Pediatric “5 Ts” (Cyanotic CHD) #2: Transposition of the Great Arteries — “Two Vessels Switched”

1. Overview

D-transposition of the great arteries (d-TGA) is a cyanotic congenital heart defect characterized by ventriculoarterial discordance: the aorta arises from the morphologic right ventricle (RV) and the pulmonary artery arises from the morphologic left ventricle (LV).

In the usual form of d-TGA, the atrioventricular connections remain concordant:

• Right atrium → tricuspid valve → RV → aorta
• Left atrium → mitral valve → LV → pulmonary artery

The essential physiologic consequence is that the pulmonary and systemic circulations are arranged in parallel rather than in series. Oxygenated pulmonary venous blood tends to recirculate through the lungs, while deoxygenated systemic venous blood returns repeatedly to the systemic circulation. Survival therefore depends on communication and mixing between these two circuits.

The arterial switch operation (ASO) has become the standard anatomic repair for most patients with d-TGA, replacing historical atrial switch procedures and allowing the morphologic LV to function as the systemic ventricle. Contemporary survival is excellent, but coronary anatomy, associated intracardiac lesions, and late neoaortic and coronary complications remain important determinants of management and lifelong surveillance [1].

2. Segmental Anatomy

The defining anatomic feature of d-TGA is ventriculoarterial discordance.

Systemic pathway

Systemic venous blood follows:

Systemic veins → RA → RV → Ao → systemic circulation

Thus, relatively deoxygenated systemic venous blood is returned directly to the body.

Pulmonary pathway

Pulmonary venous blood follows:

Pulmonary veins → LA → LV → PA → pulmonary circulation

Thus, highly oxygenated pulmonary venous blood is recirculated through the lungs.

This differs fundamentally from the normal circulation, in which the pulmonary and systemic circuits are connected sequentially.

Although the aorta is commonly anterior and rightward relative to the pulmonary artery in classic d-TGA, the precise spatial relationship of the great arteries can vary. More important surgically are the relationships among the semilunar valves, coronary ostia, ventricular septum, pulmonary outflow tract, and aortic arch.

Associated lesions—including VSD, LV outflow tract obstruction (LVOTO), coarctation, arch hypoplasia, and coronary anomalies—convert a physiologically simple transposition into a substantially more complex surgical problem.

3. Parallel Circulation: The Central Physiologic Concept

Normal circulation is arranged in series:

Systemic veins → right heart → lungs → left heart → systemic circulation

In d-TGA, two parallel loops are created.

Systemic circuit

Body → RA → RV → Ao → body

Pulmonary circuit

Lungs → LA → LV → PA → lungs

Without communication between these circuits, adequate systemic oxygen delivery is impossible.

Therefore, the severity of cyanosis in d-TGA is determined primarily by the adequacy of effective mixing, rather than simply by the amount of pulmonary blood flow. This distinguishes TGA from lesions such as tetralogy of Fallot, in which cyanosis is often predominantly related to restriction of pulmonary blood flow and right-to-left shunting.

A neonate with TGA may have substantial pulmonary blood flow and well-oxygenated pulmonary venous blood but remain profoundly cyanotic because that oxygenated blood cannot adequately enter the systemic circulation.

This makes neonatal d-TGA fundamentally a mixing emergency.

4. Sites of Mixing

Mixing between the pulmonary and systemic circulations can occur at three major levels:

1. Atrial level — PFO or ASD
2. Ventricular level — VSD
3. Great-artery level — PDA

The physiologic contribution of each communication depends on its size and resistance to flow, as well as the pulmonary and systemic vascular resistances.

Atrial-Level Mixing

The atrial septum is often the most clinically important site of mixing in the early neonatal period.

Pulmonary venous blood returning to the LA must gain access to the right-sided circulation to reach the aorta. A sufficiently large PFO or ASD permits effective bidirectional exchange between the parallel circuits.

A restrictive atrial communication can therefore be immediately life-threatening. In one review of neonates anatomically suitable for ASO who died before surgery, all 12 preoperative deaths occurred in patients with TGA with intact ventricular septum and severely restrictive PFOs; inadequate mixing with profound hypoxemia was considered responsible for 11 deaths [2].

Thus, a small interatrial communication should not be regarded simply as an anatomic finding. In TGA, it may represent a critical obstruction to systemic oxygen delivery.

Ventricular-Level Mixing

A VSD creates an additional pathway for communication between the two circuits.

A larger VSD can improve mixing and increase systemic arterial oxygen saturation. However, as pulmonary vascular resistance decreases after birth, a large VSD may also permit substantial pulmonary blood flow and lead to pulmonary overcirculation.

Consequently, infants with TGA and a large VSD may present with less dramatic cyanosis but subsequently develop:

• Tachypnea
• Pulmonary congestion
• Feeding intolerance
• Poor weight gain
• Heart failure

The presence of a VSD also has major operative implications because its location, multiplicity, relationship to the outflow tracts, and association with LVOTO may determine the definitive surgical pathway.

Ductal-Level Mixing

A PDA provides communication between the pulmonary artery and aorta. Maintaining ductal patency can therefore contribute to pulmonary–systemic mixing.

However, ductal patency alone does not guarantee adequate systemic oxygenation. A critically restrictive atrial septum may remain physiologically limiting even when the ductus is widely patent.

5. Postnatal Transition and Clinical Deterioration

The physiology of d-TGA can change rapidly during the first hours after birth.

With normal neonatal transition:

• Pulmonary vascular resistance falls.
• Pulmonary blood flow increases.
• Pulmonary venous return to the LA increases.
• The ductus arteriosus begins to constrict.
• The foramen ovale may become functionally restrictive.

As a result, a neonate who initially appears relatively stable may rapidly develop profound cyanosis when intercirculatory mixing becomes inadequate.

Supplemental oxygen cannot correct the fundamental circulatory arrangement. Even if pulmonary venous oxygen content increases, oxygenated blood remains preferentially confined to the pulmonary circuit unless it can cross into the systemic circulation.

The clinical question is therefore not simply, “How low is the saturation?” but rather, “Is mixing sufficient to maintain systemic oxygen delivery?”

Perfusion, lactate, acid-base status, hemodynamic stability, and echocardiographic assessment of the atrial communication must be interpreted together.

6. Initial Stabilization

Preoperative management has one principal physiologic objective:

Maintain adequate communication between the systemic and pulmonary circulations until definitive repair.

Prostaglandin E1

Prostaglandin E1 (PGE1) is commonly used to maintain or reopen the ductus arteriosus and thereby promote additional intercirculatory mixing [3].

Its benefit is physiologically plausible and supported by extensive clinical experience, although the evidence base is predominantly observational rather than randomized. In a European multi-institutional series of 613 patients undergoing ASO, 69% received prostaglandin before surgery [4].

Importantly, PGE1 should not be regarded as universally sufficient treatment. In the preoperative mortality series described above, every patient received PGE1, yet severe restriction of the atrial communication resulted in persistent inadequate mixing [2].

Furthermore, contemporary management does not require routine continuation of PGE1 in every stable neonate if adequate mixing is already present and early definitive repair is planned [1].

7. Balloon Atrial Septostomy

When atrial-level mixing is inadequate, balloon atrial septostomy (BAS) is the principal catheter-based intervention.

The catheter is advanced through the PFO into the LA, the balloon is inflated, and the balloon is withdrawn forcefully across the atrial septum to enlarge the communication by disrupting the septum primum.

The purpose is not to correct ventriculoarterial discordance. Rather, BAS converts a restrictive atrial communication into a low-resistance pathway that permits more effective mixing between pulmonary venous and systemic venous blood.

BAS is particularly indicated when there is:

• Echocardiographic evidence of a restrictive atrial septum
• Persistent severe hypoxemia
• Metabolic acidosis
• Poor systemic perfusion
• Hemodynamic instability despite medical stabilization

A contemporary perioperative strategy favors BAS when both restrictive atrial anatomy and clinically inadequate oxygenation or instability are present, rather than routine septostomy in every patient [3].

In the preoperative mortality series, urgent BAS was performed in approximately two-thirds of critically affected infants and improved oxygenation, emphasizing the importance of rapid recognition of inadequate mixing [2].

Thus:

Restrictive atrial communication + persistent hypoxemia or instability → urgent BAS

A stable neonate with adequate atrial mixing, by contrast, may proceed directly to early ASO without routine BAS [1].

8. Timing of Definitive Repair and LV Conditioning

For d-TGA with intact ventricular septum, ASO is generally performed during the first week of life after appropriate stabilization [3,5].

This timing is related not only to the urgency of cyanosis but also to the physiology of the LV.

Before birth, the LV pumps against high fetal pulmonary vascular resistance and is capable of generating systemic-level pressure. After birth, pulmonary vascular resistance falls rapidly. Because the LV in unrepaired TGA ejects into the pulmonary circulation, its afterload decreases and the ventricle progressively loses the mass and geometry required to sustain systemic pressure.

Early ASO therefore takes advantage of a systemically conditioned neonatal LV.

Delayed presentation introduces concern for LV deconditioning. Primary ASO can still be successful beyond the usual neonatal period in selected patients, but later repair is associated with greater physiologic risk and may require more intensive perioperative support, including mechanical circulatory support in some cases [5].

9. Coronary Anatomy: A Central Surgical Determinant

Coronary transfer is one of the technically most important components of the ASO.

The coronary ostia are excised from the native aortic root and reimplanted into the neoaortic root. Successful transfer requires:

• Adequate coronary mobilization
• Tension-free implantation
• Avoidance of kinking or torsion
• Preservation of ostial geometry
• Avoidance of external compression

Historical surgical series established coronary anatomy as an important determinant of operative risk.

In a series of 432 neonates undergoing anatomic correction, coronary anatomy was the principal determinant of operative survival on univariate analysis [6]. A multi-institutional study of 513 neonates similarly identified specific coronary patterns—particularly configurations involving abnormal origin and an intramural course—as important risk factors [7].

Single coronary and intramural coronary patterns remain particularly important because coronary transfer may require individualized reconstruction [1,5].

Postoperatively, unexplained ventricular dysfunction, ischemic electrocardiographic changes, ventricular arrhythmias, rising lactate, or low cardiac output should therefore raise concern for compromised coronary perfusion.

10. Arterial Switch Operation

The ASO restores the pulmonary and systemic circulations to a normal series configuration.

After repair:

RA → RV → PA → lungs → LA → LV → Ao → body

The morphologic LV becomes the systemic ventricle, while the RV supports the pulmonary circulation.

Major components include:

1. Transection of the aorta and pulmonary trunk
2. Excision and mobilization of the coronary buttons
3. Coronary transfer to the neoaortic root
4. Reconstruction of the neoaorta
5. Repositioning of the pulmonary arteries, commonly using the LeCompte maneuver
6. Reconstruction of the neopulmonary root and pulmonary artery
7. Closure of associated VSD when required
8. Concomitant arch reconstruction when indicated

The operation therefore corrects the fundamental ventriculoarterial discordance rather than simply creating additional sites of mixing.

11. TGA With VSD and Aortic Arch Disease

Associated lesions substantially increase the complexity of TGA repair.

In the European multi-institutional experience, 30% of 613 patients undergoing ASO had a VSD. Overall operative mortality was higher in patients with VSD than in those with intact ventricular septum, and associated arch pathology and coronary anomalies were also identified as risk factors on univariate analysis [4].

Similarly, the neonatal series of 432 patients demonstrated progressively greater operative complexity when VSD and coarctation accompanied TGA [6].

When anatomy is suitable, VSD closure and arch reconstruction can be incorporated into the same definitive neonatal operation. The important principle is that associated lesions must be understood as part of the entire ventricular–arterial and systemic-outflow anatomy rather than as isolated abnormalities.

12. TGA With VSD and LV Outflow Tract Obstruction

TGA with VSD and significant LVOTO represents a distinct surgical subgroup.

A conventional ASO may not be appropriate when the pulmonary root—the future systemic outflow after an arterial switch—is significantly obstructed.

The definitive strategy depends on:

• VSD location and size
• Relationship of the VSD to the great arteries
• Severity and mechanism of LVOTO
• Pulmonary valve anatomy
• Ventricular geometry
• Coronary anatomy

Potential strategies include an ASO-based repair when obstruction is amenable to relief, intraventricular rerouting with an RV–PA conduit such as the Rastelli procedure, or an aortic root translocation strategy such as the Nikaidoh procedure in selected anatomy.

A series examining TGA/VSD/LVOTO found that individualized selection of repair according to anatomy produced favorable functional outcomes, but patients undergoing intraventricular rerouting experienced recurrent RV outflow tract interventions related to conduit or RVOT dysfunction [8].

Thus, there is no single universal operation for TGA/VSD/LVOTO. Anatomy determines the pathway.

13. Early Postoperative Management

After ASO, postoperative management focuses on maintaining adequate cardiac output while identifying complications related to myocardial protection, coronary transfer, and reconstructed outflow tracts.

Important surveillance includes:

• LV and RV systolic function
• Coronary perfusion
• Systemic arterial pressure and pulse pressure
• Lactate and systemic perfusion
• Neoaortic valve function
• Main and branch pulmonary artery gradients
• Residual intracardiac shunts
• Rhythm and conduction
• Pulmonary vascular resistance

Inotropic support and afterload reduction may be required during early myocardial recovery [5].

Particular vigilance is required when ventricular dysfunction is unexpected or disproportionate, because postoperative dysfunction or arrhythmia may represent coronary insufficiency rather than nonspecific myocardial stunning [1].

14. Long-Term Outcomes and Surveillance

Long-term survival and quality of life after contemporary ASO are generally excellent [1,5].

Nevertheless, ASO should be understood as an anatomic correction that creates new surgical interfaces and long-term surveillance requirements rather than a procedure that eliminates all future cardiovascular risk.

Major late concerns include:

• Coronary artery obstruction or ischemia
• Neoaortic root dilation
• Neoaortic valve regurgitation
• Supravalvar or branch pulmonary artery stenosis
• Ventricular dysfunction
• Arrhythmias
• Need for cardiovascular reintervention
• Neurodevelopmental and exercise-related limitations

Coronary insufficiency and clinically important arrhythmias are relatively uncommon after ASO but may have serious consequences. Neoaortic root dilation and regurgitation also require serial assessment as the ASO population ages [1].

Lifelong congenital cardiology surveillance is therefore necessary even in asymptomatic patients with apparently excellent repair.

15. Clinical Decision-Making Framework

A practical approach to neonatal d-TGA can be organized around four questions.

1. Is systemic oxygen delivery adequate?

Assess:

• Oxygen saturation
• Perfusion
• Lactate
• Acid-base status
• Hemodynamic stability

2. Is mixing adequate?

Evaluate communication at:

• PFO/ASD
• VSD
• PDA

A restrictive atrial communication associated with persistent hypoxemia or instability requires urgent attention.

3. Is the LV prepared for systemic work?

In the typical neonate, early ASO preserves the normally conditioned LV. Delayed presentation requires specific assessment of LV preparedness and increases operative complexity [5].

4. What anatomy determines the operation?

Define:

• Coronary origins and proximal course
• Ventricular septal anatomy
• Presence and mechanism of LVOTO
• Pulmonary valve and root anatomy
• Aortic arch anatomy
• Associated congenital lesions

These findings determine whether the patient undergoes a straightforward ASO, ASO with VSD/arch reconstruction, or an alternative strategy for TGA with VSD/LVOTO.

16. Key Surgical and Physiologic Concept

The central principle of d-TGA is:

The primary problem is not simply insufficient pulmonary blood flow; it is separation of the systemic and pulmonary circulations into two parallel circuits.

Therefore:

TGA → parallel circulation → survival depends on mixing → stabilize mixing → restore circulation in series

The practical sequence is:

Recognize TGA → assess systemic oxygen delivery → evaluate atrial/ventricular/ductal mixing → maintain ductal patency when useful → perform BAS when atrial mixing is restrictive and clinically inadequate → define coronary and associated anatomy → perform early definitive repair.

Most neonates with uncomplicated d-TGA can proceed to early ASO with excellent outcomes. The major deviations from this straightforward pathway are produced by restrictive atrial mixing, delayed presentation with LV deconditioning, complex coronary anatomy, VSD/LVOTO, and aortic arch disease.

This framework links the anatomy directly to physiology and surgical decision-making: mixing determines immediate survival, while ventricular conditioning and associated anatomy determine the definitive operative strategy.

References

[1] Villafane J, Lantin-Hermoso MR, Bhatt AB, Tweddell JS, Geva T, Nathan M, et al. D-transposition of the great arteries: the current era of the arterial switch operation. Journal of the American College of Cardiology. 2014. doi:10.1016/j.jacc.2014.06.1150.

[2] Soongswang J, Adatia I, Newman C, Smallhorn JF, Williams WG, Freedom RM. Mortality in potential arterial switch candidates with transposition of the great arteries. Journal of the American College of Cardiology. 1998. doi:10.1016/S0735-1097(98)00310-6.

[3] Lorts A, Krawczeski CD. Perioperative care of a child with transposition of the great arteries. Current Treatment Options in Cardiovascular Medicine. 2011. doi:10.1007/s11936-011-0138-5.

[4] Sarris GE, Chatzis AC, Giannopoulos NM, Kirvassilis G, Berggren H, Hazekamp M, et al. The arterial switch operation in Europe for transposition of the great arteries: a multi-institutional study from the European Congenital Heart Surgeons Association. Journal of Thoracic and Cardiovascular Surgery. 2006. doi:10.1016/j.jtcvs.2006.01.065.

[5] Benscoter A, Ryan A, Tweddell J. Transposition of the great arteries and the arterial switch operation. Critical Heart Disease in Infants and Children. 2019. doi:10.1016/B978-1-4557-0760-7.00057-7.

[6] Serraf A, Lacour-Gayet F, Bruniaux J, Touchot A, Losay J, Comas J, et al. Anatomic correction of transposition of the great arteries in neonates. Journal of the American College of Cardiology. 1993. doi:10.1016/0735-1097(93)90834-N.

[7] Kirklin JW, Blackstone EH, Tchervenkov CI, Castaneda AR. Clinical outcomes after the arterial switch operation for transposition: patient, support, procedural, and institutional risk factors. Circulation. 1992. doi:10.1161/01.CIR.86.5.1501.

[8] Bierbach B, Arenz C, Suchowerskyj P, Schroth S, Blaschczok J, Asfour B, et al. Current mid-term outcome with an integrated surgical strategy for correction of d-transposition of the great arteries with ventricular septal defect and left ventricular outflow tract obstruction. European Journal of Cardio-Thoracic Surgery. 2016. doi:10.1093/EJCTS/EZW058.