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Atrial Septal Defect and Partial Anomalous Pulmonary Venous Connection
Definition
An atrial septal defect (ASD) is a hole of variable size in the atrial septum. A patent foramen ovale that is functionally closed by overlapping of limbic tissue superiorly and the valve of the fossa ovalis inferiorly (in response to the normal left-to-right atrial pressure gradient) is excluded. ASDs generally permit left-to-right shunting at the atrial level. Partial anomalous pulmonary venous connection (PAPVC) is a condition in which some but not all pulmonary veins connect to the right atrium or its tributaries, rather than to the left atrium. The term connection is preferred to the term “return,” because connection is anatomic and return is governed by hemodynamic factors. PAPVCs may occur as isolated anomalies or may be combined with ASDs.
These two groups of anomalies are considered together in this chapter because they manifest similar physiology and result in similar clinical findings. Total anomalous pulmonary venous connection is considered in Chapter 31 . ASDs typically occur in association with other cardiac anomalies, and these are considered in chapters dealing with those anomalies.
Historical Note
Clinical recognition of an ASD has been possible only in about the past 70 years. Among the 62 recorded autopsy cases of ASD analyzed by Roesler in 1934, only one had been correctly diagnosed during life. By 1941, Bedford and colleagues were able to make the diagnosis clinically in a number of patients. When cardiac catheterization came into general use during the late 1940s and early 1950s, secure diagnosis became possible. The first descriptions of PAPVC are attributed to Winslow in 1739 and Wilson in 1798. The first diagnosis of PAPVC during life was reported by Dotter and colleagues in 1949.
A number of ingenious closed methods for repair of ASDs and related conditions were proposed and studied experimentally in the productive and expansive surgical era following the end of World War II in 1945. In 1948 in Toronto, Murray reported closing an ASD in a child by external suturing. Several other closed methods had clinical application, including Bailey and colleagues’ “atrioseptopexy” and Søndergard's purse-string suture closure. However, limited applicability of these methods was always apparent, and they were soon abandoned.
Hypothermia, induced by surface cooling, and inflow occlusion for repair of ASDs was introduced during the early 1950s (see Historical Note in Section I of Chapter 2 ). Lewis and Taufic reported the first successful open repair of an ASD with this method in 1953. At about the same time, Gross invented the ingenious atrial well technique, a semi-open approach in which a rubber open-bottomed well or cone was sutured to an incision in a clamp-exteriorized portion of the right atrial wall. When the clamp was released, the blood rose into the well, and through this pool of blood, the surgeon could place sutures under digital control for direct or patch closure of the defect. Gibbon started the era of open heart surgery in 1953 when he successfully repaired an ASD in a young woman using a pump-oxygenator. Although these three methods—hypothermia and inflow occlusion, atrial well, and cardiopulmonary bypass (CPB)—were all used during the late 1950s and provided similar early results, by the late 1960s almost all surgeons used CPB exclusively for these repairs. Percutaneous catheter techniques for closing a fossa ovalis ASD using a polyester double umbrella device were introduced by King and Mills in 1974.
The first reported treatment for a type of PAPVC was lobectomy in 1950. In 1953, Neptune and colleagues reported repair using a closed technique in 17 patients with PAPVC of the right lung to the right atrium associated with ASD. It is not certain who first repaired the sinus venosus syndrome, but the malformation was clearly illustrated by Bedford and colleagues in 1957. Repair of PAPVC to the inferior vena cava was performed by Kirklin and colleagues at Mayo Clinic in 1960 and was also subsequently reported by Zubiate and Kay in 1962. Correction of anomalous connection of the left pulmonary veins to the left brachiocephalic vein and other forms of PAPVC was reported from the Mayo Clinic in 1953 and later in 1956.
Morphology
Types of Atrial Septal Defect
As viewed from the right atrial side (see Fig. 1-2 in Chapter 1 ), the normal atrial septum may have defects in almost any location ( Box 30-1 ). Although the morphology of these defects has been known since the early descriptions by Robitansky in 1875, the advent of open heart surgery emphasized their surgically important aspects ( Fig. 30-1 ).
Box 30-1
Types of Atrial Septal Defect a
a Fossa ovalis, posterior, and most confluent defects can be classified as secundum type .
-
Fossa ovalis defect b
b Varies in size from small valvar-incompetent foramen ovale ASD to complete absence of septum primum tissue with resultant ASD extending to inferior vena cava.
-
Posterior defect
-
Coronary sinus defect
-
Sinus venosus defect
-
Confluent defect
-
Ostium primum defect (absence of atrioventricular septum)
Fossa Ovalis Defect
The most common ASD is the fossa ovalis type, also called foramen ovale type or ostium secundum defect . This defect lies within the perimeter inscribed by the limbus anteriorly, superiorly, and posteriorly ( Fig. 30-2 ). The smallest defects are essentially valvar incompetent foramina ovale that occur beneath the superior limbus, between it and the valve (floor) of the fossa ovalis. The floor of the fossa ovalis (remnant of septum primum) may in this situation have multiple fenestrations of various sizes ( Fig. 30-3 ). When more of the floor of the fossa ovalis is absent, a larger fossa ovalis defect is present. When all fossa ovalis tissue is absent, the ASD is confluent with the orifice of the inferior vena cava (IVC). The eustachian valve of the IVC then overhangs the ASD and must not be mistaken for its inferior edge at operation. Size of this type of ASD is also affected by any hypoplasia of the limbus that may be present. When the limbus is quite hypoplastic anteriorly, there is only a thin rim of tissue above the atrioventricular (AV) valves (formerly this was called an intermediate defect and was sometimes confused with an ostium primum defect). The limbus may also be hypoplastic superiorly or posteriorly.
Normally the IVC–right atrial junction is partly to the left of the plane of the limbus, so that when the floor of the fossa ovalis is absent and an ASD of fossa ovalis type extends to the IVC, the caval ostium overrides (or straddles) the defect onto the left atrium. This defect results in some right-to-left shunting of IVC blood to the left atrium in virtually all patients with a large fossa ovalis–type ASD (as documented in experimental studies ) and severe shunting with cyanosis in a few patients. Also, the position of the normally connected right pulmonary veins next to the atrial septal remnant results in preferential left-to-right shunting of their venous drainage.
Posterior Defect
A defect in the most posterior and inferior part of the atrial septum, with absence, hypoplasia, or anterior displacement of the posterior limbus, is termed a posterior ASD . The orifices of the right pulmonary veins usually open directly into the area of the defect, but true anomalous pulmonary venous connection of the right lung frequently coexists. In the pure form of this type of ASD, the tissue of the fossa ovalis (including the posterior limbus) is present, and the ASD is an oval defect posterior to this tissue ( Fig. 30-4 ).
Sinus Venosus Defect
The ASD that occurs in sinus venosus syndrome (subcaval defect, superior vena caval defect) is located immediately beneath the orifice of the superior vena cava (SVC), superior to the limbic tissue, and is usually associated with anomalous pulmonary venous connection of the right superior pulmonary vein to the SVC near or at the SVC–right atrial junction. The lower margin of the defect is a sharply defined crescentic edge of atrial septum, whereas its upper margin is devoid of septum, being continuous with the posterior SVC wall, which in turn is continuous with the upper edge of the left atrium. The SVC usually overrides the atrial septum onto the left atrium to some extent (see “ Sinus Venosus Malformation [Syndrome] ” later in this chapter).
Coronary Sinus Defect
Coronary sinus ASDs are part of unroofed coronary sinus syndrome . When the sinus is completely unroofed and no partition is present to separate it from the left atrium, the ostium of the coronary sinus is a hole in the atrial septum that permits free communication between left and right atria (see Chapter 33 ). Occasionally a fenestration may exist in this partition in the midportion of the coronary sinus, particularly in hearts with tricuspid atresia, or rarely the fenestration may be almost at the ostium of the coronary sinus ( Fig. 30-5 ).
Confluent Defect
Large ASDs may represent a confluence of two of the defects already described. Thus, a fossa ovalis defect coexisting with absence of the posterior limbus can present as a very large ASD with no septal remnant posteriorly. Another confluent defect occasionally seen is a combination of coronary sinus and fossa ovalis ASDs.
Ostium Primum Defect
An ASD occurs anterior to the fossa ovalis (and the anterior limbus) when the AV septum is absent. Such defects are called AV septal defects, AV canal defects, or ostium primum atrial septal defects and are considered in Chapter 34 . When essentially the entire atrial septum is absent (common atrium), the defect includes absence of the AV septum (see “Atrial Septal Deficiency and Interatrial Communications” under Morphology in Chapter 34 ).
Types of Partial Anomalous Pulmonary Venous Connection
Sinus Venosus Malformation
The most common type of PAPVC is the defect present in sinus venosus malformation, in which PAPVC coexists with a superior caval ASD. In sinus venosus malformation, the right upper and middle lobe pulmonary veins (right superior pulmonary vein) attach to the low SVC or the SVC–right atrial junction, an arrangement present in about 95% of patients with a superior caval ASD. Most often, the anomalous pulmonary venous connection is through two anomalous veins from upper and middle lobes, one superior to the other, but there may be three or rarely four veins, with the uppermost entering the SVC near the azygos vein entry. Infrequently, only part of the right superior vein connects anomalously, with the inferior (right middle lobe) portion of that vein connecting to the left atrium. Rarely, both the right superior and right inferior pulmonary veins connect anomalously to the low SVC or SVC–right atrial junction ( Fig. 30-6 ).
The lowermost part of the SVC that receives the anomalous veins is usually wider than normal, although it may be relatively small, particularly when there is also a well-formed left SVC, which is not uncommon. The SVC typically overrides the atrial septum to some extent and enters partly into the left atrium, resulting in a right-to-left shunt of some SVC blood to the left atrium. In a few patients, SVC overriding is severe enough to produce a large right-to-left shunt and marked cyanosis. The overriding may also be complete, so that the SVC drains directly and completely into the left atrium.
The relationship between anomalous connection of the SVC to the left atrium without an ASD and sinus venosus ASD is indicated by connection of pulmonary veins from the right upper lobe to the cardiac end of the SVC in some patients with PAPVC. This relationship also occurs in patients with no ASD but in whom the pulmonary veins from the right upper lobe are connected to the cardiac end of the SVC, with the SVC connected to the left atrium by a large opening, and to the right atrium by a small opening.
Rarely, a typical high superior caval ASD is present without anomalous pulmonary venous connection; right pulmonary veins connect to the left atrium but more superiorly than normal.
Right Superior Pulmonary Vein to Superior Vena Cava
Occasionally the entire right superior pulmonary vein connects to the SVC without an associated superior caval ASD. The connection is then usually well above (superior to) the SVC–right atrial junction, and the lower part of the SVC is not dilated. Rarely, even when no superior caval ASD is present, the connection may be in the typical low position of sinus venosus syndrome. At times, only a portion of the right superior pulmonary vein draining one or two segments of the right upper lobe connects directly to the SVC. The PAPVC may be isolated or associated with a fossa ovalis ASD.
Right Pulmonary Veins to Right Atrium
Right pulmonary veins may connect directly to the right atrium, either in toto, where they may connect as two or three separate veins, or only through the superior (or rarely inferior) right pulmonary vein. This anomaly may exist as an isolated defect, without an ASD or with only a patent foramen ovale, with the plane of the atrial septum altered from coronal to near-sagittal because of leftward displacement of its lateral attachment. The plane of the right pulmonary vein is actually altered minimally from normal. Because the posterior limbus is present in such defects, the veins are clearly anomalously connected to the right atrium. In ASDs with absence of posterior limbus (posterior ASD), and at times in large fossa ovalis ASDs, the plane of division between right and left atria posteriorly can be questionable, and thus the atrial connection of the right pulmonary veins in this area is debatable. In such defects, however, true anomalous connection of the right pulmonary veins may be present (see Fig. 30-4 ).
Right Pulmonary Veins to Inferior Vena Cava (Scimitar Syndrome)
An anomalous right pulmonary vein, generally draining the entire right lung but occasionally only the middle and lower lobes, may descend in a cephalad-to-caudad direction toward the diaphragm, more or less parallel to the pericardial border but with a crescentic (scimitar) shape, and then curve sharply to the left just above or below the IVC–right atrial junction. The anomalous pulmonary venous trunk usually passes anterior to the hilum of the right lung but occasionally is posterior to it. Entrance into the IVC is just superior to the hepatic vein orifices. The atrial septum may be intact, or a fossa ovalis ASD may be present. Occasionally the anomalous vein also connects to left atrium, and rarely scimitar syndrome can exist with connection of the anomalous vein only to left atrium. Pulmonary venous drainage is then normal. (Rarely, the left lung may connect via a scimitar-shaped vein to the IVC. )
Right-sided scimitar syndrome occurs as an isolated malformation in a minority of cases. In most patients, anomalies of the right lung are also present. The most common anomaly is right lung hypoplasia, which is associated with a marked mediastinal shift and dextroposition of the heart, and in its severe form with the entire heart lying in the right side of the chest. Blood supply to the hypoplastic right lung comes mainly from a branch of the abdominal aorta in the region of the celiac axis, which ascends through the inferior pulmonary ligament to supply the lower lobe, or more often the entire right lung. A small pulmonary artery may be present, but often the central and hilar portions of the right pulmonary artery are absent. Occasionally a true right lower lobe bronchopulmonary sequestration may exist, with secondary intrapulmonary cyst formation.
Associated cardiac anomalies are often present in scimitar syndrome. In one study, for example, 11 of 13 infants had associated malformations, seven of whom had left-sided hypoplastic conditions. Diaphragmatic anomalies occurred in about 20% of the cases reviewed by Kiely and colleagues. These defects included herniation of the right lung through the foramen of Bochdalek and abnormal attachments of the diaphragm.
Rare Connections of Right Pulmonary Veins
Rarely, right pulmonary veins connect anomalously to the azygos vein or coronary sinus, with or without a fossa ovalis ASD.
Left Pulmonary Venous Connections
Left pulmonary veins may connect to the left brachiocephalic vein by way of an anomalous vertical vein. Anomalous drainage is usually from the entire left lung, but may be only from the left upper lobe. A fossa ovalis ASD coexists in some patients, and in others the atrial septum is intact. Rarely, left pulmonary veins connect anomalously to the coronary sinus, a right-sided SVC, or the right atrium.
Bilateral Partial Pulmonary Venous Connection
Partial but bilateral anomalous pulmonary venous connection is rare. The most common variant is probably the defect in which the atrial septum is intact, the left superior pulmonary vein attaches to the left brachiocephalic vein by way of an anomalous vertical vein, and the right superior pulmonary vein attaches to the SVC–right atrial junction. In another form, a common pulmonary venous chamber is present (see “Pulmonary Venous Anatomy” under Morphology in Chapter 31 for definition), and some veins from both lungs connect to it. All but one lobe or only one lobe from each side may connect to the sinus. The common venous sinus may connect to the right atrium or brachiocephalic vein.
Cardiac Chambers in Atrial Septal Defect and Related Conditions
Typically in ASD and related conditions, the right atrium is greatly enlarged (at least grade 3 or 4 on a scale of 1 to 6) and thick walled. The left atrium is not enlarged. This discrepancy occurs in the absence of any flow or pressure restriction between the two, speculatively because the right atrial wall is more distensible than the left.
Right ventricular (RV) diastolic size is increased, often greatly, because of volume overload imposed by the left-to-right shunt. Whereas normal RV diastolic dimensions are between 0.6 and 1.4 cm · m 2 , in patients with large left-to-right shunts at atrial level they average 2.66 cm · m 2 and may be as large as 4 cm · m 2 . Consequently, the cardiac apex is often formed by the RV.
Morphologically, the left ventricle (LV) is normal or slightly decreased in size. However, important LV dynamic abnormalities are present in most patients (see “ Mitral Prolapse ”).
Mitral Valve and Atrial Septal Defects
Mitral Prolapse
Mitral valve prolapse occurs in association with fossa ovalis ASD, sinus venosus syndrome, and probably other types of ASDs and related conditions that result in left-to-right shunts at the atrial level. Prevalence of true prolapse is about 20%, increasing with age and with magnitude of the pulmonary-to-systemic blood flow ratio ( 
).
Schreiber and colleagues have clarified a previously confused subject by relating mitral valve prolapse to abnormalities of LV shape in patients with ASD. Alteration in LV configuration results from leftward shift of the ventricular septum, a process that begins as a slight decrease in the normal rightward convexity and progresses with time to flattening and then reversal, with a resultant central bulge into the LV. This process is a response to RV enlargement, which is secondary to volume overload. This etiologic basis of mitral valve prolapse is supported by its decreased degree or elimination in most cases by ASD closure, with return of LV geometry to normal.
Mitral Regurgitation
Mitral prolapse in ASD can lead to mitral regurgitation, as does ordinary mitral prolapse. True prevalence of regurgitation in unselected patients varies because older patients and those with larger pulmonary blood flows have a higher prevalence of this abnormality and prolapse. Prevalence of mitral regurgitation severe enough to require correction at the time of ASD repair is about 5% or less. The data of Leachman and colleagues strongly suggest that this type of mitral prolapse can also precipitate chordal rupture, as it can in Barlow syndrome.
Cleft Mitral Leaflets
Cleft anterior or posterior mitral leaflets that cause mitral regurgitation are reported to occur occasionally in patients with ASD. However, judging from some of the illustrations of such “clefts,” they may simply be spaces between commissural and main leaflets in prolapsed valves.
Lungs and Pulmonary Vasculature
Pulmonary arteries are considerably dilated and elongated when pulmonary blood flow is increased. This dilatation involves even the smallest branches, which tend to compress the smaller airways, with resultant retention of secretions and bronchiolitis.
Hypertensive pulmonary vascular disease develops infrequently in patients with ASD, and then usually not until the third or fourth decade of life (see “Pulmonary Vascular Disease” under Morphology in Section I of Chapter 35 ). This contrasts sharply with ventricular septal defects (VSDs), complete AV septal defects, and patent ductus arteriosus, in which pulmonary vascular disease may be present early in life. In ASD, pulmonary vascular disease is caused mainly by secondary thrombosis in the dilated pulmonary artery branches, with changes in the intima and media of vessels usually playing a minor role. Haworth has suggested, however, that an increase in pulmonary arterial smooth muscle may be the only finding.
Associated Cardiac Conditions
ASDs and related conditions may coexist with almost all varieties of congenital heart disease, but such cases are not considered here unless the left-to-right shunt at atrial level is the dominant hemodynamic lesion. A wide spectrum of cardiac anomalies coexist with ASD as the dominant lesion ( Table 30-1 ).
Table 30-1
Associated Cardiac Anomalies in Patients with Atrial Septal Defect or Partial Anomalous Pulmonary Venous Connection a
| Anomaly | No. | % of 443 |
|---|---|---|
| Left superior vena cava | 24 | 5 |
| Mild or moderate pulmonary artery stenosis | 16 | 4 |
| Peripheral pulmonary artery stenosis | 4 | 1 |
| Azygos extension of inferior vena cava | 4 | 1 |
| Small ventricular septal defect | 2 | 0.01 |
| Small patent ductus arteriosus | 2 | 0.01 |
| Mild coarctation of aorta | 2 | 0.01 |
| Small coronary artery–pulmonary trunk fistula | 2 | 0.01 |
| Anomalous right subclavian artery | 2 | 0.01 |
| Dextrocardia (isolated) | 1 | 0.005 |
a Data from 443 patients undergoing repair at GLH from 1957 to 1983. Some patients had more than one anomaly, so the figures are not cumulative.
Valvar heart disease may coexist with hemodynamically important ASDs. Six cases with moderate or severe rheumatic mitral stenosis and a hemodynamically significant ASD (Lutembacher syndrome) were observed among 443 patients with an ASD at GLH (1957-1983). Eleven cases of moderate or severe mitral regurgitation were observed; in three, regurgitation was rheumatic in origin. Both mitral stenosis and regurgitation increase left-to-right shunting.
Tricuspid regurgitation of variable severity frequently complicates ASDs in older patients with heart failure, the mechanism generally being RV and tricuspid anular dilatation.
Related Conditions
Rarely, ASD may occur in patients with Marfan, Turner, Noonan, or Holt-Oram syndromes.
Clinical Features and Diagnostic Criteria
Symptoms, clinical features, and signs in ASD and related conditions producing left-to-right shunting at the atrial level are related largely to size of the left-to-right shunt. Thus, in general, when 
is less than 1.5, there are neither signs nor symptoms of the shunt, and this is often true with a 
up to 1.8. When 
is larger than this, signs of the shunt are usually present, and symptoms appear eventually (see “ Changes in Pulmonary/Systemic Blood Flow Over Time ” later under Natural History). Infants present an exception to these generalizations. Their clinical features are often atypical; for example, splitting of the second heart sound is unrelated to 
.
Determinants of Interatrial Shunting
Left-to-right shunting across a nonrestrictive (>2 cm in an adult) ASD under ordinary circumstances is a function of the relative compliance (reflected in the diastolic pressures) of RVs and LVs. RV compliance in particular is unpredictable and is one factor causing variability in 
. A compliant distensible RV (in association with a normal pulmonary vascular bed) will permit a large shunt; a less compliant one (such as may result from pulmonary hypertension or from morphologic RV changes occurring later in life ) permits a more modest shunt. LV compliance tends to decrease with age, which tends to increase 
as patients become older. Shunting is increased by systemic hypertension when this results in decreased LV compliance.
Mitral regurgitation or stenosis increases 
. When the ASD is small and flow is restrictive, left-to-right shunting is limited. Even then, mitral stenosis may elevate left atrial pressure sufficiently that a large left-to-right shunt through all phases of the cardiac cycle results, leading to a soft continuous murmur.
Symptoms
Symptoms may be absent for several decades, but when they occur, they consist of effort breathlessness and a tendency toward recurrent respiratory tract infections. Palpitation from paroxysmal atrial tachycardia or atrial fibrillation may occur later in life. Older adults may present with chronic heart failure with fluid retention, hepatomegaly, and severe cardiac cachexia. Occasionally an infant with ASD and a large left-to-right shunt, often in association with PAPVC, may have heart failure with tachypnea, but this is uncommon. In such infants, other associated malformations may contribute to the heart failure.
Atypical presentations occur. Rarely, an unequivocal history of cyanosis may bring a patient with an uncomplicated ASD to medical attention. For example, a large fossa ovalis ASD extending to the IVC may cause streaming of blood from the IVC into the left atrium, with resultant cyanosis. This coincides with occasional bidirectional shunting in patients with otherwise uncomplicated ASDs, usually older patients. For the same anatomic reasons (see Morphology , earlier), patients may present with paradoxical emboli or cerebral infarctions. This presentation occurred in 9 (2%) of a Mayo Clinic series of 546 patients. Infrequently the presentation may be modified by presence of severe pulmonary hypertension, in which case cyanosis, effort intolerance, and hemoptysis may be present.
Signs
Clinical signs diagnostic of a large shunt at the atrial level ( 
> 1.8 to 2.0) are:
- ▪
Overactive left parasternal systolic lift
- ▪
Fixed splitting of the second heart sound throughout the respiratory cycle (absent when large
is from PAPVC with an intact atrial septum) - ▪
A soft pulmonary midsystolic flow murmur (in second and third left intercostal spaces)
- ▪
A mid-diastolic tricuspid flow murmur (in fourth and fifth left intercostal spaces) present in borderline situations only on inspiration
This last sign is occasionally absent, however, particularly in older patients and in those with a larger shunt.
In addition, an extremely large shunt produces a more marked left-sided precordial RV lift, occasionally some prominence of the left anterior chest wall, and leftward displacement of the cardiac apex. Many such patients are short and thin. When heart failure is present, jugular venous pressure is elevated, the liver is enlarged, and there is gross cardiomegaly.
Tricuspid regurgitation produces systolic liver pulsation and a greater tendency to ascites and peripheral edema. Important pulmonary hypertension is evident clinically by accentuation of the second heart sound and a more marked RV and pulmonary artery lift. A pulmonary regurgitation murmur may be heard, as well as a murmur of tricuspid regurgitation.
Chest Radiography
Chest radiography reflects the large 
. The right atrium and right ventricle are large. The pulmonary trunk shadow in the upper left portion of the cardiac silhouette is enlarged, and right and left pulmonary arteries are enlarged to the periphery of the lung field. In general, pulmonary vascular markings are increased, or plethoric. The shadow of the transverse aortic arch is abnormally small. Patients with heart failure may have interstitial pulmonary edema and areas of pulmonary consolidation and atelectasis. These signs are probably secondary to compression of smaller airways by enormously enlarged small pulmonary vessels.
The chest radiograph may suggest the specific anatomic diagnosis. Occasionally the right superior pulmonary vein can be identified lying more superiorly than normal ( Fig. 30-7 ), leading to suspicion of sinus venosus syndrome. A crescentic shadow more or less parallel to the right-sided heart border ( Fig. 30-8 ) suggests the diagnosis of anomalous pulmonary venous connection of right pulmonary veins to IVC (scimitar syndrome).
Electrocardiography
Electrocardiogram (ECG) almost always shows the pattern of incomplete right bundle branch block and a clockwise frontal loop. Left axis deviation and a counterclockwise loop strongly suggest an AV septal defect, although this pattern occurs in about 10% of patients with fossa ovalis ASDs.
Echocardiography
Clinical diagnosis of ASD, particularly of the foramen ovale type, can be confirmed by visualizing the defect directly using two-dimensional echocardiography ( Fig. 30-9 ). Echocardiography also gives indirect evidence of ASD in demonstrating RV volume overload, which includes increased RV diastolic size and abnormal (flat or paradoxical) septal motion. However, two-dimensional echocardiography has been unreliable (<75% sensitivity) in detecting anomalous connection of right pulmonary veins to SVC. More often these sinus venosus defects can be detected by transesophageal echocardiography (TEE). TEE also can localize fossa ovalis defects to the high, low, or posterior septum. AV septal defects can be separated from secundum defects, and localization of subcaval defects and anomalous pulmonary venous connection is usually possible. Addition of Doppler color flow interrogation allows a reasonable estimate of 
.
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