA.
A balloon catheter is passed across the foramen ovale into the left atrium.
The balloon is inflated and rapidly withdrawn across the atrial septum.
This enlarges the atrial communication.
The purpose is not to correct the transposition.
It is to allow sufficient mixing of oxygenated and deoxygenated blood to keep the child alive until definitive surgery can be performed.
The technique replaced earlier surgical methods such as the Blalock–Hanlon atrial septectomy as a means of creating atrial-level mixing.
Historical Surgical Treatment
The history of TGA is particularly important because it mirrors the development of congenital cardiac surgery itself.
Surgeons initially recognised that if the arteries could not safely be moved, perhaps the blood could be redirected inside the heart instead.
This led to the atrial switch operations.
The Senning Procedure
In 1957, Swedish surgeon Åke Senning performed the first successful atrial switch operation.
Using the patient’s own atrial tissue, a series of baffles redirected venous blood at atrial level.
Systemic venous blood arriving from the venae cavae was redirected towards the mitral valve and left ventricle.
Because the left ventricle was connected to the pulmonary artery, this blood was then sent to the lungs.
Pulmonary venous blood was redirected towards the tricuspid valve and right ventricle.
Because the right ventricle was connected to the aorta, oxygenated blood then reached the systemic circulation.
The arteries themselves remained transposed.
The operation therefore produced a physiological correction, rather than an anatomical correction.
The Mustard Procedure
Canadian surgeon William Thornton Mustard subsequently developed another atrial switch technique.
His successful procedure was introduced clinically in the early 1960s.
Instead of constructing the entire pathway from atrial tissue, the Mustard operation used a baffle to redirect venous return within the atria.
Again:
Systemic venous blood → Left ventricle → Pulmonary artery
and
Pulmonary venous blood → Right ventricle → Aorta
The Mustard operation became widely used and, for many years, was a standard operation for TGA.
Why Did the Mustard and Senning Operations Fall Out of Favour?
The circulation worked.
But the right ventricle remained the systemic ventricle.
A right ventricle is designed to pump against the relatively low resistance of the pulmonary circulation.
Following an atrial switch it had to maintain systemic arterial pressure for life.
Over decades, some patients developed:
- systemic right ventricular dysfunction;
- tricuspid regurgitation;
- atrial arrhythmias;
- sinus-node dysfunction;
- baffle obstruction;
- and baffle leaks.
Many adults alive today underwent Mustard or Senning procedures as children and therefore require specialist adult congenital cardiac follow-up.
These operations remain historically important and clinically relevant, even though they are no longer the routine first choice for simple neonatal d-TGA.
The Arterial Switch — Jatene Procedure
The modern solution is fundamentally different.
Rather than redirecting blood inside the atria, surgeons correct the anatomical problem itself.
Brazilian cardiac surgeon Adib Domingos Jatene performed the first successful arterial switch operation in 1975.
The operation is consequently often known as the Jatene procedure.
Today, the arterial switch is the standard surgical treatment for most babies with d-TGA.
What Happens During an Arterial Switch?
The principle sounds simple:
Put the arteries back where they belong.
The aorta and pulmonary artery are divided above their valves.
The pulmonary root, arising from the left ventricle, becomes the new aorta — the neoaorta.
The original aortic root, arising from the right ventricle, becomes the new pulmonary root — the neopulmonary artery.
The great arteries are then reconstructed in their anatomically appropriate circulations.
Thus:
Left ventricle → Neoaorta → Body
and
Right ventricle → Neopulmonary artery → Lungs
The two parallel circulations have been converted back into a normal circulation in series.
The Coronary Arteries: The Difficult Part
Simply switching the large vessels is not enough.
The coronary arteries originate from the original aortic root.
If that root is converted into the pulmonary root, the coronary arteries would no longer receive systemic arterial blood.
They therefore have to be detached from their original positions and reimplanted into the neoaorta.
This is one of the technically critical stages of the arterial switch.
The coronary anatomy varies from patient to patient, and unusual coronary patterns can significantly increase surgical complexity.
The Lecompte Manoeuvre
During many arterial switch operations, the pulmonary artery bifurcation is brought forward so that it lies anterior to the ascending aorta.
This is known as the Lecompte manoeuvre.
It allows reconstruction of the pulmonary arteries without excessive tension and has become an important component of modern arterial switch surgery.
Timing of Surgery
For straightforward d-TGA, definitive repair is generally undertaken during the neonatal period.
Timing matters because before the arterial switch, the left ventricle is pumping into the low-pressure pulmonary circulation.
After the switch, that same ventricle must suddenly become the systemic ventricle.
In a newborn, the left ventricular muscle is still well prepared for systemic pressure because foetal ventricular pressures were similar.
If repair is substantially delayed, the left ventricle may progressively lose muscle mass because it has been working against pulmonary rather than systemic resistance.
This historically created problems in patients presenting late.
TGA With a VSD
Not all TGA anatomy is identical.
A Ventricular Septal Defect may coexist with transposition.
A VSD can improve mixing before surgery, but it also changes pulmonary blood flow and the surgical anatomy.
During an arterial switch the VSD may also need to be closed.
Other patients may have associated pulmonary or left ventricular outflow obstruction, creating considerably more complex surgical problems.
The Mustard Procedure Today
The Mustard operation is historically important, but it should not be described simply as an obsolete failure.
It allowed generations of children with TGA to survive into adulthood.
Many are now middle-aged or older.
The long-term challenges of the systemic right ventricle mean these patients form an important group within modern adult congenital cardiology.
Understanding Mustard physiology is therefore still clinically relevant.
Anaesthetic and Theatre Considerations
Before repair, the major concern is mixing.
A high oxygen concentration alone cannot correct inadequate mixing between two parallel circulations.
The anaesthetic team therefore needs to understand:
- atrial communication;
- ventricular communication;
- ductal patency;
- pre- and post-ductal saturation;
- systemic perfusion;
- acid-base status;
- and pulmonary/systemic vascular resistance.
A profoundly cyanosed TGA baby with inadequate mixing may require urgent atrial septostomy before definitive surgery.
During an arterial switch, cardiopulmonary bypass and myocardial protection are required.
Particular attention is paid to:
- coronary artery anatomy;
- adequacy of coronary transfer;
- ventricular function;
- pulmonary artery reconstruction;
- neoaortic valve function;
- and residual intracardiac abnormalities.
A technically perfect arterial connection is of little value if a transferred coronary artery is obstructed.
After the Arterial Switch
The arterial switch restored the left ventricle to its appropriate role as the systemic ventricle and removed many of the fundamental long-term disadvantages of the atrial switch.
However, these patients still require lifelong congenital cardiac surveillance.
Potential long-term issues include:
- neoaortic root dilation;
- neoaortic valve regurgitation;
- supravalvar pulmonary stenosis;
- branch pulmonary artery stenosis;
- coronary artery problems;
- and ventricular dysfunction.
Successful neonatal repair therefore does not mean congenital cardiac follow-up is no longer required.
TGA in One Sentence
Transposition of the Great Arteries is a congenital abnormality in which the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, creating two parallel circulations whose survival initially depends upon mixing between them.
The immediate question is:
Where can the blood mix?
The definitive modern answer is:
restore the great arteries to the appropriate ventricles with an arterial switch.
Key Points
TGA is a classic cyanotic congenital heart defect.
The aorta arises from the right ventricle and the pulmonary artery from the left ventricle.
The systemic and pulmonary circulations therefore operate in parallel.
Survival before repair depends upon mixing through an ASD/PFO, VSD or PDA.
Balloon atrial septostomy can improve mixing before surgery.
The Senning and Mustard procedures were historical atrial-switch operations that redirected blood without correcting the arterial anatomy.
The arterial switch or Jatene procedure is now the standard operation for most d-TGA.
The coronary arteries must be transferred to the neoaorta during the arterial switch.
Patients repaired by either arterial or atrial switch require lifelong congenital cardiac follow-up.
Glossary of Terms
Arterial Switch Operation — ASO
The modern anatomical repair for most d-TGA in which the aorta and pulmonary artery are transferred to their appropriate ventricular outflows and the coronary arteries are reimplanted into the neoaorta.
Also known as the Jatene procedure.
Atrial Septal Defect — ASD
An abnormal communication between the atria.
In TGA, an atrial communication permits essential mixing between the parallel pulmonary and systemic circulations.
Atrial Switch
A physiological repair in which blood is redirected within the atria while the great arteries remain transposed.
The Mustard and Senning procedures are examples.
Balloon Atrial Septostomy — BAS
A catheter procedure that enlarges the communication between the atria to improve mixing.
Often called a Rashkind procedure.
Baffle
A surgically created internal pathway used to redirect blood.
Baffles are fundamental to atrial-switch operations such as Mustard and Senning.
Blalock–Hanlon Procedure
A historical surgical atrial septectomy performed to improve mixing between the two parallel circulations before definitive repair.
Cardiopulmonary Bypass — CPB
A technique in which a heart-lung machine temporarily maintains circulation and gas exchange during cardiac surgery.
Cyanosis
Bluish discoloration resulting from increased quantities of deoxygenated haemoglobin in arterial blood.
d-TGA
Dextro-Transposition of the Great Arteries, in which the aorta arises from the morphological right ventricle and the pulmonary artery from the morphological left ventricle.
Jatene Procedure
Another name for the arterial switch operation, named after Brazilian surgeon Adib Domingos Jatene.
Lecompte Manoeuvre
Repositioning of the pulmonary artery bifurcation anterior to the aorta during arterial-switch reconstruction.
Mixing
The movement and combination of oxygenated and deoxygenated blood between the otherwise separate parallel circulations of TGA.
Mustard Procedure
An atrial-switch operation using an intra-atrial baffle to redirect systemic and pulmonary venous blood.
Neoaorta
The vessel functioning as the aorta following an arterial switch. It is formed from the original pulmonary root.
Neopulmonary Artery
The reconstructed pulmonary artery following an arterial switch, based on the original aortic root and pulmonary arterial reconstruction.
Parallel Circulation
A circulation in which systemic and pulmonary blood travel in two separate loops rather than sequentially through lungs and body.
This is the fundamental physiological abnormality of d-TGA.
Patent Ductus Arteriosus — PDA
Persistence after birth of the foetal communication between the pulmonary artery and aorta.
In TGA it can contribute to mixing between the two circulations.
Patent Foramen Ovale — PFO
Persistence of the foetal atrial communication after birth.
In TGA it can provide an essential route for mixing.
Prostaglandin E1 — PGE1
A medication used to maintain ductal patency in duct-dependent congenital heart disease.
In TGA it may improve mixing between the two parallel circulations.
Pulmonary Vascular Resistance — PVR
Resistance to blood flow through the pulmonary circulation.
Rashkind Procedure
See Balloon Atrial Septostomy.
Senning Procedure
An atrial-switch operation using the patient’s own atrial tissue to redirect systemic and pulmonary venous blood.
Systemic Right Ventricle
A morphological right ventricle supporting the systemic circulation.
This occurs after Mustard or Senning atrial-switch repair and is an important source of long-term complications.
Transposition of the Great Arteries — TGA
A congenital abnormality in which the ventricular connections of the aorta and pulmonary artery are reversed.
Ventricular Septal Defect — VSD
An abnormal communication between the ventricles.
In TGA it may improve mixing but can also alter haemodynamics and surgical management.
