Abstract
Congenitally corrected transposition is a rare condition characterized by atrioventricular and ventriculoarterial discordance. This creates a physiologically “corrected” circulation but leaves the morphologic right ventricle (mRV) in the systemic position.
Management is dependent on associated lesions, the commonest of which is ventricular septal defect (VSD) and the presence or absence of pulmonary/subpulmonary obstruction. The wide spectrum of associated lesions is further complicated by the unpredictable behavior of the mRV and tricuspid valve in the systemic position—all of which combine to create a complex picture of variable presentations, age-groups, and management priorities, varying from neonatal intervention in pulmonary atresia to the occasional patient with no associated lesions who may remain symptom-free into old age.
Surgical management includes the following:
Physiologic repair: treatment of the associated lesions alone (e.g., VSD closure or tricuspid valve repair), leaving the mRV in the systemic position. There is risk of cardiac failure in the midterm to long term.
Anatomic repair: treatment of associated lesions but also switching the atrial and arterial pathways to restore the left ventricle (LV) to the systemic position. These are the “double-switch” procedures involving either an arterial switch or a Rastelli type of procedure. These are technically challenging but carry the best long-term freedom from heart failure.
Pulmonary artery banding: may be necessary to “retrain” the morphologic LV for subsequent double-switch procedure but also has a therapeutic role in splinting the interventricular septum and reducing tricuspid regurgitation. Controversy remains over the ideal age and duration of banding.
It is a fascinating condition with ongoing debate regarding the best way to manage symptomless patients (due to the unpredictable nature of the systemic mRV) and the long-term outcomes of anatomic repair.
Key Words
congenitally corrected transposition, arterial switch, atrial switch, Senning, Mustard, pulmonary artery banding, double-switch, Rastelli
This rare condition accounts for 0.5% of all congenital heart disease and is characterized by the combination of atrioventricular (AV) and ventriculoarterial (VA) discordance. This extraordinary and unique morphologic arrangement results in a physiologically “corrected” circulation in that the systemic venous blood passes into the lungs (but via a morphologic left ventricle [mLV]) and the pulmonary venous return is directed to the systemic circulation (via a morphologic right ventricle [mRV]). The condition was first described by Rokitansky in 1875 and is typified by levo – or l-transposition in which the transposed aorta sits anterior and to the left of the pulmonary artery (PA). In its pure form this is a corrected circulation and produces no symptoms, some patients living a completely normal life.
However, the condition is typified by the existence of associated defects such as ventricular septal defect (VSD) and outflow-tract obstruction (present in 85% of cases). Most cases require surgical treatment either to address these associated defects or due to the unpredictable performance of the mRV and the tricuspid valve within the systemic circulation.
Despite being a rare condition, there is a huge heterogeneity in the underlying morphology and the pattern of associated lesions, meaning that classification is complex and that there is a similar heterogeneity in terms of clinical presentation and symptoms.
Morphology and Anatomic Features
The full name of the condition is congenitally corrected transposition of the great arteries (ccTGA); there is AV and VA discordance, and the aorta is anterior and usually to the leftward side. There is usually normal atrial situs, and the equivalent Van Praagh classification is S, L, L. Although the majority have normal atrial situs, 5% to 8% of cases have situs inversus, which is much commoner than in most cardiac conditions.
Abnormal positioning of the heart is also common, with dextrocardia or mesocardia in 25% of cases. The AV valves always correspond to the ventricular morphology; thus the right atrium leads into the mLV through a mitral valve, and the left atrium leads into the mRV through a tricuspid valve. As a consequence, there is reverse offsetting of the AV valves on a four-chamber view because the tricuspid valve maintains its relationship of being slightly closer to the apex than the mitral valve. Furthermore, more exaggerated apical displacement of the septal leaflet of the tricuspid valve can be seen in ccTGA—so-called Ebsteinoid tricuspid valve (although not associated with the failed delamination or extreme leaflet anomalies seen in true Ebstein anomaly), which predisposes to tricuspid valve dysfunction.
The commonest associated defect is VSD, present in 85% of cases and usually perimembranous but variable in size. However, the most important classification is with respect to the presence or absence of left ventricular outflow tract obstruction (LVOTO)—either as pulmonary stenosis or atresia—which is almost always associated with a VSD. Approximately half of all cases will fall into this group and will therefore be cyanosed. There is considerable geographic variability in this feature, being much commoner in the Far East, whereas unobstructed LVOT is commoner in the Western hemisphere.
Coarctation and aortic arch hypoplasia occur in 10% of patients in the presence of VSD and unobstructed LVOT. A list of associated features is shown in Table 69.1 .
| Frequency (%) | |
|---|---|
| Atrial situs | |
| Normal | 80-85 |
| Inversus | 10-15 |
| Isomeric | 2-4 |
| Position | |
| Levocardia | 70-75 |
| Mesocardia | 10-15 |
| Dextrocardia | 15-20 |
| VSD | 70-80 |
| LVOTO | 40-80 |
| Pulmonary atresia | 3-8 |
| Arch hypoplasia/CoA | 5-15 |
| Ebsteinoid tricuspid valve | 10-20 |
| Heart block | 10-15 |
| DORV | 3-5 |
The conduction system and AV node are very abnormal in ccTGA. The AV node is displaced anteriorly, away from the triangle of Koch and near the root of the right atrial appendage. The bundle then runs a long and circuitous course anterior to the root of the pulmonary valve ( Fig. 69.1 ), usually running down the superior and lateral border of any perimembranous VSD. This long and abnormal course of the conduction system predisposes patients with ccTGA to heart block, some presenting with block at birth but up to 40% developing block as part of the natural history of the condition. Note, however, that in cases of situs inversus the morphology of the heart is now I, D, D, and the conduction system reverts to its normal position.
Physiology and Natural History
The broad spectrum of associated defects and the unpredictable performance of the systemic mRV means there is a wide range of ages and modes of presentation. It is best to consider patients in two groups, those with stenosis/atresia of the subpulmonary outflow and those with unobstructed pulmonary outflow.
Unobstructed Pulmonary Outflow
If there is no VSD, then these patients may be completely free of symptoms and may not require any intervention. However, most do have a VSD, and presentation is related to the size of the defect—large defects cause high-output congestive cardiac failure and usually present in infancy with respiratory distress and failure to thrive. Moderate-size defects may cause a lesser degree of heart failure. Up to 10% of these patients with VSD have associated coarctation and/or arch hypoplasia and present in the neonatal period with circulatory collapse when the ductus closes.
Pulmonary Stenosis/Atresia
These patients usually have an associated large VSD, and presentation depends on the degree of obstruction. Mild obstruction can cause no early symptoms and patients can be well balanced, analogous to an acyanotic Fallot. More severe forms of obstruction will cause an increasing degree of cyanosis with cases of pulmonary atresia being duct dependent neonates.
In addition to these two categories, patients may present with congenital heart block, which can be present at birth or develop insidiously during childhood. The behavior of the systemic mRV is very unpredictable but usually is normal during infancy. Even patients with no associated defects may develop mRV dysfunction and tricuspid regurgitation (TR) during childhood at virtually any age, with some only presenting in adulthood with exercise intolerance and breathlessness on exertion.
In view of this great heterogeneity in both anatomy and mRV function, the natural history of the condition is difficult to define. Unrestricted VSDs and pulmonary atresia are fundamentally life-threatening lesions, need early intervention, and so skew the natural history. However, even accounting for these cases, the fundamental question in ccTGA is the natural history of the systemic mRV, which is unpredictable from one individual to another but is generally much worse than the normal population. Even with no other significant defects, over half the population with ccTGA will develop congestive heart failure by their late 30s. The cause is a complex combination of systemic mRV failure and increasing TR, which are closely interlinked, and is related to the fact that the right ventricle (RV) is not an efficient shape to function in the systemic circulation and has a different coronary blood supply and the tricuspid valve is equally not designed to work at such pressures with its septal attachments, meaning that it is more prone to dysfunction as the ventricle dilates and the septum moves away from the free wall.
Despite the fact that the vast majority of patients with ccTGA will need surgical intervention, there remains a small group (probably <5%) that remains well with preserved mRV function into old age.
Diagnosis
The condition and all the associated lesions can usually be established by detailed transthoracic echocardiography. Each component of the anatomy and physiology should be carefully assessed on echocardiography, paying particular attention to the position of the cardiac chambers and to the function and morphology of the AV valves. The position of the VSD should be carefully delineated together with the positioning of a PA band and the presence of any associated pulmonary root dilation and/or pulmonary incompetence. A chest x-ray examination may reveal an abnormal position of the heart in some cases and the narrow superior mediastinum typical of transposition but is not diagnostic. An electrocardiogram is essential to look for rhythm abnormalities because first- and second-degree heart block may be present even in patients thought to have normal rhythm, and they are predictors of subsequent complete block.
Cardiac catheterization is not usually required for diagnosis but may be essential in assessing patients for surgery, especially if they had required a PA band (see later). Magnetic resonance imaging (MRI) is not necessary in most patients except when there is concern regarding the relationship of the aorta to the VSD in consideration of a Rastelli type of repair. Because the position and size of the VSD can be variable, MRI reconstructions and even three-dimensional modeling can be helpful to guide repair and surgical technique.
Indications for Surgery
Patients with pulmonary atresia or severe pulmonary stenosis require a systemic pulmonary shunt (usually a Blalock-Taussig shunt) as a neonate. Patients with a large VSD and unobstructed pulmonary blood flow may need PA banding to control heart failure. The central question in the management of ccTGA is the concept of “physiologic” verses “anatomic” repair. The former implies treating any associated lesions on their own merit but leaving the mRV as the systemic ventricle: this would include simple VSD closure or, in the setting of ccTGA/VSD/pulmonary stenosis (PS), closing the VSD and placing a valved conduit between the subpulmonary ventricle (the mLV) and the pulmonary arteries. Technically these are the simpler options, but they should be considered only if there is good mRV function and minimal TR. Anatomic repair involves repairing any associated lesions but also restoring the mLV to the systemic circulation—this requires an arterial switch (or Rastelli type of procedure in the setting of ccTGA/VSD/PS) together with an atrial switch (Senning or Mustard procedure).
Assessment and decision making become much more complicated in the scenario of a patient with ccTGA and intact septum (or small VSD) who develops mRV dysfunction and/or significant TR. These patients are likely to develop congestive cardiac failure within 5 years and need proactive intervention in the form of preemptive PA banding to retrain the mLV and splint the interventricular septum to preserve tricuspid valve function. If the mLV responds well to the band, then a double-switch procedure may be performed within the next 6 to 18 months. The age of these patients at presentation is crucial, and the younger the age, the more likely they are to respond to this retraining of the mLV. However, older children or adolescents may have lost the fundamental plasticity in ventricular remodeling to be able to respond to banding and may never be suitable for anatomic repair, being better diverted to management on a heart failure program and consideration for transplantation if required. A small subset of adolescents with well-preserved mRV function and moderate or greater TR may benefit from isolated tricuspid valve replacement but need careful surveillance of mRV function. Tricuspid valve repair in this setting has been universally disappointing, and direct replacement is recommended.
The justification for intervention in symptomless patients is a cause for considerable debate, and if there is well-preserved mRV function, then they should be left well alone and managed expectantly. A summary of the treatment decisions is shown in Fig. 69.2 .
Surgical Management
Systemic to Pulmonary Artery Shunt
Neonates or infants with severe cyanosis may require a systemic shunt. This is preferably performed through a midline sternotomy, and in the setting of normal situs we prefer to place a right modified Blalock-Taussig shunt because this is easy to access at subsequent surgery. If the arterial duct is still patent, then this is ligated at the same procedure to prevent competitive flow. A thin-walled expanded polytetrafluoroethylene (ePTFE [Gore-Tex]) tube is used as an end-to-side graft between the underside of the distal innominate artery and the superior surface of the right PA; a 3.5-mm shunt is usually used except in older children (>3 months), when a 4-mm shunt can be selected. The pericardium is loosely closed at the end of the procedure to aid with future resternotomy.
Physiologic Repair
Simple VSD closure can be performed in cases for which anatomic repair is not preferred, approaching the VSD through the right atrium and working through the mitral valve. Care has to be taken with suture placement to avoid the conduction tissue, which will run over the superior and around the lateral margin of the defect (see Fig. 69.1 ). Sutures in these areas should be placed from the right side of the defect (i.e., passing the needle holder inside the VSD with the needle placed in the mRV side) so as to avoid the bundle as it runs along the edge of the VSD on the mLV side of the septum. Physiologic repair can also be considered in cases of ccTGA/VSD/PS by simply closing the VSD and placing an mLV-PA conduit. Care has to be taken when performing the ventriculotomy on the mLV not to damage the papillary muscles of the mitral valve—this can be helped by looking first through the mitral valve to identify their position and so guide placement of the ventriculotomy.
Anatomic Repair
The surgery restores the mLV to the systemic circulation, which is established by switching over the atrial inflows (the Senning and Mustard procedures), switching over the arterial outflows (either arterial switch or Rastelli procedure, dependent on the LVOT anatomy), and repairing any associated defects. These anatomic repairs are much more complex procedures than the physiologic repair but have the advantage of repairing the circulation while also establishing the mLV as the systemic ventricle and therefore excluding the unpredictable function of the tricuspid valve and mRV from the systemic position.
The procedures can be considered in their two main groups, dependent on whether the LVOT is normally developed or is stenotic or atretic.
Double-Switch Procedure.
The LVOT is of good size with a normal-size pulmonary valve. These patients have usually had a PA band placed previously, either to train the mLV or, in the presence of a large VSD, to prevent pulmonary overcirculation. Preoperative assessment must include careful assessment of the mLV to ensure it is fully prepared to support the systemic circulation (see under “ Pulmonary Artery Banding ”). Surgery combines atrial switch (the Senning is described here, but the Mustard can be used) with arterial switch and closing any associated VSD.
The ascending aorta is cannulated as high as possible, and bypass is established with bicaval cannulation, with very low inferior vena cava (IVC) cannulation to facilitate the atrial switch. The branch pulmonary arteries are fully mobilized and controlled with Silastic slings, and Waterston’s groove is fully developed. With the cross-clamp applied and the heart arrested the initial incisions are made for the Senning (see later) before proceeding to the arterial switch. The principles of the arterial switch are exactly as they are in a neonatal switch, but the great vessels tend to be slightly more side-to-side than the typical anteroposterior relationship seen in the neonate with d-transposition of the great arteries (d-TGA). The aorta is transected well above the level of the sinotubular junction and the coronary buttons excised on as large a button of aortic tissue as possible. The buttons are then mobilized to ensure they transfer comfortably—the posterior coronary (equivalent of the right coronary artery) usually runs directly backward and transfers readily. The anterior coronary (equivalent of the left coronary artery) needs to be carefully mobilized because the circumflex branch runs close to the aortic wall ( Fig. 69.3 ).
