1. Answer: D. Congenital heart disease is found in about 0.5%-0.8% of live births. Tetralogy of Fallot is the fourth most common form of congenital heart disease (but the most common cyanotic form), comprising about 10% of the total cases. Transposition of the great arteries would be the next most common in frequency, accounting for about 5%. Truncus arteriosus, tricuspid atresia, and total anomalous pulmonary venous return are relatively rare, accounting for only about 1%-2% of the total cases of congenital heart disease, respectively. A common mnemonic for remembering this has been that all forms of cyanotic disease begin with the “T.” This is a reasonable way to remember the ones listed above, but this trick does not hold true completely. Additionally, the initial presentation for patients with “cyanotic” congenital heart disease, may not always be clinical cyanosis, even though these patients may eventually become cyanotic over time.

2. Answer: A. Patients with a complete AVSD are at risk for the development of irreversible pulmonary hypertension (Eisenmenger syndrome) if unrepaired in early childhood. This phenomenon occurs when the pulmonary vascular bed is subjected to elevated pressures, as seen when a large ventricular septal defect (VSD) is present. Most complete AVSDs feature a large VSD component. A restrictive VSD, by definition would restrict both flow and pressure to the right heart and should not damage the pulmonary vascular bed. In this setting, pulmonary pressure and resistance usually remain normal. Therefore a shunt through a restrictive VSD should be left to right, not right to left. Atrioventricular valve regurgitation is a common finding in AVSDs. However, mild regurgitation should be of limited, if any, physiologic significance. PAPVCs are more commonly associated with a sinus venosus type ASD, not a primum atrial septal defect seen in AVSDs. PAPVCs are also most commonly associated with left-to-right atrial level shunt, and should not impact the ventricular level shunting. While a so-called “goose-neck” deformity can be seen in patients with an AVSD, this feature may or may not produce left ventricular outflow tract obstruction, but would not effect a right-to-left ventricular level shunt.

3. Answer: A. The anatomic description is consistent with truncus arteriosus. There is also a malalignment VSD with override of the semilunar valve, in this case referred to as the “truncal” valve. Up to 33% of patients with truncus arteriosus will have the DiGeorge syndrome, characterized by abnormal facies, thymic hypoplasia and parathyroid hypoplasia or aplasia resulting in hypocalcemia. A large proportion (70%) of patients with DiGeorge syndrome have microdeletions of 22q11. This is so prevalent that DiGeorge syndrome and 22q11 deletion have become almost synonymous, although that is not completely accurate. Truncus arteriosus is not generally a ductal-dependent lesion, as the pulmonary artery blood supply is usually vigorous. Ductal-dependent lesions include defects which compromise great artery flow such as aortic or pulmonary valve atresia. Atrioventricular valve atresia will usually require an atrial septal defect for survival. The oxygen saturation is likely to slightly decrease in truncus arteriosus, due to mixing at the great artery level. However, patients with truncus arteriosus are not usually profoundly cyanotic and may have near normal saturations. Surgical repair is often carried out as a newborn, and should not be deferred for longer than a few months, if at all, due to a fairly high incidence of pulmonary vascular obstructive disease or Eisenmenger syndrome, if it is not corrected early. Due to the absence of a main pulmonary artery, surgical repair involves closure of the VSD and placement of a right ventricle to pulmonary artery conduit—an important point for the future since the conduit may require multiple replacement operations during the lifetime of the patient.

4. Answer: C. The tricuspid regurgitant signal predicts a pressure in the right ventricle of about 90 mm Hg, using the modified Bernoulli equation. A patient with tetralogy of Fallot will generally have a large VSD which has no real chance of closure or restriction. Therefore, the pressures in the right and left ventricles will equalize at systemic levels and the finding of a high tricuspid regurgitant velocity is expected in all cases. It is very unlikely (but not impossible) that pulmonary hypertension could develop in tetralogy patients. However, the usual presence of pulmonary valve and subvalvular stenosis generally protects the pulmonary arterial bed from hypertension. The presence of a loud murmur in the pulmonary area supports the presence of this finding.

5. Answer: A. This scenario describes a patient with transposition of the great arteries and VSD. The VSD is usually large in this scenario, so pulmonary pressures will be at systemic levels and pulmonary “hypertension” is technically present. This does not imply the presence of pulmonary vascular obstructive disease, which would be unlikely in an infant. Coarctation of the aorta, even if present, is an answer of no significance. Most patients with transposition actually have somewhat increased pulmonary blood flow with newborn transposition, despite the cyanosis. Total anomalous pulmonary venous return may produce cyanosis, but would require a right-to-left atrial shunt and there is nothing in this scenario which points in that direction.

6. Answer: E. Detection of congenital heart disease can occur for many reasons. An increasing number of patients are detected prenatally by ultrasound examination. Even though these problems are “cyanotic” problems, cyanosis may be relatively mild as a clinical clue and go undetected. All of the diagnoses listed in the question might go undetected for a period of time. Transposition of the great arteries usually presents with profound cyanosis as a newborn. However, those patients with transposition and VSD may be less cyanotic and may present later with a heart murmur. This scenario is rare. Tetralogy of Fallot frequently presents as an asymptomatic heart murmur. However, the heart murmur is not subtle and usually brings the patient to attention in the first few weeks of life. A patient with tetralogy of Fallot with pulmonary atresia might be the silent exception. Tricuspid valve atresia may also present later in infancy with a heart murmur, but it is very rare for these patients to escape detection until adulthood. Supracardiac total anomalous pulmonary venous return will frequently have very mild, if any, clinical cyanosis. The heart murmur is soft and can be subtle. Some patients will escape detection in the first months of life and a very few patients have been known to escape detection until adulthood, but this is also fairly rare. Ebstein anomaly can be very mild and patients may have little if any murmur. If there is no ASD, then they will not be cyanotic or have much exercise intolerance. Ebstein’s can present with severe cyanosis as a newborn. However, mild forms of Ebstein anomaly represent the most subtle of the choices and the most likely answer.

7. Answer: E. The spectrum of anomalies producing cyanosis is fairly broad and all of the reasons given may account for the clinical phenomenon of cyanosis. Realize that clinically cyanosis is due to a bluish discoloration of the skin. This is seen when the deoxyhemoglobin levels exceed about 4-5g%. Therefore, cyanosis may also be affected by the overall hemoglobin levels in the body since an oxygen saturation of 70% (30% of the hemoglobin will be desaturated in the “deoxy” state) will reach the 5 g% level much easier in a polycythemic patient than in an anemic patient. Patients who have decreased pulmonary flow and an intracardiac shunt, will be cyanotic because the amount of saturated pulmonary vein flow returning to the heart is diminished. Additionally, patients with simple shunts or even mixing lesions will not necessarily be cyanotic since they will have exuberant pulmonary flow and excessive pulmonary venous return. However, the answer is based more on the frequency of anomalies producing cyanosis. Tetralogy of Fallot is common and has decreased pulmonary flow. Added to this are numerous other complex defects for which pulmonary stenosis is also a component. Patients with cyanosis due to great artery position or pulmonary vein anomalies are much rarer.

8. Answer: C. This patient has hypoxemia on pulse oximetry, which may be difficult to appreciate as clinical cyanosis on visual examination. A saline contrast injection can establish the diagnosis of a persistent left superior vena cava to an unroofed coronary sinus as contrast appears in the left atrium. The key element here is the unroofing of the coronary sinus, which allows for right-to-left shunting of systemic venous blood flow directly into the left atrium, causing the desaturation. It also places the patient at risk for paradoxical embolism. The presence of a left superior vena cava connecting to the coronary sinus without the unroofing does not result in cyanosis, as the deoxygenated systemic blood passes through the coronary sinus and into the right atrium, regardless of whether or not there is a bridging vein connection to the right superior vena cava (which is irrelevant to the shunting). In these instances no right-to-left shunt exists, and cavitations will not appear in the left atrium. While total anomalous pulmonary venous connections mix with the systemic veins and may cause desaturation due to right-to-left atrial level shunting, this would not result in the detection of cavitations in the left atrium on saline contrast study.

9. Answer: E. Anomalous pulmonary venous connections are to the systemic venous system and may occur in a variety of ways. The most common is connection to the innominate vein (supracardiac). There may also be connections to the right atrium or coronary sinus. Connections below the diaphragm to the hepatic veins, through the liver, are frequently severely obstructed, creating a surgical emergency. A connection to the left atrium would be normal, and not anomalous, making this the answer.

10. Answer: B. There are a number of congenital defects which may present with a large malalignment VSD and overriding great vessel. The most common of these would be tetralogy of Fallot. However, the great vessel is not always the aorta. Other “lookalikes” for this particular view might be truncus arteriosus, double-outlet right ventricle, D-transposition with VSD, or pulmonary atresia with VSD. In each case, the parasternal view may be similar, but the key is identifying the position and status of the pulmonary artery. This is the key for both the proper diagnosis as well as aiding in predicting the clinical course. The presence of an ASD, and the position of the arch may be important adjunctive items to add to the overall imaging picture, but they do not have great clinical significance. The size of the VSD in this situation is virtually always one of a large and unrestricted flow. Tricuspid valve Doppler adds little in this situation.

11. Answer: A. All forms of tricuspid valve atresia must have some type of atrial communication to decompress the right atrium and allow for egress of blood from that chamber. Tricuspid atresia does not have to be ductal dependent. Ductal dependence is more likely to occur in instances of outflow obstruction rather than inflow obstruction. In cases of tricuspid atresia with transposition, there is frequently aortic coarctation present and this might become ductal dependent if it is severe. However, ductal dependency is generally a phenomenon of early infancy, which comes to light after ductal closure in the first day or two of life—not a very common likelihood in a 2-month-old patient. Tricuspid atresia will present anatomically in two forms: (1) normally related great arteries (75%) and (2) transposition of the great arteries (25%). With either great artery position, there is complete mixing of pulmonary venous and systemic venous blood at atrial and ventricular levels and saturations overall will depend upon the amount of pulmonary blood flow. In this instance the saturation is fairly high, suggesting vigorous pulmonary blood flow, but one cannot specifically distinguish between normal or transposed great arteries based on that finding alone. The arch position is irrelevant, but it is very rare to see a right aortic arch with tricuspid atresia.

12. Answer: B. The atrial switch operation, accomplished by either the Mustard or the Senning procedure, was a common surgery performed for D-transposition of the great arteries (D-TGA) during the 1960s through the late 1980s. In D-transposition of the great arteries, the right ventricle gives rise to the aorta and the left ventricle gives rise to the pulmonary artery, creating two parallel circuits of blood flow. The route of the systemic blood flow is from the vena cavae to the right atrium, then to the right ventricle out to the aorta to the body and finally back to the vena cavae. The route of pulmonary venous blood flow is from the pulmonary veins to the left atrium and the left ventricle, then out to the pulmonary artery and the lungs, and then back to the pulmonary veins (see Fig. 31-12A,B). The atrial switch operation allows for systemic and pulmonary circulation to exist in (normal) series, rather than in parallel circulation, therefore physiologically correcting the transposition defect. Blood flow following the Mustard procedure is as follows: vena cavae through the systemic venous baffle, directed to the mitral valve into the left ventricle then to the pulmonary artery and lungs to the pulmonary veins through the pulmonary venous baffle to the tricuspid valve into right ventricle to aorta out to the body and back to the vena cavae (see Fig. 31-12C,D). Obstruction to pulmonary venous return can occur within the surgically created pulmonary venous baffle, which courses between the pulmonary veins and the tricuspid valve (answer B). Obstruction of the systemic venous baffle can occur. However, this would create systemic, rather than pulmonary venous, congestion. The LeCompte procedure (which repositions the branch pulmonary arteries in the relatively limited space between the aorta and the chest wall) is often used during the arterial switch (Jatene procedure) and is not performed during the Mustard or Senning procedure. A complication of the Jatene procedure is branch pulmonary artery stenosis. Placement of a right ventricle to pulmonary artery conduit can be performed when there is right ventricular outflow tract obstruction in D-TGA with ventricular septal defect, but was not pertinent in this scenario and is not used in conjunction with a Mustard operation. Coronary artery issues can be seen following the arterial switch (Jatene procedure), due to issues with transferring the coronary arteries from the native aorta to the neoaortic root. The coronary arteries are not moved in an atrial switch (Mustard) operation, so coronary artery complications are uncommon in this setting.

13. Answer: D. Truncus arteriosus is a lesion in which the aorta and the pulmonary arteries arise from a common arterial trunk, which overrides a usually large ventricular septal defect (VSD). Surgery occurs within the first few weeks of life and consists of separating the pulmonary arteries from the aorta, placing a right ventricular to pulmonary artery conduit, and patch closure of the ventricular septal defect. This is often referred to as a Rastelli operation. The conduit requires multiple replacements throughout a lifetime as it does not grow with the patient and is subject to calcification causing various levels of obstruction to right ventricular outflow. Conduit stenosis is often first detected echocardiographically on spectral Doppler interrogation of the conduit itself or by estimating elevated right ventricular systolic pressures via tricuspid regurgitation jet velocities (choice D). Patients with unrepaired truncus arteriosus are at risk for the development of pulmonary hypertension prior to surgery given the presence of a large VSD and the arterial communication between the systemic and pulmonary circulations (common arterial trunk). The operation surgically separates the two circulations, the pulmonary pressure should become normal after VSD closure, and it is therefore unlikely for pulmonary hypertension to continue or to progress. Truncal valve stenosis would result in obstruction to systemic outflow following surgery but will not raise right ventricular pressures. A modified Blalock-Taussig (BT) shunt is a common form of surgical palliation of inadequate pulmonary blood flow in infancy. Truncus arteriosus usually presents with excessive pulmonary blood flow and it would be unlikely for a patient with this defect to have a BT shunt at any time. Worsening pulmonary vein stenosis is unlikely given that the pulmonary veins are not involved in surgical management of truncus arteriosus.

14. Answer: B. This patient has pulmonary hypertension, which is severe based on the elevated right ventricular systolic pressures. However, the pulmonary artery pressures could be expected to be elevated due to the presence of a large PDA. The patient is also profoundly hypoxic with a saturation of 60%. With this level of hypoxemia and otherwise normal anatomy (excepting the PDA) one can presume a pulmonary issue, likely due to persistently elevated pulmonary resistance, and shunting of blood at the ductal level should be completely right to left, away from the pulmonary bed. It would also be typical to see right-to-left shunting at the atrial level under these conditions. This situation actually occurs relatively frequently in newborn nurseries and may be provoked by severe respiratory problems such as meconium aspiration. A continuous left-to-right shunt would be expected in a patient with a ductus arteriosus in whom the pulmonary vascular resistance has begun to fall, which usually occurs normally in the first few hours of life. Bidirectional shunts may occur with a large ductus, and the predominance of flow in one direction or the other is dictated by the relative pulmonary or systemic vascular resistance at that time. However, bidirectional shunting would not produce the profound hypoxemia seen here.

15. Answer: E. Patients with transposition of the great arteries have two parallel circulations and survive with areas where the systemic and pulmonary circulations can mix. These include the atrial septum, the ventricular septum and the ductus arteriosus. The atrial septum is by far the most effective place for infants with transposition to have mixing. If the atrial defect is small and constricted, this will likely result in more cyanosis, even if the ductus arteriosus is widely patent. Infants with a large VSD may also have problems mixing at the ventricular level. In this case, the ductus arteriosus should be examined, but prostaglandin is generally an effective drug and keeps the ductus widely patent. Checking the dosage, route of delivery and status is always a good idea and would be a close second to the atrial septum as a place to check right away. The aortic arch should not have any bearing on cyanotic or hypoxemic issues. Anomalies of systemic and pulmonary venous return may be involved in cyanosis, but are less likely as answers here.

16. Answer: D. This patient has tetralogy of Fallot. Figure 31-1A shows malaligned ventricular septal defect and overriding of aorta. Figure 31-1B shows right ventricular outflow tract obstruction in the form of infundibular (subpulmonary) narrowing. Tetralogy of Fallot is a conotruncal anomaly that is classically defined as having the following four components: (1) right ventricular hypertrophy, (2) ventricular septal defect, (3) overriding aorta, and (4) pulmonary stenosis. Tetralogy occurs in approximately 9% of children born with congenital heart defects. The ventricular septal defect in TOF is usually large and nonrestrictive. Only in rare cases, will it be restrictive. In the given figure, the VSD appears to be typically large and nonrestrictive. This will result in equalization of pressure between the ventricles. Since the left ventricle will always pump systemic pressure, this will also be the pressure in the right ventricle.

17. Answer: A. The patient has hypoplastic left heart syndrome (HLHS), which is a ductal-dependent lesion. The image in Figure 31-2A is an apical 4-chamber view which demonstrates two atria with a single ventricle and single AV valve. The left atrium is small and there is no obvious second AV valve. Figure 31-2B is a suprasternal view showing a diminutive ascending aorta. Figure 31-13 is the same image with three arrows outlining the extremely hypoplastic ascending aorta and the aortic arch labeled which confirms that this is HLHS. This tiny, diminutive ascending aorta is a classic appearance when an atretic aortic valve is present. HLHS typically consists of varying degrees of aortic stenosis or atresia, mitral stenosis, coarctation of the aorta, and hypoplasia of the left ventricle and aortic arch. The ductus arteriosus must be kept open to supply blood to the systemic circulation, until a Norwood (stage I) palliative surgery can be performed. Initiation of 100% FiO₂ is not recommended, as it will increase pulmonary blood flow and decrease systemic blood in the face of systemic (aortic arch) obstruction. As there are varying degrees of leftsided obstruction, cardiac output is maintained by circumventing the majority of the left heart. This is accomplished by allowing the pulmonary venous return to cross the atrial septum, travel through the right atrium, tricuspid valve, right ventricle, and main pulmonary artery to the ductus arteriosus and out to the descending aorta, also bypassing the coarctation and hypoplastic aortic arch. Therefore a left-to-right atrial level shunt is essential to maintaining cardiac output, not a right-to-left atrial level shunt as suggested in answer C. Moderate to severe atrioventricular valve regurgitation impair cardiac output and is not well tolerated in HLHS. A right aortic arch is very rare with this anatomy.

18. Answer: B. This patient has Ebstein anomaly of the tricuspid valve. This lesion was initially described in 1866 by the German physician Wilhelm Ebstein. Characteristic pathologic findings include apical displacement of the septal and posterior leaflets of the tricuspid valve into the right ventricle to varying degrees. The arrows in Figure 31-14 show the tricuspid valve annulus. The portion of the right ventricle between the true valve annulus and the apically displaced valve leaflets forms an “atrialized” portion of the right ventricle that is continuous with the true right atrium. Although the atrialized portion of the right ventricular anterior wall may be thin, the distal unaffected portion of the right ventricular wall is usually normal in thickness.

In Ebstein anomaly, the effective right ventricular size is reduced depending upon the severity of the tricuspid valve displacement. However, it is not associated with hypoplasia of right ventricle. Choice C is incorrect as the tricuspid valve is seen to be displaced apically. The anterior leaflet of tricuspid valve cannot be seen in apical 4-chamber view. Instead, the posterior and septal leaflets are very well seen in Figure 31-14.

19. Answer: C. This patient has D-transposition of great arteries. Transposition is defined as a connection of the aorta to the right ventricle and the pulmonary artery to the left ventricle. This abnormal ventricular arterial connection is also termed “ventriculoarterial discordance” and probably results from abnormal conotruncal septation. Transposition occurs in approximately 4%-8% of children born with congenital heart defects. “D-transposition” is a term which refers to the way the conotruncal septum rotates in utero (“D” for dextro) and has been commonly applied to this entity. In transposition, the aorta arises from the right ventricle, usually in a position which is anterior and rightward of the pulmonary valve (Fig. 31-4B in this case). The two great arteries course parallel to one another; a distinctly different arrangement from the normal pulmonary artery crossing over the aortic root (Fig. 31-4A in this case).

Figure 31-15 demonstrates the position of the great arteries as commonly seen from the echocardiographic perspective of the parasternal shortaxis view. Great artery position is a key to identify the type of transposition. Normally, the pulmonary artery wraps around the aorta anteriorly as it courses posteriorly (demonstrated in Figure 31-15B). With D-transposition the great arteries assume a parallel course, and the aorta is located anterior and rightward of the centrally located pulmonary artery (demonstrated in Figure 31-15A). In L-transposition, the great arteries are again parallel, but the aorta is anterior and leftward (demonstrated in Figure 31-15C). It is important to note that in normally related great vessels, the pulmonary valve and aortic valve are not in the same plane while in any form of transposition, they are usually in the same plane as shown in Figure 31-4B as D and C, respectively because of the generally more parallel course of the great arteries in transposition.

20. Answer: A. Figure 31-5A illustrates a malalignment ventricular septal defect with overriding of aorta. Figure 31-5B shows the narrowing of right ventricular outflow tract, hypoplastic pulmonary valve, and hypoplastic main pulmonary artery. This is consistent with the diagnosis of tetralogy of Fallot. The oxygen saturation is determined by how much blood goes across the RVOT into the pulmonary artery and thereby the degree of RVOT obstruction. It is important to note that RVOT obstruction can be at any level and can be at multiple levels—subvalvar, valvar, and/or supravalvar. The degree of aortic override is usually about 50% but has been observed to range from 15% to 95% in one echocardiographic study. However, neither the ventricular septal defect nor the degree of overriding of the aorta determines the level of saturation in TOF. An atrial septal defect/patent foramen ovale is present in most cases of TOF. The presence of an ASD is important to know for surgical purposes, but it does not have any role in determination of oxygen saturation in TOF.

Based on the severity of right ventricular outflow tract obstruction, TOF can present in mainly three different forms. These clinical/anatomic variables (shown in Fig. 31-16) include: (a) “Pink” tetralogy, (b) Classic tetralogy, and (c) Pulmonary atresia/VSD (tetralogy with pulmonary atresia). It is important to recognize that these descriptions refer to a starting point at the time of diagnosis and are helpful in trying to predict the subsequent clinical course. In the “pink” form of TOF, there is minimal narrowing/RVOT obstruction. These infants will have very little or no cyanosis at all and may even exhibit some symptoms of pulmonary overcirculation. The right ventricular outflow obstruction in tetralogy tends to change and worsen with time, causing more severe restriction of pulmonary blood flow. In the classic form of TOF, there is less blood flow to the lungs, the peripheral saturation will be lower than normal and some infants are cyanotic. These patients may need a shunt surgery (Blalock-Taussig shunt) in the early period of their life to provide adequate pulmonary blood flow. Patients may start life “pink” and progress to a cyanotic stage with time. However, in practical terms this happens infrequently in the modern age, because infant surgery is generally available to address this trend. Early primary repair is often carried out within the first few months of life. Patients with pulmonary atresia are ductal dependent and represent a very complex spectrum of tetralogy patients. There may be many anatomic variables in this group depending upon the anatomy of the pulmonary blood supply.

21. Answer: D. Ebstein anomaly is a severe deformity of the tricuspid valve which results from failure of the normal development of the septal and posterior leaflets. These leaflets become displaced apically and adherent to the septum and wall of the right ventricle, respectively. The anterior leaflet becomes enlarged and “sail-like” with variable attachments to the trabecular portion of the right ventricle and outflow area. The apical displacement of the valve reduces the effective volume of the right ventricle available for pumping function. In addition, the Ebstein’s valve usually is quite insufficient. All of these factors contribute to tricuspid regurgitation, poor forward flow, and the potential for right-to-left shunt through and ASD or foramen ovale. Thus, patients with severe Ebstein anomaly may be profoundly cyanotic, particularly as newborns. This cyanosis usually resolves after several weeks once pulmonary resistance starts to drop after the birth of the child. Patients with less severe form of Ebstein anomaly may be clinically quite well for many years till young adulthood such as given in the case.

This is not a case of tricuspid atresia as the tricuspid valve is seen, but displaced apically. Patients with pulmonary hypertension (primary or secondary to Eisenmenger syndrome) can present with low saturation and limitation of exercise capacity, but they do not have the anatomical abnormality of the tricuspid valve as shown in the figure. A large ASD can lead to dilation of the right atrium and right ventricle secondary to a large left-to-right shunt. Less than 7%-10% of large ASDs can develop Eisenmenger syndrome in their late adulthood. Nonetheless, they also will not have such anatomical abnormality of tricuspid valve. Uhl’s anomaly is an extremely rare congenital heart defect characterized by an almost total absence of the right ventricular myocardium. Less than 20 cases had been reported so far. It was first described in 1980. In Uhl anomaly, the tricuspid valve is normal.

22. Answer: A. The image shown demonstrates a large VSD with an overriding great vessel. The differential diagnosis for this image includes defects with an anterior malalignment ventricular septal defect (choices B, C, and D) and truncus arteriosus (choice E). This image is not consistent with a diagnosis of interrupted aortic arch with normally related arteries and ventricular septal defect, which is associated with posterior malalignment of the ventricular septum.

Anterior malalignment of the ventricular septum results in a large VSD and some degree of obstruction to the anterior outflow. The obstruction could be to the systemic outflow if the great arteries are D-malposed, such that the aorta is anterior and rightward of the pulmonary artery. Pulmonary atresia with ventricular septal defect and tetralogy of Fallot both feature an obstructed anterior outflow tract to varying degrees. Truncus arteriosus will commonly display a large VSD and with an overriding semilunar (truncal) valve when imaged from the parasternal long-axis view. Parasternal long-axis views of an interrupted aortic arch with normally related great arteries would most commonly demonstrate an obstructed posterior (systemic) outflow tract, often with subaortic stenosis and aortic valve hypoplasia. A VSD is commonly present with interrupted aortic arch.

23. Answer: C. This infant has truncus arteriosus. The parasternal long-axis view shows the malalignment VSD and overriding great vessel, similar to many other conotruncal abnormalities. Figure 31-17A shows that the great artery divides into two segments, typical of truncus arteriosus. Color flow Doppler can help to illuminate the division of the great artery as shown in Figure 31-17B. The arrows demonstrate the division of the common trunk into the anterior aorta and the more posterior pulmonary artery segment. This is often better seen from a parasternal short-axis view as shown in Figure 31-17C. Truncus arteriosus is an uncommon congenital heart defect of the outflow tract of the heart. A single arterial vessel gives rise to the systemic, pulmonary and coronary arteries. By definition, there is always a large malalignment VSD and the presence of a truncal valve instead of separate pulmonary and aortic valves. During normal embryology, the common arterial trunk undergoes septation to allow the aorta to arise from the left ventricle and the main pulmonary artery from the right ventricle. Absence or abnormal conotruncal septation leads to persistence of truncus arteriosus.