Transposition of the Great Arteries (TGA)

Transposition of the great arteries (TGA) is the tenth most common congenital heart lesion overall and the second most common cyanotic lesion after tetralogy of Fallot, with an estimated incidence of roughly 31.5 per 100,000 live births.

A Terminology Point Worth Getting Right: Avoid “D-TGA”

This is worth addressing directly, since the term is genuinely common but imprecise. “D-TGA” is explicitly described in current literature as a nonspecific term that’s better avoided. The letter “D” can refer to two different things that don’t always coincide: the ventricular loop (D-loop vs. L-loop, a description of how the embryonic heart tube looped during development) and the spatial position of the aortic valve relative to the pulmonary valve. When precision about looping specifically is needed, “D-loop TGA” is the more accurate term.

The actual definition of TGA is segmental, not positional: it’s ventriculoarterial discordance — the aorta arising from the morphologic right ventricle, the pulmonary artery from the morphologic left ventricle — occurring together with atrioventricular concordance (right atrium to right ventricle, left atrium to left ventricle, as in a normal heart). Earlier definitions emphasized the abnormal position of the great arteries instead, which had the effect of pulling other, genuinely different malformations (like double-outlet right ventricle) into the “transposition” category — exactly the kind of ambiguity the connection-based definition avoids. Defined this way, TGA can occur with either usual (solitus) or mirror-image (inversus) atrial arrangement, but not with atrial isomerism.

This distinction matters clinically, not just semantically: it’s what separates TGA from congenitally corrected transposition (ccTGA), which combines ventriculoarterial discordance with atrioventricular discordance as well (double discordance) — a genuinely different lesion, with a different physiology (the double discordance restores a physiologically “corrected” flow pattern), not simply a variant of TGA.

The Spatial Relationship: Real, But Not Definitional

The aorta’s position relative to the pulmonary artery is worth documenting precisely, but — per the point above — isn’t what defines the lesion. In usual (solitus) atrial arrangement, the aorta is most often anterior and rightward relative to the pulmonary artery, though directly anterior or even anterior-and-leftward relationships occur; rarely, the aorta sits posterior and rightward (the so-called “normal” relationship, in the sense that this particular spatial arrangement resembles the normal heart’s, even though the connections themselves are still transposed). In mirror-image atrial arrangement, the aorta is most often left-sided and anterior. Similarly, the usual infundibular pattern (a subaortic conus, and fibrous continuity between the pulmonary and mitral valves) is common but not a defining feature either — bilateral infundibula, and the reverse arrangement (fibrous continuity between the aortic and mitral valves) in “normal relationship” hearts, both occur.

Epidemiology and Etiology

TGA affects males more than twice as often as females — a notably different sex distribution from lesions like patent ductus arteriosus, which skews female. Chromosomal anomalies are notably uncommon in TGA — a genuine point of contrast with tetralogy of Fallot and truncus arteriosus, where 22q11 deletion is common enough to warrant active screening; in TGA, 22q11 deletion is distinctly rare. Familial recurrence does occur (roughly 10% of patients have a relative with some form of congenital heart disease), rare single-gene mutations have been identified, and some families show a mix of TGA, heterotaxy syndrome, and ccTGA linked to laterality-gene mutations. Recognized environmental associations include maternal diabetes, gestational retinoic acid exposure, in vitro fertilization, certain pesticide exposures, and some antiepileptic medications — none of which are individually strong predictors, but worth knowing as a risk factor profile distinct from the conotruncal-defect genetic pattern.

Anatomy Relevant to Surgical Timing

The ventricular septum in most hearts with TGA is straight, lacking the curvature typical of a normal heart — reflecting parallel, rather than crossing, right and left ventricular outflow tracts. The right ventricular wall thickens rapidly relative to the left in the immediate postnatal period in patients with an intact ventricular septum, since the RV — not the LV — is doing systemic work. This has a direct surgical consequence: an arterial switch operation requires the LV to take over systemic pumping duty immediately afterward, and if it has “deconditioned” too far from supporting only the low-pressure pulmonary circuit, it may not tolerate the abrupt systemic workload — this is why timing of the arterial switch matters so much in the first weeks of life. In roughly 90% of cases, a subaortic conus is present together with absence of a subpulmonary conus, producing fibrous continuity between the mitral and pulmonary valves — essentially the reverse arrangement from a normal heart.

Coronary Artery Anatomy: A Genuine Risk-Stratifying Feature

Coronary pattern in TGA is described with the Leiden classification, naming the aortic sinuses relative to an observer positioned in the non-facing (noncoronary) sinus looking toward the pulmonary trunk — one sinus always falls to that observer’s right (sinus 1), the other to their left (sinus 2), regardless of the great arteries’ spatial relationship to one another. This convention is genuinely convenient for a surgeon looking down at the valve, but can be disorienting for an echocardiographer viewing from the opposite perspective — worth being aware of directly rather than assuming the labeling will feel intuitive on echo. The “usual” pattern (LAD and circumflex from the left-facing sinus, RCA from the right-facing sinus) occurs in roughly 65% of cases; a single left coronary artery, inverted patterns, and partially inverted patterns each account for a further few percent.

This isn’t merely descriptive — pooled data across multiple studies link intramural coronary arteries or a single coronary artery pattern specifically to increased mortality risk during the arterial switch operation, making preoperative coronary mapping a genuine risk-stratification exercise, not just a surgical planning convenience.

Associated Anomalies

• Ventricular septal defect — commonly cited around 40–50% of cases, most often from malalignment of the outlet septum (deviated into the right ventricle in this setting, letting the pulmonary valve override) — see Ventricular Septal Defect for the isolated-lesion anatomy this parallels. With greater degrees of override, this anatomy merges into the double-outlet right ventricle spectrum with a subpulmonary VSD (the Taussig-Bing anomaly).
• Left or right ventricular outflow tract obstruction, from a range of mechanisms including abnormal mitral valve attachment to the outlet septum or discrete fibrous tissue tags.
• Aortic arch anomalies — hypoplasia, coarctation, interruption, or (rarely) a double aortic arch.
• Anomalous systemic and pulmonary veins (persistent left SVC; partial or total anomalous pulmonary venous connection), atrioventricular canal defect, and AV valve anomalies (stenosis, a cleft leaflet, abnormal chordal attachments, or straddling) are all recognized, less common associations.
• Leftward juxtaposition of the right atrial appendage occurs in roughly 2% of cases, usually alongside other complex findings such as dextrocardia or tricuspid atresia.

Pathophysiology

The defining physiologic problem is that systemic and pulmonary circulations run in parallel rather than in series: deoxygenated systemic venous blood returns via the right atrium and ventricle straight back out to the aorta, while oxygenated pulmonary venous blood returns via the left heart straight back to the pulmonary artery — each circuit recirculating its own blood rather than exchanging with the other. Survival depends entirely on mixing between the two circuits, through an ASD/PFO, VSD, and/or PDA; without adequate mixing, severe hypoxemia and acidosis develop rapidly.

Clinical Findings

Presentation is typically in the immediate neonatal period with cyanosis. A systolic murmur at the left sternal border (often from an associated VSD) and a diastolic flow murmur at the apex (from increased mitral inflow with significant mixing) may be present. ECG shows right axis deviation and RV hypertrophy, reflecting the RV’s systemic workload. Chest X-ray classically shows cardiomegaly with a narrow, oval cardiac silhouette (the “egg on a string” appearance) and increased pulmonary vascular markings.

Echocardiographic Findings

Echocardiography is the primary imaging modality for TGA — most patients are referred for surgery based on echocardiographic diagnosis alone, and it’s also typically used to guide balloon atrial septostomy when needed. A complete study should prioritize: segmental anatomy; systemic and pulmonary venous connections; the atrial septum (location, size, and flow direction/velocity of any interatrial communication, with mean transatrial gradient by CW Doppler); atrial appendage juxtaposition; AV valve morphology (cleft, straddling, abnormal attachments) and function; biventricular size and function; and, when a VSD is present, its type, size, and whether single or multiple.

A Systematic, View-by-View Approach

• Subxiphoid views are the primary window and allow rapid, accurate diagnosis. After confirming abdominal situs, a sweep in the subxiphoid long-axis (frontal) plane from posterior to anterior directly demonstrates the ventriculoarterial discordance: the main pulmonary artery is seen arising from the LV and bifurcating first, and as the transducer tilts further anteriorly, the aorta comes into view arising from the rightward, superior aspect of the RV. Coronary artery origins from the aortic root are often visible from this same window, helping confirm which vessel is the aorta. The great arteries are seen running in parallel rather than crossing as in a normal heart, and this view can also reveal atrial appendage juxtaposition. Rotating the transducer roughly 90° clockwise brings up the subxiphoid short-axis (oblique sagittal) plane, swept from the bicaval view right through to the level of the ventricular apex — this shows the atrial septum, AV valve attachments, and both ventricular outflow tracts, and again demonstrates the parallel great arteries clearly.

  Determining the adequacy of the interatrial communication can be a genuine clinical urgency, since it directly informs the need for balloon atrial septostomy — subxiphoid sweeps are the best view for localizing and sizing this communication. Adequacy is judged by a combination of size and the infant’s pulse oximetry/PO₂, not size alone, and preoperative atrial-level shunting is almost always left-to-right.

• Apical views assess ventricular inflow and outflow and the ventricular septum, and are ideal for AV valve stenosis or regurgitation by color and spectral Doppler, as well as providing a good Doppler angle for LVOT obstruction assessment. Careful interrogation of the apical septum specifically is important to exclude muscular VSDs — a genuinely easy region to miss. Worth knowing as a view-selection guide: the apical view best highlights the basal (inlet) and apical septal segments, but doesn’t optimally show the anterior-superior septum — for that region, and for the posterior-inferior and mid-septal segments, the subxiphoid and parasternal short-axis views are more helpful.

• Parasternal long-axis shows both great arteries longitudinally in the same plane as they dive posteriorly — a parallel arrangement worth recognizing as not unique to TGA; double-outlet RV can produce the same appearance, so this finding alone doesn’t clinch the diagnosis. This view also shows fibrous continuity between the mitral and pulmonary valves when present (confirming absence of a subpulmonary conus), the aortic valve’s anterior position supported by its infundibulum, and can suggest left atrial appendage juxtaposition (seen as an oval structure between the main pulmonary artery and left atrium).

• Parasternal short-axis shows the spatial relationship between the semilunar valves — most commonly aortic anterior and rightward relative to pulmonary, though side-by-side, directly anterior, or leftward relationships all occur. A high left parasternal position often optimizes this view. Where obtainable, this view lets you assess commissural alignment between the aortic and pulmonary valve leaflets — genuinely important for planning coronary artery transfer during the arterial switch operation, not just a descriptive detail. This view can’t always be obtained, particularly when the aortic valve sits more superior relative to the pulmonary valve than usual. Biventricular size, wall thickness, and systolic/diastolic function should be assessed across multiple views; systolic function is usually preserved in infants with TGA unless significant hypoxia or metabolic acidosis is present. 3D echocardiography doesn’t add a great deal to preoperative assessment in TGA with an intact ventricular septum specifically, though it may add more value when a VSD is present.

• Suprasternal views assess ductal patency and aortic arch anatomy, relevant given the arch anomalies discussed above.

VSD Imaging in TGA: A Genuine Technical Limitation Worth Knowing

VSD location and type in TGA follow the same classification used for concordant hearts, but the parasternal short-axis view — normally excellent for VSD visualization — is specifically less useful in TGA, precisely because of the abnormal great artery relationship. Multiple views are needed instead: subxiphoid long- and short-axis, apical, parasternal short-axis, and tilted long-axis planes together, rather than relying on any single plane. Lowering the color Doppler Nyquist limit is genuinely important for delineating VSDs in TGA, given how often LV and RV pressures are equal or near-equal in this condition — a standard velocity scale can otherwise fail to show the shunt at all. Defects with anterior malalignment of the outlet (conal) septum are usually visible in most planes, but the subxiphoid short-axis view is particularly well suited to characterizing the displaced conal septum and any associated RV outflow obstruction, and is also used to measure the obstruction gradient by PW and CW Doppler. RV outflow tract narrowing found this way should prompt a careful look at the aortic arch specifically — the two findings are worth checking together rather than in isolation.

How to Diagnose TGA by Echo

The diagnosis rests on one finding — ventriculoarterial discordance with atrioventricular concordance — and everything else on the exam either confirms it, excludes a look-alike, or answers the questions surgery will ask. Because echocardiography is the imaging modality of choice, and most patients go to surgery on echo alone, the diagnostic study has to be complete.

The Core Findings

Question	What confirms TGA	Best view
Are the AV connections concordant?	Right atrium to tricuspid valve to morphologic RV; left atrium to mitral valve to morphologic LV	Subxiphoid, apical four-chamber
Which vessel arises from the LV?	The pulmonary artery — it bifurcates into left and right branches	Subxiphoid long-axis sweep
Which vessel arises from the RV?	The aorta — its root gives rise to the coronary arteries	Subxiphoid long-axis sweep, parasternal short-axis
Do the great arteries cross?	No — they run in parallel	Subxiphoid short-axis, parasternal long-axis
Is there mixing?	An interatrial communication, a VSD, and/or a PDA	Subxiphoid, suprasternal, high left parasternal

A Practical Sequence

1. Establish situs and the venous connections from the subxiphoid window, starting with the abdominal great vessels, then the atrial arrangement, atrial septum, and any juxtaposition of the atrial appendages.
2. Confirm AV concordance by identifying each ventricle by its own morphology rather than its position — see Congenitally Corrected Transposition for the features that tell an RV from an LV, and for the look-alike lesion where the connections are discordant at both levels.
3. Sweep the subxiphoid long-axis plane from posterior to anterior. The first vessel seen arises directly from the LV and bifurcates — the pulmonary artery. Tilting further anteriorly shows the second vessel arising from the rightward, superior aspect of the RV — the aorta. This sweep is itself the diagnosis.
4. Confirm which vessel is which by what it does, not where it sits. The pulmonary artery bifurcates; the aortic root gives rise to the coronary arteries. The arch can then be followed from the suprasternal notch.
5. Document the relationship and the outflow anatomy. The great arteries run in parallel; the semilunar valve relationship on parasternal short-axis is usually aortic anterior and rightward but is variable (see above). Look for mitral–pulmonary fibrous continuity, which shows there is no subpulmonary conus.
6. Look for obstruction of the LV (pulmonary) and RV (aortic) outflow tracts, including abnormal mitral valve attachments to the outlet septum or tissue tags, and screen the arch for coarctation or interruption.
7. Define how the circulations mix. Size the atrial communication, show its direction and mean gradient, and look for a VSD (type, number, size) and a PDA.
8. Map the coronary arteries (below).
9. Assess ventricular function, AV valve morphology (cleft, straddling, abnormal attachments), and — for a patient presenting after about 2 months of age — LV size, wall thickness, and mass, since the LV loses its suitability to support the systemic circulation as it deconditions. Also note any elevation of pulmonary vascular resistance.

Imaging the Coronary Arteries: A Specific Technique

The views below apply the general approach described in Coronary Arteries, which also covers the pitfalls of coronary imaging (dropout, still frames, and linear mimics).

• Parasternal short-axis view. For the left coronary artery, rotate slightly clockwise to a transverse view with the index mark at about 3 o’clock, which shows its origin and proximal course. For the right coronary artery, rotate slightly counterclockwise, with the index mark at about 1 o’clock.
• Parasternal long-axis view through the aortic root. In usual coronary anatomy, this shows the origin of the RCA from the posterior sinus. Sweeping toward the left shoulder shows the LCA bifurcating into the LAD and circumflex.
• Course relative to the great arteries. When the circumflex arises from the right posterior-facing sinus (sinus 2), its posterior course is seen on short-axis and confirmed on the apical four-chamber and subxiphoid views. An anterior course, or combined anterior and posterior courses, occurs in single-coronary and inverted patterns.
• Color Doppler. Lowering the Nyquist limit lets coronary flow toward the myocardium be seen.
• Why it matters: an intramural coronary course raises operative risk for the arterial switch, so it must be identified beforehand.

Look-Alikes and Pitfalls

• Double-outlet right ventricle can produce the same parallel-vessel appearance on parasternal long-axis, so that finding alone doesn’t clinch TGA. What separates them is which ventricle each great artery is connected to — and with greater degrees of override, the two merge into a spectrum (the Taussig-Bing anomaly).
• Congenitally corrected transposition also has ventriculoarterial discordance; the difference is that its AV connections are discordant too.
• Position is not the definition. The aorta is usually anterior and rightward, but other relationships — including a posterior-and-rightward aorta that resembles a normal heart — occur in TGA. Rely on the connections.
• Small VSDs hide. The parasternal short-axis view is less helpful than usual in TGA, and muscular defects are easy to miss — sweep the whole septum, lower the Nyquist limit, and use several windows.
• Don’t stop at the diagnosis. A complete study for a surgeon includes the atrial communication, VSD, obstruction, arch, coronaries, and AV valves.

Prenatal Diagnosis

TGA can be diagnosed in the second trimester, and in some cases screening detects it as early as 13–14 weeks. A screening ultrasound based solely on the four-chamber view is inadequate — the four-chamber view may look normal in a fetus with TGA unless there is a large VSD. More cephalad views that include the outflow tracts show the parallel vessels, with the pulmonary artery arising from the LV and the aorta from the anterior, rightward aspect of the RV. Small VSDs, especially muscular ones, can be missed. If an anterior malalignment VSD is found, assess the arch for coarctation or interruption; with a posterior malalignment VSD, the main and branch pulmonary arteries may be small. Check the atrial septum for a restrictive foramen ovale or an intact septum, which can cause severe cyanosis at birth; a hypermobile septum may help predict the need for urgent balloon septostomy, although the accuracy of individual markers is limited.

Treatment Considerations

• The arterial switch operation (Jatene procedure) is now the standard repair — the great arteries are transected and reconnected to the correct ventricle, and the coronary arteries are reimplanted into the neo-aorta (the original pulmonary root). This restores normal physiologic flow, unlike the atrial-level repairs it replaced.
• Atrial switch procedures (Mustard or Senning) — historically the standard approach, redirecting blood at the atrial level via a baffle so that systemic venous return reaches the (still morphologically right) systemic ventricle, and pulmonary venous return reaches the (still morphologically left) subpulmonary ventricle. These fell out of favor because the right ventricle continues supporting the systemic circulation long-term — a workload it isn’t built for — leading to progressive systemic RV dysfunction, tricuspid (systemic AV valve) regurgitation, and arrhythmia over time. Most adults currently living with TGA had an atrial switch, since arterial switch didn’t become the standard until the late 1980s — an important context for interpreting an adult TGA patient’s anatomy and expected complications.
• The Rastelli procedure — for TGA with both a VSD and pulmonic stenosis, using an intracardiac patch to route LV output across the VSD to the aorta, combined with a valved conduit connecting the RV to the pulmonary artery.

Postoperative Follow-Up

After arterial switch: watch for supravalvular stenosis of either great vessel at the surgical anastomosis sites, and coronary-reimplantation-related complications (myocardial ischemia from a kinked or stenotic reimplanted coronary). A specific, well-documented late finding is dilation of the neo-aortic root — the original pulmonary root and valve, now functioning as the systemic aortic valve — with associated aortic regurgitation; see Aortic Regurgitation for the broader severity-assessment framework relevant here. Ventricular function (both ventricles) and coronary flow should be assessed at every follow-up study.

After atrial switch (Mustard/Senning): systemic (morphologic) RV function is the central long-term concern, alongside systemic AV (tricuspid) valve regurgitation and arrhythmia surveillance. The atrial baffle itself needs deliberate evaluation for leaks (residual interatrial shunting) and obstruction of either the systemic venous or pulmonary venous limb — both channels should be interrogated specifically with color and pulsed Doppler rather than assumed patent.

Clinical Importance

Getting the segmental definition right — ventriculoarterial discordance with atrioventricular concordance, not a description of vessel position — is what correctly separates TGA from its look-alike, ccTGA, and from other malformations that share only the “abnormally positioned great arteries” feature without the same connection pattern. See Tetralogy of Fallot and Truncus Arteriosus for the related conotruncal defects that, unlike TGA, carry a strong 22q11 deletion association worth actively screening for.