Introduction
The congenital malformation characterized by origin of the arterial trunks from morphologically inappropriate ventricles has probably been the source of as much confusion and controversy as any other single topic in pediatric cardiology. When Matthew Baillie described the first case, he had no problems with nomenclature, describing the entity in accurate fashion as a “singular malformation.” Since then, the entity has most usually been described simply as transposition . However, it has been defined in many different ways and has been used to describe many different types of congenital cardiac malformations. Arguments have centered on whether the morphologic arrangement of the arterial trunks should be defined primarily according to their ventricular origin or on the basis of an anterior location of the aorta relative to the pulmonary trunk. Such disagreements have been nonproductive. In the clinical setting, it is the origin of the arterial trunks from the ventricles that is paramount because this feature determines the patterns of flow of blood through the heart, irrespective of the relationships of the arterial trunks. The combinations of segmental connections then produce the variations still described in terms of “transposition.” We will describe the combination to be discussed in this chapter simply as transposition, including only those patients having the specific combination of concordant atrioventricular and discordant ventriculoarterial connections ( Fig. 37.1 ).
Historical Background
To the best of our knowledge, it was John Farre who first used the term transposition in the context of a cardiac malformation. Subsequent to this description, it was usually considered that the aorta needed to be anterior to justify diagnosis, combined with support of the aortic valve by a muscular infundibulum. However, some segmental combinations associated with an anterior aorta produce physiologic correction of the circulatory patterns, such that the systemic venous blood reaches the lungs despite being pumped by the morphologically inappropriate ventricle. In two such combinations, there is also so-called anatomic correction, in that the aorta takes its origin from the morphologically left ventricle despite its anterior position. In two additional variants, the circulations are in parallel rather than in series, despite the fact that the aorta arises in concordant fashion from the morphologically left ventricle. It was this use of “corrected” in an anatomic sense, being dependent on the definition of transposition as an anterior aorta, that produced the potential for confusion. Further problems arose when it was recognized that the aorta could arise inappropriately from the morphologically right ventricle when in posterior position and with its valve in fibrous continuity with the mitral valve. It makes little sense, when considering clinical presentation, to describe a patient with an anterior aorta arising from the left ventricle as having “transposition” when the atrioventricular connections are also concordant, just as it makes little sense to suggest that patients with discordant ventriculoarterial connections, but with a posterior aorta, do not have transposition. Thus, over recent years, there has been an increasing groundswell of opinion in favor of defining transposition on the basis of the origin of the great arterial trunks from morphologically inappropriate ventricles. Therefore, the system that we currently use bases the use of “transposition” on the combination of specific atrioventricular and ventriculoarterial connections. This is the solution now adopted by the International Working Group on Nomenclature and that which was advocated by the consensus statement prepared on behalf of the Society of Thoracic Surgeons. The approach recognizes the need also to describe, when appropriate, both arterial relationships and infundibular morphology. In this way, all potentials for confusion are avoided.
Prevalence and Etiology
Transposition was responsible for up to 20% of cardiac deaths in infancy prior to the era of surgical correction. Although no definite etiologic factors have been identified, the condition is held to be more frequent in infants of diabetic mothers. Maternal intake of alcohol or poor nutrition during pregnancy may increase the risk of transposition in the offspring, whereas addition of folic acid to the maternal diet has been associated with reduced risk.
Genetics
Males are affected two to three times as frequently as females. Transposition does not seem to coassociate with known major genetic disorders, and most cases are sporadic. The increased presence of copy number variations in patients with transposition has allowed for identification of candidate genes. In general, genes affecting laterality and the nodal pathway have been associated with the disease.
Anatomy and Morphogenesis
As emphasized, it is the basic combinations of concordant atrioventricular and discordant ventriculoarterial connections that produce the entity we call transposition ( Fig. 37.1 ). The anatomic situation can be complicated by the presence of a ventricular septal defect, an obstruction within the left ventricular outflow tract, both of these malformations, or by other associated malformations. Cases with an intact interventricular septum and no obstruction of the left ventricular outflow tract are usually considered simple, even if they are complicated by other lesions such as persistent patency of the arterial duct.
Basic Segmental Combinations
The combination of segmental connections ( Fig. 37.2 ) can be found with either usual or mirror-imaged atrial arrangements ( Fig. 37.1 ). However, this specific combination cannot exist when there is isomerism of the atrial appendages. Hearts with isomeric atrial appendages can, of course, have biventricular atrioventricular connections, right-hand topology, and discordant ventriculoarterial connections. Such hearts are closely related to transposition as it is defined for the purposes of this chapter, but the patients with isomeric atrial appendages have grossly abnormal venoatrial connections as their major feature (see Chapter 26 ). In the arrangement as defined for the purposes of this chapter, the atrial anatomy is basically normal, although most frequently the oval foramen is patent, or there is a deficiency of the floor of the oval fossa. Even if the flap valve overlaps the rim of the oval fossa, it is flimsy and can be ruptured easily by balloon septostomy. In keeping with this normal atrial anatomy, the sinus and atrioventricular nodes are in their anticipated position.
Unlike the atrial chambers, ventricular morphology is subtly different from normal. The ventricular septum is much straighter than usual, not showing the multiple curves so typical of the normal heart. The pulmonary valve is not wedged as deeply between the mitral and tricuspid valves as is the aortic valve in the normal heart. This, in turn, means that the area of off-setting of the leaflets of the atrioventricular valves is much less marked, as is the area occupied by the membranous septum. Another consequence of this abnormal arrangement is that the ratio of the dimensions of the inlet and outlet components of the ventricular mass are abnormal in favor of the outlet dimension, although not to the extent seen in atrioventricular septal defects (see Chapters 31 and 32 ). At birth, the walls of the morphologically left ventricular wall are marginally thicker than those of the right ventricle. The right ventricular mural thickness then rapidly increases in the first 2 years of life, becoming much thicker than that of the left ventricle. The most obvious external abnormality is the relationship of the aorta to the pulmonary trunk. In the majority of patients with an intact ventricular septum, the aortic root is to the right of the pulmonary trunk in hearts in the setting of usual atrial arrangement ( Fig. 37.2 ) and to the left in the mirror-imaged variant ( Fig. 37.3 , left panel). Patients are found, nonetheless, with usual atrial arrangement and intact ventricular septum when the aorta is to the left ( Fig. 37.3 , right panel). Rarely, the aorta may be right sided and posterior, even when the ventricular septum is intact.
When the ventricular septum is intact, the aorta almost always has a complete muscular infundibulum, while the leaflets of the pulmonary valve are in fibrous continuity with those of the mitral valve ( Fig. 37.2 ). Variations in this infundibular morphology (e.g., the presence of bilaterally complete muscular infundibulums) are not seen nearly as frequently when the ventricular septum is intact as when there is a ventricular septal defect (discussed later).
Coronary Arteries
Whenever the position of the aortic root is abnormal, the origins of the coronary arteries deviate from those found in the normal heart. Without exception, the arteries continue to arise from one or other, or both, of those aortic sinuses that face, or are adjacent to, the pulmonary trunk ( Fig. 37.4 ).
The epicardial course of the arteries arising from these adjacent sinuses is much less predictable. It is best to use a descriptive approach to account for the multiple variations, taking note separately of the origins of the three major coronary arteries from the aortic sinuses and their course relative to the vascular pedicle. Particular attention should be paid to any intramural course, along with the location of the artery supplying the sinus node. Although the coronary arteries always arise from one or both of the aortic sinuses that are adjacent to the pulmonary trunk, the position of these sinuses in space can vary according to the relationship of the arterial trunks. Therefore, when describing the sinusal origin, it is advantageous to use a system that is independent of spatial relationships. This can be achieved by considering the aortic sinuses as they would be visualized, figuratively speaking, by an observer standing in the nonadjacent aortic sinus and looking toward the pulmonary trunk ( Fig. 37.5 ). Irrespective of the relationships of the arterial trunks, the sinuses supporting the coronary arteries are always located to the right and left hands of the observer. The sinus as seen to the right is universally known as sinus 1, while that to the left is sinus 2 . All patterns are then accounted for on the basis of whether the right, circumflex, and anterior interventricular arteries arise from sinus 1 or from sinus 2. Most usually, the arteries arise within the aortic sinuses or at the level of the sinutubular junction, albeit usually eccentrically placed within the sinus. In some instances, the arteries can take a high origin above the sinutubular junction. Of more significance is the arrangement when the arteries take a tangential course through the aortic wall, crossing the attachments of the valvar leaflets at the sinutubular junction. This is the so-called intramural origin. In addition to sinusal origin ( Fig. 37.5 ), epicardial course is also important, in particular retropulmonary or anteroaortic location of any of the three major coronary arteries. The artery to the sinus node is of further significance. It can arise from the initial course of either the right or the circumflex coronary arteries, or it can take a direct origin from one or other of the facing aortic sinuses. However, its most important variation is when it crosses the lateral margin of the right atrial appendage. In such a lateral position, it is at surgical risk during a standard atriotomy.
Ventricular Septal Defect
The most significant, and frequently occurring, associated lesion in transposition is a ventricular septal defect. As with such defects found in the setting of concordant ventriculoarterial connections, these may be small, large, or multiple, and they can be located within any part of the ventricular septum ( Fig. 37.6 ). The most characteristic defects are those that open beneath the ventricular outlets, with the muscular outlet septum being malaligned relative to the rest of the ventricular septum and located within the right ventricle. Such defects, which occupy a subpulmonary position when seen from the left ventricle, may have a muscular posteroinferior rim ( Fig. 37.7B ) or may extend to become perimembranous.
The outlet septum can also be malaligned into the left ventricle. This then narrows the left ventricular outflow tract, producing pulmonary stenosis in association with overriding and biventricular connection of the aortic valve ( Fig. 37.7B ). In the more common malalignment defects, as shown Fig. 37.7A , the pulmonary valve overrides the septum. With ever-greater degrees of overriding, and increasing connection of the leaflets of the pulmonary valve within the right ventricle, the spectrum of anomalies culminates in double outlet from the right ventricle with subpulmonary interventricular communication. This whole series is often called the Taussig-Bing complex. We divide the series at its midpoint, including only those with less than half the circumference of the overriding pulmonary valve connected within the right ventricle as examples of transposition. Equally significant from the surgical standpoint are those defects that extend to open to the inlet of the right ventricle. These are hidden beneath the septal leaflet of the tricuspid valve, complicating their surgical repair. When the defect extends to open into the right ventricular inlet, there is the potential for straddling and overriding of the tricuspid valve. In this setting, the muscular ventricular septum no longer extends to the crux, and the atrioventricular conduction axis takes origin from an anomalous posteroinferior atrioventricular node. Apart from in the presence of overriding of the tricuspid valve, the conduction tissue is carried on the left ventricular aspect of the septum, and the posterocaudal rim of the defect is most vulnerable during surgical correction. As with isolated defects, the conduction axis is better protected when the posterior limb of the septomarginal trabeculation fuses with the ventriculoinfundibular fold ( Fig. 37.7A ). However, if a muscular defect opens to the inlet, the axis of conduction tissue will be found in anterocephalad position. There can be other types of defects, such as multiple muscular defects, solitary apical muscular defects, or doubly committed defects roofed by the conjoined leaflets of the aortic and pulmonary valves. They are less common. The aorta is also rarely found posteriorly and to the right in the presence of a ventricular septal defect. With this arrangement, there is usually aortic-mitral valvar continuity through the roof of the defect. Bilateral infundibulums are also seen more frequently in association with a defective ventricular septum, most frequently when the arterial trunks are side by side. Other anomalies are also more frequent in association with ventricular septal defect, including juxtaposition of the atrial appendages, aortic stenosis, and aortic coarctation.
Obstruction of the Left Ventricular Outflow Tract
Any lesions that produce obstruction to the outflow tract of the morphologically left ventricle, and which in the normal heart produce aortic obstruction, will produce subpulmonary obstruction in the setting of transposition. Such lesions can be found at the valvar or subvalvar level ( Fig. 37.8 ).
Isolated valvar obstruction is rare, being more common in combination with subvalvar obstruction, which may be dynamic, fixed, or both. Dynamic obstruction is produced by bulging of the ventricular septum. In its most severe form, this is reminiscent of hypertrophic obstructive cardiomyopathy. When septal bulging itself is less severe, obstruction of the left ventricular outflow tract is frequently exacerbated by a fibrous ridge located on the septal bulge. This can progress to form a complete subvalvar shelf, the ridge extending onto the facing surface of the mitral valvar leaflet. The extent of fibrous stenosis can also be more elongated, giving a tunnel lesion. Other rarer forms of stenosis are produced by anomalous attachment of the tension apparatus of the mitral valve across the outflow tract or by aneurysms of fibrous tissue tags bulging into the outflow tract. All can exist with an intact ventricular septum or in association with a ventricular septal defect. However, when there is a septal defect, there is another most significant type of stenosis, namely a malalignment and deviation of the muscular outlet septum into the left ventricle. This combination narrows the subpulmonary outflow tract in association with overriding of the aortic valve ( Fig. 37.7B ). Most of these fixed stenoses present major problems in surgical removal, either because of their own intrinsic morphology (e.g., a papillary muscle) or because of their proximity to vital structures, such as the left ventricular conduction tissues or the left coronary artery in the transverse sinus. If removal is attempted, the safest area for resection is the area occupied by the muscular outlet septum.
Associated Malformations
Other malformations can coexist either when the ventricular septum is intact or in association with a ventricular septal defect or obstruction of the left ventricular outflow tract. Persistent patency of the arterial duct is particularly significant because it loads the left ventricle, producing a thicker wall. Other significant anomalies are infrequent but can occur. They include stenosis of the subaortic outflow tract, aortic coarctation, or anomalous pulmonary venous connections.
Morphogenesis
Two concepts have been advanced to explain the development of transposition. The first suggests that the anomaly is the consequence of inappropriate separation of the arterial pole of the heart. As a result, the sixth aortic arches, destined to supply the pulmonary arteries, are connected to the morphologically left ventricle, while the fourth aortic arches take their origin from the morphologically right ventricle. It was proposed that this can occur if the cushions septating the intermediate and proximal parts of the developing outflow tract fuse in straight rather than in their normal spiral fashion. The second theory puts the seat of maldevelopment in the proximal parts of the developing outflow tracts. This hypothesis suggests that, rather than the subaortic portion becoming connected to the left ventricle, as occurs in normal development, it is the subpulmonary component, together with the pulmonary trunk, which is thus connected. This concept was first proposed by Keith, who invoked differential absorption of the inner heart curvature as the causative mechanism. The concept was revitalized under the guise of differential conal growth. So as to explain all the known variants of transposition, including the arrangement in which the aorta arises posteriorly and to the right, it is necessary to invoke maldevelopment within both the proximal outflow tracts and the outflow cushions. Initial evidence from animal models showing discordant ventriculoarterial connections, such as the Pitx2 knockout mouse, supports this notion of the combination of fusion of the cushions in straight rather than spiraling fashion, with the subpulmonary component of the proximal outflow tract then transferred to the developing left ventricle ( Fig. 37.9 ).
Pathophysiology
Circulatory Physiology
The predominant physiologic abnormality in transposition is that the systemic venous return is recirculated to the body through the right ventricle and the aorta, while the pulmonary venous return is recirculated to the lungs through the left ventricle and pulmonary trunk. Thus, although in the normal heart the systemic and pulmonary circulations are in series, in transposition they are set up as two separate and parallel circuits. In the fetus, this arrangement results in more desaturation of the upper body than is usual but, although not associated with other significant circulatory instability, may be partially responsible for abnormal central nervous system findings later in life (see Chapter 76 ). Kept separated, the oxygenated pulmonary venous blood cannot reach the systemic circuit, and the systemic venous return cannot circulate to the lungs. This results in severe systemic arterial desaturation following birth and is incompatible with life. Although this is easy to understand, it is the lifesaving communications between these two circuits that add significant complexity to the physiology.
Determinants of Systemic Arterial Oxygenation and Mixing
Systemic flow (Q S ) is all of the blood coursing through the systemic circulation. Effective systemic flow (Q ES ) is defined as the portion of Q S that contains saturated pulmonary venous blood. Likewise, pulmonary flow (Q P ) is the entirety of the blood in the pulmonary circulation, whereas effective pulmonary flow (Q EP ) is that desaturated portion, having completed its course through the systemic circulation, now directed toward the lungs. In the normal heart, where the cardiac connections are concordant and no shunts exist, Q P = Q S = Q EP = Q ES . In transposition, if no shunts existed, Q EP and Q ES would both equal 0 and the patient would die. Fortunately, two areas of communication between the circuits typically exist in the newborn. One is at the patent arterial duct. The other is through the atrial septum. In addition, a ventricular septal defect, present in approximately 40% of patients, may exist as well.
In the first few hours of life, when pulmonary vascular resistance is high, blood will shunt predominately from the pulmonary (saturated) circulation to the systemic (desaturated) circulation. This may result in the interesting finding of “reversed differential cyanosis,” whereby the blood in the lower extremities is more highly saturated than the in upper extremities. This phenomenon lasts for as long as there is pulmonary to aortic shunting at the ductal level. It may be prolonged in the setting of transposition with preductal coarctation (or interruption) of the aorta, whereby the distal aortic pressure is reduced by the obstruction, driving saturated blood from pulmonary artery to aorta. In the absence of such obstruction as the pulmonary vascular resistance drops, transductal shunting takes the opposite course, with desaturated aortic blood entering the pulmonary circulation, contributing to Q EP . In turn, pulmonary venous return to the left atrium is increased, and the resultant elevation in left atrial pressure serves as the impetus to drive the saturated left atrial blood across the second communication, the oval foramen or atrial septal defect, to join the systemic circulation, thereby contributing to Q ES . Once the arterial duct has closed, it is the bidirectional flow across the atrial septum that maintains Q EP and Q ES . Interventions to augment flow at the ductal and atrial levels allow for the successful stabilization of the newborn with transposition.
Ventricular Septal Defect
The presence of a ventricular septal defect can have a variable influence on the circulation. This in part depends on the size of the defect, as well as on the presence of interatrial mixing, and the pulmonary vascular resistance. A large interventricular communication may contribute to beneficial circulatory mixing, but the presence of unrestricted flow to the lungs may result in symptomatic heart failure. If surgery is not performed during infancy, this can predispose to early pulmonary vascular disease, which is generally evident by 6 to 12 months of age. The presence of additional pulmonary or subpulmonary obstruction further modifies these relationships and may limit Q P . Patients who are well balanced due to pulmonary obstruction are generally asymptomatic, having a protected pulmonary circulation, adequate mixing, and a “well-trained” left ventricle.
Left Ventricular Pressure and Mass
The essence of transposition is such that the left ventricle supports the pulmonary circulation. In the neonatal period, when the pulmonary vascular resistance is high, left ventricular work and impedance are elevated. In a patient with an intact or virtually intact ventricular septum, the normal postnatal reduction in pulmonary vascular resistance results in a progressive reduction in left ventricular pressure and work, so that as a result, left ventricular myocardial mass progressively falls. This reduction in left ventricular pressure and myocardial mass will be attenuated in the presence of a large ventricular septal defect.
The effects of this postnatal transition are of crucial importance to the operative approach. In a patient with transposition with an intact ventricular septum, who has traversed this postnatal transition, any procedure (including the arterial switch operation), in which the left ventricle is called on to support the high resistance systemic circulation will result in left ventricular failure in the early postoperative period. Thus, as will be discussed, the arterial switch operation is performed in early postnatal life, before the postnatal transition is complete. Older patients will require either a preliminary procedure to increase the load on the left ventricle, such as banding of the pulmonary artery to “train” the left ventricle in preparation for definitive surgery, or alternatively may require a period of left ventricular support with ventricular assistance, in the early postoperative period. Finally, an atrial switch procedure may be desirable for some of these patients.
Clinical Diagnosis
Presentation
The increasing frequency of antenatal diagnosis of transposition allows the clinical team to deliver the infant in a specialist center or arrange prompt transfer for those born outside of such centers. As a result, time-sensitive and potentially lifesaving interventions such as the initiation of a prostaglandin infusion and balloon atrial septostomy can be expedited and the potential for hemodynamic instability reduced.
Most newborns with transposition have a normal birth weight. They have been noted to have smaller occipitofrontal circumferences at birth than controls, which may reflect the abnormal hemodynamics in utero, with relatively desaturated blood circulating to the brain and upper body. Undiagnosed infants can present with cyanosis, with or without clinical evidence of circulatory insufficiency. As discussed, Q EP and Q ES depend on the extent of circulatory mixing. In the extreme, some infants may present shortly after birth with cyanosis and circulatory collapse, which is most often related to inadequacy of the interatrial communication. Infants with a widely patent arterial duct and a large unrestrictive ventricular septal defect may present with only minimal cyanosis and, instead, later develop signs of pulmonary overcirculation.
Clinical examination during infancy reveals a variable degree of cyanosis, which is incompletely responsive to supplemental oxygen. The adequacy of the peripheral pulses generally reflects the overall circulatory state. Infants with real or pending circulatory collapse will tend to be profoundly desaturated, with globally reduced peripheral perfusion, cool extremities, and weak pulses. Decreased pulses noted only in the lower extremities should alert the clinician to an associated coarctation or interruption of the aorta. An upper to lower extremity blood pressure gradient may be present. Cardiac auscultation typically reveals a single second heart sound attributable to the relative positions of the aortic and pulmonary valves. There may be an audible continuous murmur from the arterial duct or a systolic murmur related to a ventricular septal defect. Large septal defects will eventually result in findings consistent with pulmonary overcirculation including tachypnea, dyspnea, hepatomegaly, feeding difficulty, and failure to thrive.
Chest Radiography
The chest radiograph may or may not be abnormal. Cardiac size is often normal, whereas the pulmonary vascular markings may be reduced, normal, or increased, reflecting the volume of flow to the lungs. In approximately one-third of neonates, the mediastinum is narrow, a result of the anteroposterior relationship of the arterial trunks. This classic finding has been described as resembling an “egg on a string.”
Electrocardiography
In most newborns, the electrocardiogram is normal. During early infancy prior to surgical intervention, the electrocardiogram may begin to reflect right ventricular hypertrophy and later demonstrate right-axis deviation. A superior axis in the neonatal period suggests associated abnormalities of the tricuspid valve, particularly straddling or overriding.
Fetal Imaging
In the modern era, transposition is being diagnosed more frequently in utero than it has in the past; however, even recent series demonstrate that less than half of patients have an antenatal diagnosis. The efficacy of fetal screening is based on the premise that early detection will provide opportunities for parental choice and allow optimal neonatal care and preoperative management. In some series, reductions in both preoperative and postoperative morbidity, as well as mortality, have been demonstrated in neonates after an antenatal diagnosis of transposition. However, the data are mixed, with other studies reporting no such benefit.
In mothers referred for comprehensive fetal echocardiography performed by specialist sonographers, the lesion can be detected with high levels of accuracy ( Fig. 37.10 , ). A systematic segmental approach is recommended. A detailed assessment of the adequacy of intracardiac mixing, particularly at the atrial level, is usually performed. Restriction to flow at the atrial level is a highly specific predictor of the need for emergency neonatal care, but its sensitivity is too low to permit detection in all fetuses. Findings of a small oval foramen relative to the total septal length and reversal of flow through the arterial duct during diastole have been associated with the need for an early atrial septostomy. There are mixed data regarding whether increased mobility of the flap of the septum primum is similarly predictive. Four-dimensional fetal echocardiography may provide an advantage over cross-sectional imaging in that it creates an en face view of the four cardiac valves, thus enhancing the definition of the spatial relationships of the great arteries and, in turn, improving the probability of predicting an abnormal distribution of the coronary arteries. Cardiac magnetic resonance may be used to detect reductions in flow at the atrial and ductal levels and excessive flow through aortopulmonary collateral arteries.
Postnatal Echocardiography
The echocardiographic examination of the neonate with transposition requires a complete assessment of the anatomy, using a sequential approach, an examination of the circulatory physiology, particularly of the adequacy of mixing, and an estimation of the ability of the left ventricle to support the systemic circulation. Echocardiographic confirmation of the diagnosis requires demonstration of concordant atrioventricular and discordant ventriculoarterial connections. The atrioventricular connections can readily be examined using subcostal and apical four-chamber views. Anterior angulation of the transducer then demonstrates the discordant ventriculoarterial connections. Specifically, the bifurcating pulmonary trunk can be demonstrated originating from the left ventricle, and the aorta, which is usually anterior, from the right ( Fig. 37.11 ). Parasternal imaging reveals the relationships of the arterial trunks. Typically, the aortic valve is positioned anterior and to the right of the pulmonary valve ( Fig. 37.12 ).
Having demonstrated the basic anatomic connections, other aspects can readily be appreciated. Juxtaposition of the atrial appendages (most commonly left juxtaposition of the right appendage) can be demonstrated from apical, parasternal, or subxiphoid windows ( ). Additional subxiphoid, apical, and parasternal views, combined with the use of color Doppler interrogation, should be used to confirm or exclude the presence of a ventricular septal defect and demonstrate the direction of associated shunting. A detailed examination of the pulmonary valve and the left ventricular outflow tract is an essential part of the preoperative assessment because any significant obstruction to the left ventricular outflow may influence surgical management. The atrioventricular valves should be assessed carefully to identify any abnormal attachments of the tendinous cords.
A detailed assessment of the origin and course of the coronary arteries should routinely be performed prior to surgical intervention. This requires interrogation from multiple views and is aided by a familiarity with the common arrangements in transposition ( Fig. 37.13 ). Determining intramurality of a coronary artery is challenging but is suggested when the vessel passes between semilunar valves or has a high take-off from the aorta. Patency of the arterial duct and the direction of ductal shunting should be confirmed. These features are best visualized from the suprasternal and high sagittal views, sweeping between the aorta and pulmonary artery. The aortic arch should always be assessed in detail to exclude coarctation or interruption. As it is difficult to exclude coarctation in the presence of a widely patent arterial duct, repeat studies should be considered to reassess the arch after the prostaglandin infusion is discontinued.
The atrial septum and the degree of interatrial shunting are best visualized using subcostal windows. The demonstration of a small interatrial communication, deviation of the atrial septum towards the right atrium or high-velocity left-to-right flow on pulsed wave Doppler, should immediately suggest the presence of restriction at the interatrial communication, particularly in a severely cyanosed infant. However, it is important to note that normal septal alignment or the absence of high-velocity flow across the septum does not necessarily mean that all is well. Rather, in the presence of an inadequate atrial communication, blood may simply travel across the mitral valve into the left ventricle, never creating a left atrial to right atrial pressure gradient.
Cardiac Catheterization
Cardiac catheterization is no longer routinely performed; all necessary anatomic information may be obtained from cross-sectional echocardiography. There is still an occasional role for cardiac catheterization in the assessment of the pulmonary vascular resistance in patients for whom there is a clinical suspicion of pulmonary vascular disease, or for additional anatomic or physiologic information.
Magnetic Resonance Imaging
Although transthoracic echocardiography is typically sufficient, preoperative magnetic resonance imaging may provide useful complementary information in the assessment of the anatomy. It is particularly useful in the assessment of the mass of the left ventricle in the infant presenting late ( Fig. 37.14 ). This may assist clinicians in assessing the suitability of an individual patient for an arterial switch operation. In a novel application, four-dimensional sequences have been used to assess streaming of flow in both repaired and unrepaired patients. Further discussion of postoperative use of magnetic resonance imaging follows in this chapter.
Medical Management of Neonates
Postnatal Stabilisation
Careful and timely management of the neonate with transposition is critical to minimize preoperative mortality, which ranges from 3% to 10%. Optimal management requires thoughtful consideration of the risks and benefits related to atrial septostomy, mechanical ventilation, the use of prostaglandin, and the timing of surgery.
Neonates diagnosed in utero should be delivered, preferably at term, in a high-risk obstetric unit, with rapid access to advanced cardiac care, including the availability of atrial septostomy. Among many other benefits, term delivery allows for more complete maturation of the brain in this patient population in whom it is known that oxygen delivery to the fetal brain is significantly diminished compared with normal. Venous access should be obtained immediately after delivery, and an infusion of prostaglandin E should be at least readied, if not started empirically. The common side effects of prostaglandin should be anticipated, including apnea, hypotension, and fever. Lower starting doses (0.0125 µg/kg per minute at our institution) are usually adequate.
Prompt transthoracic echocardiography will define the nature of the anatomy and guide therapy. Patients with severe acidosis or hypoxemia, usually attributable to inadequate mixing at the atrial level, may require an urgent balloon atrial septostomy. This is the case in up to 12% of neonates. In others, a nonurgent septostomy is performed if there are signs of inadequate intracardiac mixing, including acidosis, decreased systemic perfusion, or hypoxemia, or if echocardiographic findings suggest a risk for inadequate mixing. As discussed later, the threshold for performing an elective septostomy is institution dependent. As previously mentioned, one must not be misled by the lack of a demonstrable pressure gradient from left-to-right atrium on the echocardiogram, and, if other indicators of inadequate mixing are present, a septostomy should be considered. Although traditional teaching has advised against the use of supplemental oxygen, modest amounts may be beneficial during the early postnatal hours by compensating for any alveoloarterial gradient and reducing pulmonary vascular resistance.
Atrial Septostomy
Following its introduction by Rashkind and Miller in 1966, the balloon atrial septostomy has become well accepted in the preoperative management of patients with transposition and selected other lesions. The procedure requires passage of a balloon-tipped catheter from the right-to-left atrium, crossing the oval foramen. The balloon is then inflated with saline and, with a quick, short motion, is pulled into the right atrium. Although a femoral venous approach was originally described, it is now common to use the umbilical vein to enter the heart through the venous duct. Fluoroscopic screening was previously used to guide the procedure ( Fig. 37.15 ) but has largely been replaced by cross-sectional echocardiography ( Fig. 37.16 , ). This provides a simple and reliable method for monitoring the position of the balloon and assessing the success of the septostomy. It also allows the procedure to be performed outside of the cardiac catheterization suite.
