Transposition of the great arteries is a congenital heart defect in which the aorta arises from the right ventricle and the pulmonary trunk arises from the left ventricle. This results from abnormal septation of the truncus arteriosus during the third to fourth weeks of fetal development. Transposition occurs in 19.3 to 33.8 per 100000 live births, and is the most common cardiac cause of neonatal cyanosis. It is more common in males than in females.

Transposition of the great arteries may be classified according to the presence or absence of associated cardiac abnormalities. Simple transposition refers to cases in which no other haemodynamically significant defects are present. It can also be described as transposition with intact ventricular septum, and is found in 60 per cent of patients.

Complex transposition of the great arteries indicates the co-existence of associated haemodynamically significant congenital heart defects. The two most common are ventricular septal defect, which occurs in 28 per cent of patients with transposition of the great arteries, and ventricular septal defect with left ventricular (subpulmonic) outflow tract obstruction, occurring in 12 per cent of patients with transposition of the great arteries.

The position of the great vessels in transposition of the great arteries may vary considerably, but most often the aorta arises anteriorly from the morphologically normal right ventricle, slightly to the right of the main pulmonary trunk. The pulmonary artery usually arises posteriorly and to the left of the aorta from the morphologically normal left ventricle. The relationship of the aorta to the right of the pulmonary trunk is noted by the designation dextro- or d-transposition of the great arteries. The aorta may rarely arise to the left of the pulmonary trunk (1-transposition of the great arteries) or directly posterior to it. Thus, transposition refers only to the discordance between the great vessels and the ventricles from which they arise and not to the anatomical and spatial relationships between the great vessels themselves.

The coronary arteries in transposition of the great arteries arise from the aorta, but variability in their take-off and course is common. This becomes important in the planning of surgical repair. The most frequent patterns are illustrated in Fig. 1 1716.

The basic pathophysiological abnormality in simple transposition of the great arteries is that the pulmonary and systemic circulations are aligned in parallel rather than in series, as outlined schematically in Fig. 2 1717. Desaturated systemic venous blood returns normally to the right atrium and right ventricle, but is then directed back to the systemic circulation through the aorta. Similarly, oxygen-rich pulmonary venous blood enters the left atrium and left ventricle where it is pumped, via the pulmonary trunk, to the lungs. The failure of oxygenated blood to reach the systemic circulation results in cyanosis, severe acidosis, and myocardial ischemia. Mixing of blood through a patent foramen ovale and patent ductus arteriosus may attenuate these changes and allow survival beyond the perinatal period. They also result in increased pulmonary bloodflow, which may produce pulmonary congestion and the subsequent development of pulmonary vascular disease.

The presence of a ventricular septal defect will alter the pathophysiological effects outlined above. As pulmonary vascular resistance decreases normally after birth, systemic venous blood will be shunted from the right ventricle to the left ventricle across the septal defect. Thus, some unoxygenated blood will pass through the pulmonary circulation and return to the left atrium. Increased left atrial bloodflow results in dilatation of the left atrium and stretching of the atrial septum, thereby enlarging the foramen ovale. The net result is an increased mixing of systemic and pulmonary venous blood with consequent lesser degrees of cyanosis and acidosis. The increased pulmonary bloodflow may, however, cause congestive cardiac failure and induce the subsequent development of pulmonary vascular disease.

This combination of defects, depending on the severity of the obstruction, may result in diminished pulmonary artery bloodflow and shunting of blood across the ventricular septal defect from the left ventricle to the right ventricle. The degree of obstruction may increase rapidly in the first few hours after birth, and a marked diminution in pulmonary bloodflow may occur. Simultaneously, as right ventricular volume and pressures increase, right atrial enlargement develops, leading to right-to-left shunting through the foramen ovale. The net result is marked desaturation of left atrial and left ventricular blood with consequent severe cyanosis and acidosis.

Survival of patients with transposition of the great arteries depends on mixing of blood between the systemic and pulmonary circulations. In simple transposition, this must occur through a patent foramen ovale and/or a patent ductus arteriosus. Failure of adequate mixing after birth results in death in the first 24 h of life. Physiological closure of the foramen ovale and ductus results in a 6-month mortality of 90 per cent in untreated infants. Patients with transposition of the great arteries and ventricular septal defect may survive into early childhood, but without treatment, pulmonary vascular disease and pulmonary hypertension develop. If left untreated, 34 per cent of patients with simple transposition of the great arteries and 78 per cent of patients with transposition of the great arteries and ventricular septal defect develop irreversible pulmonary vascular disease which renders them inoperable by 12 months of age. The onset of pulmonary hypertension is associated with rapid deterioration and subsequent death. Patients with transposition of the great arteries, ventricular septal defect, and left ventricular outflow tract obstruction have the worst prognosis. Rapidly worsening cyanosis occurs early in the postnatal period, resulting in poor survival without correction.

Cyanosis at or shortly after birth is commonly the initial finding in patients with transposition of the great arteries. The differential diagnosis thus includes other causes of cyanosis at birth including truncus arteriosus, tetralogy of Fallot, tricuspid atresia, and total anomalous pulmonary venous return. Transposition of the great arteries with ventricular septal defect is usually associated with less cyanosis, whereas transposition of the great arteries with ventricular septal defect and left ventricular outflow tract obstruction is associated with more severe cyanosis. In simple transposition, the only cardiac finding on physical examination may be a single accentuated second heart sound resulting from aortic valve closure anteriorly near the chest wall. Patients with associated ventricular septal defect, with or without subpulmonic stenosis, also have a holosystolic murmur. If the flow across the ventricular septal defect is large, a thrill may also be present.

Electrocardiographic findings are generally limited to right atrial and right ventricular hypertrophy. Patients with transposition of the great arteries and ventricular septal defect, with or without left ventricular outflow tract obstruction, may have both left and right ventricular hypertrophy, but this is uncommon early in infancy.

Three principal radiological changes are found in patients with transposition of the great arteries. Cardiomegaly, often with a characteristic oval-shaped heart, results from enlargement of the right ventricle which supports the systemic circulation. Narrowing of the superior mediastinum is seen as a result of the abnormal anteroposterior relationship of the aorta and pulmonary trunk. Pulmonary vascular markings are increased due to the increase in pulmonary bloodflow (Fig. 3) 1718. Similar findings occur in transposition of the great arteries with ventricular septal defect. Cardiomegaly and mediastinal narrowing are accompanied by diminished pulmonary vascular markings in patients with transposition, ventricular septal defect, and left ventricular outflow tract obstruction.

Cardiac catheterization and ventriculography have been the definitive means of diagnosing transposition of the great arteries. Origin and position of the great arteries, the presence or absence of an associated ventricular septal defect, and the presence or absence of subpulmonic stenosis can be demonstrated with relative ease (Figs 4, 5) 1719,1720. Additionally, aortic root injection or selective coronary arteriography can delineate details of the coronary artery anatomy. In neonates, the craniocaudal projection gives a clear image of the coronary artery origins and distribution.

Two-dimensional Doppler echocardiography is rapidly replacing cardiac catheterization as the means of definitive diagnosis in patients with transposition of the great arteries. This non-invasive technique can be performed easily at the bedside and obviates the need for vascular access and the use of nephrotoxic contrast materials. Echocardiography readily demonstrates the abnormally related great arteries, and the presence of ventricular septal defects, as well as the presence and degree of subpulmonic stenosis. Views through the aortic root can also demonstrate the anatomy of the proximal coronary arteries (Fig. 6) 1721. At UCLA Medical Center, echocardiography is used as the sole method of preoperative assessment in most patients with simple transposition of the great arteries and transposition of the great arteries with ventricular septal defect. Catheterization is reserved for patients with more complex anatomy, or those in whom echocardiographic assessment is deemed technically inadequate.

Appropriate palliative and definitive treatment of transposition of the great arteries varies with the specific pathophysiological derangements present.

The immediate therapeutic concern, once the diagnosis of transposition of the great arteries has been made, is to allow mixing of blood from the systemic and pulmonary circulations, thereby improving systemic oxygen saturation. The first method employed was described by Blalock and Hanlon in 1950 and involved surgical excision of a portion of the atrial septum without the use of cardiopulmonary bypass. In 1966, Miller and Rashkind revolutionized the management of these patients by introducing the non-surgical catheter technique of balloon atrial septostomy. A balloon catheter is passed across the atrial septum through the patent foramen ovale from right to left. The balloon is inflated and the catheter is forcibly withdrawn back into the right atrium, tearing the atrial septum. The procedure can be performed at the time of cardiac catheterization under fluoroscopic guidance, or at the bedside under echocardiographic guidance. The resultant large atrial septal defect almost always allows adequate mixing, and balloon septostomy has become the mainstay of initial management in transposition of the great arteries. Blalock-Hanlon atrial septectomy may rarely be required in patients who do not respond to balloon septostomy. Prostaglandin E&sub1; infusion is used to maintain patency of the ductus arteriosus during initial evaluation and therapy, but should not be considered more than a temporary measure. Severe metabolic acidosis may require treatment with sodium bicarbonate, although correction of systemic oxygen desaturation is the primary therapy for acidosis. Supportive therapy includes the use of digitalis and diuretics to treat ventricular failure.

The surgical management of simple transposition of the great arteries has undergone a rapid evolution in the last 5 to 10 years. The basic principle of definitive treatment is the redirection of bloodflow such that the systemic and pulmonary circulations are now placed in series rather than in parallel. This is accomplished by ‘switching’ bloodflow either within the atria or at the level of the great arteries.

Senning, in 1959, described the first technique for definitive repair of transposition of the great arteries by ‘switching’ bloodflow at the atrial level. The wall of the right atrium and the translocated atrial septum are used to create an atrial baffle. The procedure is illustrated in Fig. 7 1722. The right atrium is opened parallel to the interatrial groove, just anterior to the crista terminalis. The incision is extended superiorly to the base of the right atrial appendage and inferiorly toward the valve of the inferior vena cava. The left atrium is opened by dissection and incision of the interatrial groove, with a counterincision in the right superior pulmonary vein. The atrial septum is exposed through the right atriotomy, and a septal flap is created with its base laterally at the interatrial groove. The flap is then rotated posteriorly and sutured to the wall of the left atrium just posterior to the left atrial appendage and mitral valve and just anterior to the left pulmonary veins, so as to cover the orifices of the pulmonary veins. The posterior edge of the right atriotomy is next sutured to the right atrial wall anterior to the superior vena cava, and then to the remnant of the atrial septum. Inferiorly, the edge of the atrial baffle is sutured to the valve of the inferior vena cava and the right atrial wall just posterior to the coronary sinus. This baffle is thus used to divert blood from the orifices of the inferior and superior vena cavae to the mitral valve. The anterior edge of the right atriotomy is then anastomosed to the posterior edge of the left atriotomy, completing the repair.

In 1964, Mustard described an alternative method of atrial switch which was technically simpler than that developed by Senning (Fig. 8) 1723. Following right atriotomy and complete atrial septectomy, a baffle of pericardium (or synthetic graft material such as Dacron) is sutured to the atrial wall to divert flow from the caval orifices to the mitral valve. Pulmonary venous return flows lateral to the baffle into the tricuspid orifice, and then to the right ventricle and aorta.

Until about 1985, the atrial switch using either the Senning or Mustard procedures was the technique of choice for definitive repair of transposition of the great arteries. These operations were performed in patients 6 weeks of age or older, and palliation with balloon atrial septostomy and occasionally atrial septectomy was required to allow survival from birth to operation. The major decision facing the surgeon at that time concerned choosing between Senning's and Mustard's technique. Although Senning was the first to report successful repair in 1959, other centres failed to duplicate these results, largely due to the technical difficulty of this procedure at a time when cardioplegia was not used and when open heart procedures generally had a high mortality rate. This prompted the development of Mustard's simplified operation in 1964. The Mustard procedure, which has been performed with a 1 per cent early mortality rate and a 5 per cent late mortality rate, was widely adopted for the treatment of transposition of the great arteries. However, it soon became recognized that there was a significant late morbidity related to the procedure. Superior vena cava obstruction occurred in up to 10 per cent of patients and pulmonary vein obstruction, usually left-sided, occurred in 5 per cent. Many centres, therefore, returned to the Senning procedure as the technique of choice. The Senning procedure uses only autologous atrial tissue, which will theoretically grow with the patient, preventing subsequent obstruction of the pulmonary venous return. Recent reports indicate comparably low early and late mortality after either procedure.

Despite these excellent survival data, a number of major concerns remained about switching bloodflow at the atrial level, regardless of the specific technique employed. First, atrial arrhythmias have been noted in up to 40 per cent of patients after Senning or Mustard procedures, and 10 per cent require pacemaker insertion by 10 years. Loss of the atrial kick may seriously impair cardiac output, and treatment regimens including antiarrhythmic agents and pacemakers add significantly to patient morbidity. A second concern centres on the ability of an anatomical right ventricle to support the systemic circulation after an atrial switch. The bellows-like morphology of the right ventricle is ideally suited to pump high volumes at low pressures. Although short-term results have been good, doubts regarding the long-term capabilities of a systemic right ventricle are now being confirmed. Right ventricular dysfunction and failure is reported in 3 to 10 per cent of long-term survivors, and tricuspid regurgitation (an indicator of right ventricular pressure and volume overload) is present in 4 per cent. Exercise tolerance is also diminished in these patients. Finally, the necessary interval between birth and the atrial switch procedure is associated with a mortality of 5 to 10 per cent, which must be added to the early operative mortality rate in comparing this to other procedures.

Switching bloodflow at the arterial level was initially attempted early in the 1950s by Mustard. Success of this ‘anatomical’ repair for transposition of the great arteries was dependent on two principle factors: the ability to transpose the coronary arteries to the ‘neo’-aorta, and the ability of the left ventricle, previously connected to the low pressure pulmonary circulation, to maintain the systemic circulation. After his initial attempts, Mustard abandoned this operation primarily because of the technical difficulties imposed by the coronary anastomoses. While certain patterns of coronary anatomy make arterial switch difficult or impossible, improvements in microvascular surgical techniques, as well as the concerns regarding long-term results of the atrial switch, prompted renewed interest in this procedure. Jatene reported the first successful operation in a 42-day-old child with transposition of the great arteries and ventricular septal defect in 1975. The volume load from the ventricular septal defect ‘prepared’ the left ventricle for support of the systemic circulation. Yacoub reported use of the arterial switch for transposition of the great arteries with intact septum after preliminary preparation with a pulmonary artery band and systemic-to-pulmonary shunt. Eber reported the first small series of neonatal arterial switch procedures and Casteneda reported the first successful large series of arterial switches in the neonate. The technique is illustrated in Fig. 9 1724. The aorta and pulmonary trunk are divided after careful assessment of the coronary anatomy. The coronary ostia are excised with a surrounding button of aortic wall and are reimplanted into the adjacent neo-aorta, care being taken to avoid angulation, kinking, or tension. The neopulmonary artery, from which the coronary buttons have been removed, is now repaired using a ‘W’ shaped pericardial patch. Early techniques included a graft of pericardium, dura mater, or synthetic material to bridge the proximal and distal pulmonary artery. Currently, mobilization of the pulmonary arteries and displacement of the pulmonary trunk bifurcation anterior to the aorta, as described by Lecompte, allows direct anastomosis of the proximal and distal pulmonary artery. Closure of the atrial septal defect is accomplished during a brief period of circulatory arrest.

Proper timing of the operation is essential for a successful outcome after the arterial switch, largely due to changes in the pulmonary vascular resistance. Before birth, the pulmonary vascular resistance is high, and right and left ventricular size and wall thickness are similar. The normal postnatal decrease in pulmonary vascular resistance results in continued growth and hypertrophy of the systemic ventricle with a relative decrease in the wall of the pulmonary ventricle. Patients with simple transposition of the great arteries must, therefore, undergo an arterial switch in the first few weeks of life, preferably by 2 weeks of age, before the left ventricular mass becomes inadequate to maintain the systemic circulation. In rare instances (e.g. those with sepsis or delayed diagnosis), patients who are otherwise good candidates may be unable to undergo the arterial switch in the neonatal period. Pulmonary artery banding in early infancy induces a rapid increase in left ventricular wall mass, thereby ‘preparing’ the left ventricle for a delayed arterial switch within a few weeks. Construction of a systemic-to-pulmonary artery shunt concomitant with pulmonary artery banding may be necessary to provide an adequate systemic oxygen saturation.

The other critical factor determining the outcome after the arterial switch procedure is the coronary artery anatomy. As illustrated in Fig. 1 1716, the most common pattern of coronary distribution in transposition of the great arteries is ‘normal’, and easily amenable to the arterial switch. However, the coronary artery may course within the wall of the aorta, crossing the commissure and making transfer to the neo-aorta impossible. Such intramural vessels, as well as vessels which arise immediately adjacent to the commissure, are associated with an increased risk of postoperative myocardial ischaemia and death.

Although a learning curve exists, centres with experience report early mortality rate between 5 and 10 per cent for neonatal arterial switch. Several recent reports comparing the atrial and the arterial switch demonstrate a significantly better systemic ventricular ejection fraction and a decreased incidence of atrial arrhythmias in the arterial switch group, without a difference in overall mortality. Morbidity after the arterial switch procedure is related primarily to pulmonary artery stenosis and occlusion of the left anterior descending coronary artery. Stenosis of the pulmonary artery requiring reoperation is reported in up to 10 per cent of patients. The use of absorbable suture material and a generous pericardial patch reconstruction at the sites of coronary button excision allows growth of the pulmonary artery anastomosis. The incidence of late pulmonary artery stenosis has declined with its use. Occlusion of the left anterior descending artery occurs in up to 14 per cent of patients following the arterial switch and is thought to result from kinking or tension on the vessel following reimplantation. While increased experience should reduce the incidence of this complication, it should be noted that patients with coronary occlusion all demonstrate normal left ventricular function, presumably due to the excellent coronary collaterals which develop in this age group. Long-term monitoring is necessary to demonstrate the continued adequacy of such collaterals.

The neonatal arterial switch is currently the procedure of choice for simple transposition of the great arteries. The atrial switch is reserved for patients whose coronary anatomy is not amenable to correction by arterial switch. In patients who cannot undergo definitive repair in the first few weeks of life, atrial switch or pulmonary artery banding, shunt, and subsequent arterial switch are indicated.

Initial management considerations are identical to those outlined above for patients with simple transposition of the great arteries. Excessive pulmonary bloodflow resulting from shunting across the ventricular septal defect causes congestive cardiac failure and was formerly palliated by pulmonary artery banding. This may occasionally still be required, but is reserved for patients too sick for the arterial switch and for those with complex anatomy.

The general strategy in treatment of transposition of the great arteries with ventricular septal defect is redirection of bloodflow to establish the pulmonary and systemic circulations in series, and closure of the defect. The atrial switch procedures with ventricular septal defect closure were found to have poor results in this group of patients. The operative mortality rate was high and late right ventricular dilatation and failure with tricuspid regurgitation was more frequent than in patients with transposition of the great arteries and intact septum. This prompted the early adoption of the arterial switch with closure of the ventricular septal defect for this group of patients. As with simple transposition, atrial switch or arterial switch may be utilized. However, some differences exist with regard to timing of operation in transposition of the great arteries with ventricular septal defect. Systemic-to-pulmonary shunting across the defect produces volume overloading of the left ventricle with consequent maintenance of left ventricular wall thickness despite the postnatal decrease in pulmonary vascular resistance. These patients can therefore tolerate the arterial switch after the first few weeks of life; in fact, the initial successful application of the arterial switch was limited to older infants with transposition of the great arteries and ventricular septal defect. Delayed operation allows cardiac and coronary artery growth, thereby reducing the difficulty of coronary transposition. Improved early and late results were obtained as well, as increased experience in anastomosis of smaller coronary vessels. This prompted application of the arterial switch to patients with simple transposition of the great arteries in the neonatal period. Although patients with transposition of the great arteries and ventricular septal defect can undergo later repair, the success of neonatal repair for simple transposition of the great arteries and the risk of morbidity from excessive pulmonary bloodflow have resulted in the current recommendation for neonatal arterial switch. The technique is as described above, with Dacron patch closure of the defect through the right atrium during a brief period of circulatory arrest. Atrial switch is reserved for patients with coronary anatomy not amenable to arterial switch.

Patients with transposition of the great arteries, ventricular septal defect, and left ventricular outflow obstruction have diminished pulmonary bloodflow. As with the other types of transposition, initial therapy includes atrial septostomy and prostaglandin E&sub1; to maintain patency of the ductus arteriosus. These patients require construction of a systemic to pulmonary shunt to augment pulmonary bloodflow and allow survival until definitive repair can be performed.

The anatomy in these patients is not amenable to repair by either atrial or arterial switch procedures. Direct excision of the subpulmonic left ventricular outflow tract obstruction risks injury to the conduction system and is difficult to perform without left ventriculotomy which may impair ventricular performance. Instead, right ventriculotomy is performed and patch closure of the ventricular septal defect is accomplished with baffling of the left ventricular outflow through the defect into the aorta. The pulmonic valve is oversewn, and a valved conduit is placed between the right ventricle and the pulmonary trunk. The procedure, first described by Rastelli, is illustrated in Fig. 10 1725.

Operation is deferred until 3 to 5 years of age so that placement of an adult-sized conduit is possible. Although operative mortality rates were high in earlier series, recent results indicate an acceptable rate of less than 5 per cent. Morbidity is principally secondary to late conduit obstruction, usually related to sternal compression and development of a fibrointimal peel. The use of the cryopreserved homograft conduit and placement of the conduit well to the left of the sternum have significantly reduced the incidence of this complication. If obstruction or growth necessitates conduit replacement, the procedure can be performed safely, with mortality rates under 1 per cent.

Transposition of the great arteries is the most common form of cyanotic congenital heart disease in the neonate, and is associated with a grave prognosis if left untreated. Diagnosis may be suspected by physical examination and chest radiographs, and confirmed by echocardiography in most situations. Cardiac catheterization with coronary angiography may occasionally be required. Initial treatment is accomplished by balloon atrial septostomy and prostaglandin E&sub1; infusion. Neonatal arterial switch has become the accepted definitive procedure of choice for patients with transposition of the great arteries, with or without ventricular septal defect. Atrial switch may be reserved for those patients with simple transposition of the great arteries in whom arterial switch could not be performed (due to difficult coronary anatomy, delayed diagnosis, or other medical contraindications) in the neonatal period. A delayed arterial switch may be possible with prior preparation of the left ventricle by pulmonary artery banding. The Rastelli procedure is employed for transposition of the great arteries with ventricular septal defect and left ventricular outflow obstruction. Excellent early and midterm results continue to improve as more experience is obtained. A summary of the experience at UCLA Medical Center is given in Table 1 496. Confirmation of the long term benefits of this approach is expected.

In some patients, transposition of the great arteries may be associated with discordance between the atria and ventricles. Systemic venous blood returns to the right atrium, fills the morphologically left ventricle, and exits via the pulmonary artery to the lungs. Pulmonary venous return to the left atrium is directed to the morphologically right ventricle and aorta. Thus, the congenitally abnormal relationships between the atria and the ventricles correct the circulatory derangements which would otherwise be caused by the transposed great arteries (i.e., a physiological correction occurs). Congenitally corrected transposition of the great arteries is a rare congenital defect, occurring in 0.008 per cent of live births. The terms left and right ventricle will refer to morphology rather than spatial position for the remainder of this chapter.

In 1 per cent of cases, congenitally corrected transposition of the great arteries is present without associated abnormalities. The atria are normal in position and morphology. The right atrium connects to the left ventricle through the mitral valve. Fibrous continuity exists between the mitral and pulmonic valves. Similarly, the left atrium connects to the right ventricle through the tricuspid valve, which is separated by a muscular ridge from the aortic valve annulus. The left ventricle is positioned posteriorly and to the right of the right ventricle. The aorta arises anterior and to the left of the pulmonary trunk.

The coronary arterial distribution is a mirror image of normal, but the relationship between the vessels and the ventricles is unchanged. Thus, the left main coronary artery arises from a right aortic sinus of Valsalva and bifurcates into anterior descending and circumflex branches. The anterior descending artery courses in the anterior interventricular groove (located more rightward than in normal hearts) to the apex. The circumflex coronary artery passes rightward to the right atrioventricular groove with obtuse marginal branches to the left ventricular free wall. The right coronary artery arises from a left aortic sinus and enters the left atrioventricular groove, giving rise to a posterior descending artery in most cases.

The abnormal connection between atria and ventricles results in significant abnormalities of the conduction system, specifically involving the atrioventricular node and penetrating bundle. A normally located ‘posterior’ atrioventricular node is found in its usual location near the coronary sinus, but does not connect with the penetrating bundle. A second anteriorly located atrioventricular node exists in these hearts which connects to a long penetrating bundle and thence to the bundle branches, thereby allowing atrioventricular conduction.

Patients with congenitally corrected transposition of the great arteries in whom no associated defects coexist may be completely asymptomatic. Right ventricular failure and tricuspid regurgitation develops in 21 per cent of these patients as the right ventricle is required to support the systemic circulation. Tricuspid regurgitation may be caused by annular dilatation with normal leaflet morphology, or to abnormal leaflets with apical displacement similar to that found in Ebstein's malformation. The abnormal anatomy of the atrioventricular node and penetrating bundle results in an 8 to 20 per cent incidence of spontaneous atrioventricular block.

An associated ventricular septal defect is present in 60 to 70 per cent of patients with congenitally corrected transposition of the great arteries. Left-to-right shunting causes excessive pulmonary bloodflow and volume overload of the morphologic right ventricle. Atrioventricular conduction disturbances are more frequent when a ventricular septal defect is present. The coexistence of left ventricular outflow tract obstruction and ventricular septal defect produces right-to-left shunting with resultant decreased pulmonary bloodflow and cyanosis. The pathophysiological picture is identical to that of tetralogy of Fallot. Left ventricular outflow tract obstruction is most commonly annular or subvalvar in nature, but may occur at the pulmonic valve as well.

Most patients with congenitally corrected transposition of the great arteries present themselves because of the effects of the associated cardiac defects or because of heart block. Physical examination of patients with ventricular septal defect reveals a holosystolic murmur; those who also have left ventricular outflow tract obstruction present with cyanosis and an ejection murmur heard best at the right sternal border. Chest radiography is unremarkable in uncomplicated congenitally corrected transposition of the great arteries. Cardiomegaly with pulmonary plethora will be seen in patients with ventricular septal defect, and cardiomegaly with diminished pulmonary vascular markings is present in those who also have left ventricular outflow tract obstruction. Cardiomegaly and pulmonary oedema may be seen in patients with tricuspid regurgitation. The physical examination and radiographic findings will not distinguish congenitally corrected transposition of the great arteries from morphologically normal hearts with similar defects. Electrocardiography may, however: abnormal septal depolarization results in right precordial q waves and absent q waves in the left precordial leads. This pattern is directly opposite that found in hearts with normally related ventricles. Atrioventricular block, although not pathognomonic, is commonly present. A definitive diagnosis of congenitally corrected transposition of the great arteries and its associated defects can be made by echocardiography. The abnormally related ventricles and great vessels are easily identified by two-dimensional echocardiography, as are ventricular septal defects and left ventricular outflow tract obstruction. Doppler flow studies allow estimation of the left ventricular outflow tract gradient and the degree of tricuspid regurgitation. Cardiac catheterization with angiocardiography remains the ‘gold standard’ of diagnosis, but as with transposition of the great arteries, is being supplanted by echocardiography.

Congenitally corrected transposition of the great arteries does not, in and of itself, require treatment. Thus management is based on the presence of coexisting lesions.

The ventricular septal defect is managed in a manner similar to that in morphologically normal hearts. Infants with severe congestive heart failure unresponsive to medical therapy including digoxin and diuretics should undergo patch closure of the defect. Patients with minimal or no failure can undergo elective repair at about 1 year of age. The defect is usually accessible through a right atriotomy with retraction of the mitral valve leaflets for exposure. Sutures are placed on the right ventricular side of the defect, as the conduction tissue lies superficially in the anterior left ventricle.

This is a difficult problem in patients with congenitally corrected transposition of the great arteries. Since the obstruction is usually at the annular and subvalvar levels, simple valvotomy and resection is not effective. Annular enlargement and resection of subpulmonic muscle bundles has been reported, but with high rates of recurrence. Placement of a transannular patch, as in tetralogy of Fallot, is not possible since the circumflex coronary artery crosses immediately anterior to the left ventricular outflow tract. Thus, a left ventricle to pulmonary artery conduit is the procedure of choice. Conduits should be placed from the left ventricular apex around the right heart border to the pulmonary artery, thereby avoiding compression between the heart and sternum. There is often little space for such a conduit in neonates and infants; these patients would also quickly outgrow the conduit. Therefore, in affected infants and small children with severe cyanosis a systemic-to-pulmonary shunt, usually a modified Blalock-Taussig shunt, is constructed as a palliative procedure. Definitive repair with conduit placement is performed around 5 or 6 years of age.

Tricuspid valve replacement is indicated in patients with significant symptomatic tricuspid regurgitation or minimally symptomatic patients with severe regurgitation and marked right ventricular dilatation. Repair for the Ebstein's type of valve in congenitally corrected transposition of the great arteries is rarely successful and valve replacement is usually preferred. The tricuspid valve is approached through the left atrium similar to mitral valve operations in the morphologically normal heart.

The presence of atrioventricular block, either primarily or as a result of surgical repair of other lesions, requires placement of a permanent pacing system. Atrioventricular sequential pacing is preferable since the right ventricle may be more susceptible to failure and may benefit from the presence of the ‘atrial kick’. In adults, pacing systems may be implanted transvenously or epicardially at the time of operation. In children, epicardial placement is necessary.

Operation for congenitally corrected transposition of the great arteries with ventricular septal defect is associated with a 10 per cent mortality rate. Repair of left ventricular outflow tract obstruction or tricuspid replacement have a 10 to 20 per cent operative mortality rate. The incidence of postoperative atrioventricular block is 10 per cent.

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