Holes between the atrial chambers are important congenital cardiac anomalies, both because they are relatively common, and because they are more nearly completely correctable than most other cardiac malformations, the more so in the new era of catheter intervention. It may be difficult, nonetheless, to diagnose the presence of such holes, and often there is no suspicion of organic cardiac disease during infancy and early childhood. Moreover, the anatomical arrangement, on occasion, is more complex than mere deficiency of the floor of the oval fossa. It is these anatomical features that underscore our specific description of holes between the atrial chambers, rather than the more usual reference to atrial septal defects. We will emphasise the subtle differences between true atrial septal defects and interatrial communications in our initial section devoted to morphology. It is the lack of early symptoms, nonetheless, coupled with the added subtlety of the physical findings, which delay the diagnosis, sometimes until well into adult life, or even until middle or old age. In contrast, if the left-to-right shunt is large, clinical diagnosis can still be made on the basis of a meticulous cardiac physical examination. The chest radiograph and electrocardiogram usually support such diagnosis, but echocardiography and Doppler studies are diagnostic in virtually all cases. Therapeutic intervention during childhood, after documentation of an important shunt at atrial level, should now result in a normal life expectancy, free of cardiac morbidity, and with the increasing use of devices inserted on catheters, without the need for thoracotomy.
INCIDENCE
Reports of incidence of holes between the atrial chambers have varied between the lesions accounting for one-twentieth to one-tenth of all congenital cardiac anomalies. 1–3 To a large extent, these discrepancies may depend on the definition of holes between the atriums. In the autopsy room, it may be difficult to distinguish those patients with deficiencies of the floor of the oval fossa from probe patency, in which the flap of the foramen had been competent during life, with the atrial communication being only potential. Probe patency is remarkably frequent, being discovered in up to one-third of all the hearts, irrespective of age. 4,5 With the finding that probe patency may underscore some cases of migraine, 6 diagnosis of this normal finding has achieved increased significance. Irrespective of the incidence of probe patency, the variable prevalence of defects noted in the different clinical series is related to the ages of the patients within the series. Thus, a series of infants and young children will have a relatively low prevalence of a clinically apparent inter-atrial communication as the primary defect, while a series of adults with congenital cardiac disease will have a high prevalence. This is due both to the attrition of patients with other severe and life-threatening defects, and to the frequent delay in diagnosis of an atrial defect until young adulthood. The prevalence also depends upon whether clinically unimportant interatrial communications accompanying more serious defects are included in the series. Prevalence of clinically significant defects, therefore, can perhaps best be judged by the relative frequency of the necessity for surgical closure. When preparing this chapter for the second edition of this book, we noted that, over a period of 25 years, more than 6000 infants and children under the age of 19 years had undergone surgery for treatment of congenital cardiac malformations at the Children’s Hospital of Pittsburgh. Of these children, an interatrial communication was the sole, or most important, anomaly in one-tenth of the series. Surgical closure of a hole between the atriums had been undertaken in a further one-sixth, in conjunction with repair of other anomalies. The excellent records kept of autopsies performed at Children’s Hospital in Pittsburgh also permit similar observations concerning recognition at postmortem examination. So, during that 25-year period, just over 1000 infants and children with congenital cardiac malformations had come to autopsy. Of these, none had died because of a hole between the atriums, but an interatrial communication had been observed as an incidental finding in one-sixth. The oval foramen was found to be probe patent in nearly two-fifths of the specimens. Holes between the atriums are said to be more common in females, with one report suggesting a ratio of this finding in two females to every male. 7 Others, 8 in contrast, found a slight predominance of males. The defects may be found in higher prevalence in populations residing at higher altitudes. 9
AETIOLOGY
There are no known intra-uterine events which predispose to deficient atrial septation, and most cases occur sporadically, with no family history of congenital cardiac disease. There is a significant familial incidence when the atrial septal defect is associated with certain skeletal abnormalities of the forearm and hand, the so-called Holt-Oram syndrome. 10 This has now been shown to be due to mutations of the Tbx5 gene, a member of the Brachyury family of genes. 11 When observed in families, atrial septal defects are also associated with prolongation of the PR interval. 12 There is also, occasionally, a familial incidence without these associated abnormalities of the skeleton or atrioventricular conduction. In the patients undergoing surgical closure in Pittsburgh over the period of 25 years discussed above, from one to five close relatives were recognised in nine families, giving a total of 18 familial cases.
MORPHOLOGY AND CLASSIFICATION
The accurate classification of interatrial communications is important, not because the location of the defect alters haemodynamics, but rather because of differences in the incidence of associated anomalies, differences in techniques of surgical repair, and implications for the option of interventional closure. For the surgeon, when planning repair, it is important to be aware of the anomalously connecting pulmonary veins which are the essence of the sinus venosus defect, or of the atrioventricular valvar abnormalities which are part and parcel of the ostium primums defect. This latter lesion, of course, is an atrioventricular septal defect with common atrioventricular junction, but with shunting between the chambers confined at atrial level. It will be described in a chapter to follow. As an extreme example of anatomical difficulties, nonetheless, it may be difficult to recognise a hole at the mouth of the coronary sinus as an interatrial communication, and not simply enlargement of the coronary sinus, if the diagnosis has not been established prior to attempted closure, either by surgery or interventional catheterisation.
The Normal Atrial Septum
Understanding the morphology, and classification, of interatrial communications is itself dependent on a thorough knowledge of the anatomy and extent of the atrial septum. This is because, as we will emphasise, some of the defects which permit shunting of blood at atrial level are outside the confines of the atrial septum. The septum has markedly different characteristics on its morphologically right and left sides. When the right atrium is opened through a wide incision, it seems at first sight that a large expanse of the atrial walls between the orifices of the superior and inferior caval veins and the attachment of the septal leaflet of the tricuspid valve are interposed between the right and left atriums ( Fig. 25-1 ). Sectioning the heart shows that only a small part of this area interposes as a common wall between the atrial cavities, specifically the floor of the oval fossa and its antero-inferior muscular margins ( Fig. 25-2 ). The superior part of the rim, often called the septum secundum, is produced because of infolding of the atrial walls between the mouth of the superior caval vein and the insertion of the right pulmonary veins to the left atrium. This is the area known to surgeons as Waterston’s, or Sondergaard’s, groove. A substantial cleavage plane, filled by extracardiac fat in the adult heart ( Fig. 25-3 ), extends down to the margin of the fossa.
The internal aspect of this groove represents the prominent muscle bundle which separates the fossa from the orifice of the superior caval vein. It is described appropriately as the superior rim of the fossa, or the superior interatrial fold. More anteriorly, the rim continues as the antero-superior atrial wall overlying the aortic root. This part is well described as the aortic rim of the fossa (see Fig. 25-2 ). When traced rightwards as viewed in attitudinally appropriate orientation, the rim of the fossa becomes directly continuous with the wall of the inferior caval vein. It is along the antero-inferior margin of the fossa that the morphology is most complicated. This part of the rim separates the fossa from the orifice of the coronary sinus, the mouth of the coronary sinus itself being separated from the mouth of the inferior caval vein by an additional folding, called the sinus septum. The tendon of Todaro, one of the important landmarks of the triangle of Koch, runs through this area to insert into the central fibrous body (see Fig. 25-3 ). The remainder of the antero-inferior rim is another true septal structure, produced by muscularisation of the developmental component known as the vestibular spine (see below). The right atrial wall continues beyond this septal component, inserting into the vestibule of the tricuspid valve, and forming the atrial component of the muscular atrioventricular sandwich (see Fig. 25-3 ; see also Chapter 27 ). As explained in Chapter 2 , this region is a sandwich rather than a true septum. This is because an extension of the inferior atrioventricular groove produces a fibroadipose layer between the sheet of atrial musculature and the underlying ventricular mass. It is only the thin translucent floor of the oval fossa, formed from the primary atrial septum, together with the antero-inferior muscular rim of the fossa, which are true interatrial septal structures. As such, they can be removed without exiting from the cavities of the heart ( Fig. 25-4 ). When viewed from the left atrial aspect, the morphology is less complex. The floor of the fossa is smooth, but in its anterosuperior margin it is roughened and wrinkled, forming two horns where it overlaps the infolded superior interatrial groove. In the majority of normal individuals, the flap valve and rim are fused, but as emphasised, anatomical fusion does not occur in about one-third of the population, and the heart shown in Figure 25-5 has probe patency of the oval foramen ( Fig. 25-6 ).
Types of Interatrial Communication
It is on the basis of this septal morphology that we should distinguish between the different anatomical types of interatrial communications, and recognise that not all are deficiencies of the septal components. Holes between the atriums can be divided into the so-called primum and secundum types, to which need to be added the sinus venosus defects, those found at the site of the mouth of the coronary sinus, and the rare vestibular defect ( Fig. 25-7 ).
Of these various lesions, it is the alleged secundum defects that are within the confines of the oval fossa, and hence, along with the vestibular defect, 13 those which represent true deficiencies of the atrial septum. The so-called secundum defects result from deficiency of the primary atrial septum, which forms the floor of the oval fossa. The vestibular defect occurs because of inappropriate muscularisation of the vestibular spine and the mesenchymal cap that clothes the leading edge of the primary septum during cardiac development (see Chapter 3 ). The other defects do unequivocally permit interatrial shunting, and so, with the exception of the primum defect, they are rightly considered in this chapter. They are, nonetheless, more accurately described as interatrial communications.
The so-called ostium primum defect is an atrioventricular septal defect found in the setting of a common atrioventricular junction, but in which the fused bridging leaflets of the common atrioventricular valve are also fused to the scooped-out crest of the ventricular septum. Because of this, shunting across the atrioventricular septal defect occurs only at atrial level. This defect is considered in Chapter 27 . The sinus venosus defects are found in the mouths of the caval veins, most usually the superior caval vein, but also in the mouth of the inferior caval vein. The phenotypic feature of these defects is that the oval fossa itself is intact, or else its floor is deficient. 14–16 These defects exist because of anomalous attachment of one or other of the right pulmonary veins. The coronary sinus defect is at the mouth of the coronary sinus. 17 It permits interatrial shunting because of fenestration of the walls which normally separate the coronary sinus and the left atrium. This type of defect is almost always associated with drainage of the left superior caval vein to the roof of the left atrium. The rare vestibular defect is another true septal defect, which is found when the components forming the antero-inferior rim of the fossa do not muscularise appropriately during cardiac development. 13 On the basis of this introduction, we will now describe these defects in more detail.
Defects Within the Oval Fossa
These defects are by far the most common type of interatrial communication, and are true atrial septal defects. Although usually termed secundum defects , this term is justified only because the defect is present at the site of the secondary embryonic foramen, and not because there is inappropriate formation of the so-called septum secundum. As we have emphasised, this alleged septum is no more than a deep fold between the attachments of the superior caval vein to the right atrium, and the right pulmonary veins to the left atrium (see Fig. 25-3 ). The defects found within the oval fossa result from deficiency, perforation, or complete absence of its floor, which is derived from the primary embryonic septum. As we have also emphasised, the upper border of the primary septum fails to fuse with the left atrial aspect of the infolded superior rim of the oval fossa in around one-third of normal individuals, even though it overlaps the rim (see Fig. 25-6 ). This is probe patency of the oval foramen. As long as left atrial pressure is higher than right, which is the normal situation, such a happening does not permit interatrial shunting under ordinary circumstances. Probe patency, nonetheless, is known to be responsible for paradoxical embolism, and can be an important finding in those undertaking deep sea diving. The finding has also been implicated recently as a cause of some cases of migraine, 6 and for this reason there has been a vogue for closing such probe-patent foramens by non-surgical means. When measuring such foramens, it should be remembered that the potential hole is a tunnel between the edge of the flap valve and the infolded superior rim of the fossa, rather than being a hole with dimensions that can be measured as occurs for the true septal deficiencies. There is a spectrum of size for defects within the oval fossa, depending on the degree of deficiency of the primary septum. The least severe type of defect is found when the flap valve is minimally deficient, so that it fails to overlap entirely the left atrial margin of the rim of the fossa. With increasing deficiency of the flap valve ( Fig. 25-8 ), the defect becomes larger. In extreme cases there may be no floor to the fossa ( Fig. 25-9 ). Alternatively, the flap valve may be perforated, with either single or multiple perforations ( Fig. 25-10 ). When the deficiency is marked, the hole can extend towards the mouth of the inferior caval vein, which may then straddle the persisting rim to open in part to the left atrium. In the most extreme form of septal deficiency, the hole can extend from the openings of the caval veins and coronary sinus to the septal attachment of the tricuspid valve, but still with a muscular infero-anterior rim separating the right and left atrioventricular junctions. This arrangement should be distinguished from an atrioventricular septal defect with common atrioventricular junction and virtually absent atrial septum. In addition to the small rim of antero-inferior musculature persisting when the defect is due to absence of the floor of the oval fossa, the atrioventricular valvar morphology will distinguish the two types. When the defect is within the oval fossa, the left atrioventricular valve has the characteristic morphology of the mitral valve, whereas when there is a common atrioventricular junction, it has the anticipated trifoliate arrangement (see Chapter 27 ).
When defects are present within the confines of the fossa, they do not alter the basic disposition of either the sinus or atrioventricular nodes. When the inferior rim is effaced, the node will of necessity be confined to the narrow strip of myocardium between the edge of the defect and the septal attachment of the tricuspid valve. This will still be within the triangle of Koch, readily visible in Figures 25-9 and 25-10 , but the triangle itself will be narrow. Apart from this circumstance, the morphology of the atrioventricular junctional area will be normal in the presence of septal defects within the oval fossa. Although these defects may exist in isolation, they often occur in combination with many other congenital cardiac malformations.
Sinus Venosus Defects
The essence of the so‑called sinus venosus defects is that they exist outside the confines of the oval fossa. 14–16 The defects are usually found in the mouth of the superior caval vein, but can also be found at the orifice of the inferior caval vein (see Fig. 25-7 ). In the case of the superior sinus venosus defect, the orifice of the superior caval vein typically overrides a defect, which has the superior rim of the fossa as its floor. The phenotypic feature of the defect is anomalous insertion of right pulmonary veins into the wall of the superior caval vein, which creates the conduit through which the blood can shunt between the atrial chambers ( Fig. 25-11 ). Indeed, we have now encountered a heart with such a defect in which there is no overriding of the orifice of the superior caval vein. Despite the normal attachment of the caval vein to the right atrium, the anomalous attachment of the right pulmonary veins still create the interatrial communication outside the confines of the intact atrial septum ( Fig. 25-12 ). In most instances, nonetheless, as seen in the specimen shown in Figure 25-13 , the defect has a well-circumscribed inferior margin, the superior rim of the oval fossa, but does not have a roof. In these more usual circumstances, the superior caval vein is attached in part to the right atrial wall and in part to the left atrial myocardium (see Fig. 25-13 ). Usually, the lower right pulmonary vein inserts into the left atrial wall, the middle pulmonary lobe vein drains into the area of the defect, and the upper right pulmonary vein drains directly into the superior caval vein. In this setting, the superior rim of the fossa becomes a muscular tube enclosing a corridor of extracardiac fat. A probe can be passed from back to front through the tube without encroaching on the atrial cavities (see Fig. 25-13 ). The presence of a superior sinus venosus defect does not markedly affect the site of the sinus node, which is found lateral to the superior cavo-atrial junction, lying immediately subepicardially within the terminal groove. Thus, a patch placed within the atrium to reconnect the caval vein to the right side should not jeopardise the sinus node. Because of the usual overriding of the superior caval vein, and the usual presence of anomalous pulmonary veins, it may not be possible to septate the defect in such a way that normal pulmonary venous return is restored without narrowing the superior caval pathway. In these circumstances, it may be difficult to widen the caval vein without putting either the sinus node or its vascular supply at risk, since the artery to the node may pass either in front of or behind the cavoatrial junction to enter the node. Despite this caveat, most defects can be closed without jeopardising normal function of the node. Inferior sinus venosus defects are far less common. 16 They, too, are associated with an anomalous attachment of the right pulmonary veins, but in these cases, it is the anomalous attachment of the right inferior pulmonary vein that creates the extracardiac conduit ( Figs. 25-14 ). Closure of such a defect should not jeopardise either the sinus or the atrioventricular node. Inferior sinus venosus defects must, of course, be distinguished from defects within the oval fossa which extend towards the mouth of the inferior caval vein (see Fig. 25-9 ).
Coronary Sinus Defects
The coronary sinus defect consists of an interatrial communication through the orifice of the coronary sinus, with absence of the usually adjacent walls of the coronary sinus and left atrium. Most times there is also connection of a persistent left superior caval vein to the left atrial roof. 17 In essence, the anomaly is the consequence of absence of the walls which normally exist between the coronary sinus and the left atrium. In their absence, the orifice of the coronary sinus becomes an interatrial communication ( Fig. 25-15 ). In some examples of this combination, filigreed remnants of the atrial wall persist between the orifice of the coronary sinus and the termination of the left caval vein in the left atrium. More commonly, as shown in Figure 25-15 , there is no tissue separating the coronary sinus, the mouth of the left caval vein, and the cavity of the left atrium. The opening of the coronary sinus is usually large. When such a defect is large, its anterior margin encroaches on the triangle of Koch, approximating the area of the atrioventricular node. Care must be taken, therefore, when the defect is closed. In this respect, unless previously diagnosed, a coronary sinus defect may be difficult to recognise as an interatrial communication in the operating room, appearing simply to be the mouth of the coronary sinus itself.
The Vestibular Defect
The heart shown in Figure 25-16 has a defect in the antero-inferior margin of the oval fossa. The fossa itself exhibits two additional defects, and the tricuspid valve is hypoplastic, since the patient also had pulmonary atresia in the setting of an intact atrial septum. This lesion is a true atrial septal defect through the septal component of the muscular margin of the oval fossa. 13
Morphogenesis of Interatrial Communications
During normal development, the embryonic primary septum grows down to divide the primary atrium, carrying with it a cap of mesenchymal tissue ( Figs. 25-17 and 25-18 , upper panels). This cap will fuse with the atrioventricular endocardial cushions, and will then be reinforced by the vestibular spine to close the primary interatrial foramen (see Figs. 25-17 and 25-18 , middle panels). As its lower edge fuses with the cushions, its top edge breaks down to form the secondary foramen (see Fig. 25-18 , middle panel). The upper edge of the remaining primary atrial septum is then usually overlapped by the infolding of the atrial roof between the superior caval and pulmonary veins, this fold forming the superior rim of the oval fossa, often described as the septum secundum (see Figs. 25-17 and 25-18 , lower panels).
Failure of normal development of the atrioventricular junction prevents the lower edge of the primary septum from fusing with the ventricular septum. This results in the so‑called ostium primum defect, which is an atrioventricular septal defect with common atrioventricular junction, and is described further in Chapter 27 . It is maldevelopment of the primary atrial septum itself, after it has fused with the ventricular septum and the endocardial cushions, which results in defects within the oval fossa. This can either be due to deficiency of its superior edge, so that it no longer overlaps the infolded superior rim, or breakdown of the septum to a greater or lesser degree, resulting in the various types of perforate or fenestrated defects within the fossa (see Fig. 25-10 ). In the most severe defects, the muscular rim also becomes effaced. It is possible to envisage temporary effacement of the rim of the oval fossa, since this is no more than a muscular infolding. Such effacement in the presence of volume load, with subsequent restoration of the fold once the volume load has been corrected, is a likely explanation for so-called spontaneous closure of some defects within the oval fossa during the neonatal period. Sinus venosus defects are best explained on the basis that the right pulmonary veins are abnormally attached to the wall of either the superior or inferior caval veins (see Figs. 25-12 and 25-14 ). Resorption of the walls normally formed between the pulmonary and caval veins will produce a defect outside the margins of the oval fossa, usually also permitting biatrial communication of the caval vein itself. The coronary sinus defect is best explained on the basis of similar resorption of the walls which usually separate the lumen of the coronary sinus from the cavity of the left atrium (see Fig. 25-15 ). The rare vestibular defect (see Fig. 25-16 ) is likely due to improper muscularisation of the antero-inferior rim of the oval fossa, this usually requiring involvement of both the vestibular spine and the mesenchymal cap carried on the leading edge of the atrial septum. 13
Associated Anomalies
Cardiac
Certain cardiac and vascular anomalies have been reported to occur more frequently in patients with an atrial septal defect than in the general population. The atrioventricular valve is essentially a common structure in the setting of an atrioventricular septal defect with common atrioventricular junction but with shunting confined at atrial level, the so-called ostium primum defect, as considered in Chapter 27 . Hence, it is inappropriate always to consider such valves as abnormal, albeit that they often show additional malformations. Such additional malformations are less common with other varieties of interatrial communication, but do occur. A prolapsing mitral valve, for instance, was found in one-sixth of one series of patients. 18 The incidence amongst the patients undergoing surgical closure in Pittsburgh was only 2.5%. The true incidence is hard to determine, since the criterions for diagnosing prolapse are different in different institutions.
Anomalous connection of the pulmonary veins is a relatively common associated finding in patients with interatrial communications. Indeed, it is the phenotypic feature of the sinus venosus variety. This feature was reported in one-sixth of the patients undergoing surgery at Pittsburgh, but was seen in five-sixths of those said to have sinus venosus defects, compared to only 1.7% of those with a defect within the oval fossa. This statistic, however, calls into question the nature of those patients allegedly having a sinus venosus defect in the absence of anomalous connection of the pulmonary veins. It is very likely that re-examination of these patients would reveal that initially they had oval fossa defects extending towards the openings of the caval veins.
An interatrial communication may co-exist with a ventricular septal defect, persistent patency of the arterial duct, or coarctation of the aorta. The true incidence is hard to determine, since each of these defects may lead to increased left atrial pressure and size, while the interatrial communication may be due to incompetence of a stretched oval foramen. Although a small gradient across the right ventricular outflow tract is common in patients with an interatrial communication, such stenosis is usually functional, and related to increased flow. Organic pulmonary stenosis requiring surgical attention was uncommon in the series of patients undergoing surgery in Pittsburgh, occurring in less than one-twentieth.
Non-cardiac
The association of Down’s syndrome with ostium primum defects is well known. It occurred in three-tenths of patients undergoing surgery for such defects in Pittsburgh. The incidence of Down’s syndrome was only 2.9% in those having surgery for atrial septal defect in the oval fossa. The skeletal anomalies of the Holt-Oram syndrome, 10 and electrophysiological abnormalities, 12 are examples of non-cardiac anomalies, which occur in a few patients. As with associated cardiac malformations, non-cardiac anomalies are relatively rare in patients with an atrial septal defect.
PATHOPHYSIOLOGY
The pathophysiology of an atrial septal defect is related to the magnitude and direction of shunting of blood across the interatrial communication. There is usually a substantial left-to-right shunt, resulting in a high ratio of pulmonary to systemic flow. The primary determinants of the amount of shunting are the size of the defect and the relative resistance to inflow, or the compliance, of the ventricles. The latter factor is greatly influenced by pulmonary vascular resistance. The compliance of the right and left atriums themselves may also play a role, but this has been difficult to prove. As with a ventricular septal defect, the site of the defect in the atrial septum does not influence the magnitude of flow across it, although relative contributions to the shunt from the individual pulmonary veins do vary depending on the location of the defect.
Effect of Size of the Defect
The size of the interatrial communication is an important determinant of the magnitude of the shunt. Because of the lower resistances involved, an interatrial communication tends to be larger than a ventricular defect for a shunt of similar magnitude. Most interatrial communications recognised clinically are essentially non-restrictive, and approximate the area of the mitral valvar orifice. The defect imposes no more restriction to flow across it than does the mitral valve. There is, therefore, at most a small pressure gradient between the atriums. If the defect is smaller and restrictive, the flow of blood to the lungs will be limited by the size of the defect. The ratio of flows usually will be less than 2 to 1. There is no doubt that many small defects escape clinical detection because a limited left-to-right shunt is haemodynamically well tolerated, and causes no abnormalities in the physical examination.
In some cases, the interatrial communication may result from incompetence of the valve of the oval foramen, rather than a true deficiency of the atrial septum. This incompetence can lead to considerable left-to-right shunting when there are associated abnormalities that lead to increased left atrial pressure. Mitral stenosis, a dysfunctional left ventricle, aortic stenosis, coarctation of the aorta, systemic hypertension, patency of the arterial duct, and ventricular septal defect are examples. If the primary defect leading to volume or pressure overload of the left atrium can be corrected, the valve may become competent and the atrial shunt will then disappear. Some otherwise normal infants have left-to-right atrial shunting owing to transient incompetence of the valve of the oval foramen. This phenomenon was well described, 18 and subsequently confirmed by the widespread use of Doppler echocardiography in newborns and young infants. It is likely to be caused by effacement of the superior infolded rim.
Effect ofVentricular Compliance
If the atrial defect is non-restrictive, the magnitude of the shunt will be directly related to the relative resistance to filling offered by the right and left ventricles. The inflow resistance of a ventricle, or its compliance, is related to its diastolic function and distensibility. This is largely related to its mass, but other factors may also play a role in the complex determination of diastolic function. The mass, and hence the compliance, of the right ventricle is influenced by pulmonary vascular resistance. It undergoes predictable postnatal changes. At birth, right and left ventricular compliances are approximately equal, and there is little shunting across an atrial defect in either direction. Pulmonary vascular resistance and pressure generally undergo their normal dramatic fall over the first days of life in patients with an interatrial communication. This is true even in a patient with a large defect, since the pulmonary vascular bed is never exposed to systemic arterial pressure, as it is with a large ventricular septal defect or aortopulmonary communication. It takes several months, however, before the mass of the right ventricle decreases relative to that of the left ventricle. Only then is a normal adult relationship reached between the two ventricles. Hence, a significant left-to-right shunt is not expected until several months of age. Pulmonary arterial pressure at this stage is almost always normal. Although patients with an atrial septal defect have a normal regression of pulmonary vascular resistance, they are susceptible to a secondary increase in resistance if the flow of blood to the lungs remains high for many years, usually several decades. 3,18,19 As pulmonary vascular resistance rises, the pulmonary arterial pressure also increases. This causes right ventricular hypertrophy with a concomitant decrease in compliance, which leads to less left-to-right shunting. As right ventricular compliance approaches that of the left ventricle, there is little shunting across the defect. If right ventricular compliance exceeds that of the left ventricle, there will be a right-to-left atrial shunt, since the atriums will empty preferentially into the more easily filled left ventricle. If there is associated severe pulmonary stenosis, hypertrophy of the right ventricle will occur. The resulting decreasing compliance of this chamber may eventually lead to right-to-left shunting. A right-to-left atrial shunt may also be caused by tricuspid stenosis or severe tricuspid valvar regurgitation. In addition to changes in right ventricular compliance, abnormal filling characteristics of the left ventricle can also influence the magnitude of the atrial shunt. Filling of the left ventricle is impaired by mitral stenosis, or by any abnormality that leads to increased diastolic pressures in the left ventricle. This may result in marked accentuation of the left-to-right shunt across the atrial defect.
Cardiac Response to the Interatrial Communication
The usual haemodynamic characteristics of an uncomplicated interatrial communication are a large left-to-right shunt and normal pulmonary arterial pressure. Flow across the defect is phasic, and occurs predominantly in late ventricular systole and early diastole. 20 Numerous studies have documented that most patients with a typical defect within the oval fossa have a small right-to-left shunt. It occurs mainly from streaming of a portion of the return from the inferior caval vein directly across the defect into the left atrium. 19,21 This small shunt is not detectable by oximetry, so there is no systemic desaturation, but it can be demonstrated by indicator dilution techniques, 22 contrast echocardiography, 23 and Doppler studies. 24
The contribution of the pulmonary venous return from each lung to the total left-to-right shunt is unequal. In a typical large defect, four-fifths of the pulmonary venous return from the right lung shunts left to right. This is in contrast to between one-fifth and two-fifths of the pulmonary venous return from the left lung. 18,19,22 In a sinus venosus defect, it is the anomalously connected right pulmonary veins that provide most of the shunted blood. This preferential shunting from the right lung does not occur to any great extent with an ostium primum interatrial communication.
A large left-to-right shunt at the atrial level leads to enlargement of both the right atrium and the right ventricle. 25 The left atrium is of normal size despite the increased pulmonary venous return, since the atrial septal defect allows its decompression. Left ventricular dimensions are usually normal, although some studies have shown left ventricular end-diastolic volume to be less than normal. 26
Systemic cardiac output is almost always normal in children. Exercise tolerance is good, probably because the cardiovascular response to exercise favours a decrease in the magnitude of the atrial shunting. The afterload on the left ventricle is decreased by the drop in systemic vascular resistance that attends exercise, tending to facilitate left ventricular filling. The increased cardiac output also augments systemic venous return, competitively filling the right atrium at the expense of shunting across the defect. In contrast to the situation in childhood, systemic cardiac output has been found to be decreased in up to half the patients with an atrial septal defect who are older than 18 years of age. 27 Numerous studies in adults have shown significant left ventricular dysfunction, which may persist even after surgical correction. 26,28,29
Despite the greatly increased flow of blood to the lungs, pulmonary arterial pressure is rarely elevated in children, and pulmonary vascular resistance is low, frequently less than 1 Wood unit. 30 The incidence of pulmonary hypertension in children less than 20 years of age is no more than one-twentieth in most studies but increases to one-fifth of those aged from 20 to 40 years, and is found in half of the patients older than 40 years. 3,19,31,32 The incidence of elevated pulmonary vascular resistance also increases with age. 27,33 Severe elevation of resistance, and Eisenmenger’s reaction, however, is unusual, occurring in only about one-twentieth of patients. 34,35 With severe elevation of pulmonary vascular resistance, right ventricular hypertrophy may increase, and compliance decrease, sufficiently that cyanosis results from reversal of the atrial shunt. This suggests slowly progressive development of pulmonary vascular disease, presumably resulting from increased flow of blood to the lungs over many years. Heath and Edwards 36 described changes in the pulmonary vascular bed consistent with this hypothesis, including a predominance of intimal fibrosis and endothelial proliferation, but with less medial muscular hypertrophy than is seen in patients with ventricular septal defects. There are many aspects of the clinical spectrum that are not explained by this simplistic scheme. On the one hand, although it is uncommon, some children with isolated atrial septal defects do have pulmonary hypertension. On the other hand, there are adults who live into their sixth and seventh decades with markedly increased flow to the lungs who have normal pulmonary vascular resistance and normal pulmonary arterial pressure. 27,37 There must, therefore, be individual variation of pulmonary vascular reactivity to various noxious stimuli such as increased pulmonary flow, increased pressure, and so on. As yet, these features are poorly understood.
The chronic right ventricular volume overload caused by an interatrial communication is generally well tolerated for many years, particularly when there is no associated elevation of pulmonary arterial pressure. Congestive heart failure rarely occurs before the fourth or fifth decades but has been reported to be present in approximately one-third of patients greater than 40 years of age. 27,33 Rarely, an isolated defect may cause congestive heart failure in infancy. 38–40 Another cardiac consequence of the long-standing left-to-right shunt is the occurrence of atrial arrhythmias, particularly atrial flutter and fibrillation. They presumably result from chronic stretching of the atriums, and occur most commonly in patients greater than 40 years of age. 37,41,42 As with the other complications associated with inter-atrial communications, atrial arrhythmias rarely occur in childhood. Electrophysiological studies, nonetheless, have demonstrated a high incidence of subclinical dysfunction of the sinus node, along with conduction disturbances, in children prior to operative intervention. 43–46
CLINICAL FINDINGS
Presentation
Mild dyspnoea on exertion, and/or easy fatiguability, are the most common early symptoms of an interatrial communication. They are not usually present during infancy and childhood, or may be appreciated only in retrospect after a diagnosis has been made. Not infrequently, parents report increased activity and vigour after repair, even though they had considered their child to be asymptomatic prior to surgery. Infants less than 1 year of age may rarely present with congestive heart failure owing to an isolated defect. Some children have an increased number of respiratory infections. Additionally, some infants who are predisposed to respiratory compromise, such as those with bronchopulmonary dysplasia, may suffer more significant effects from the atrial shunting, and may benefit from early closure of the defect. Symptoms become much more common in the fourth or fifth decades, for reasons already discussed. Some patients, even with a large defect, may be in their sixth or seventh decades before they demonstrate dyspnoea on exertion, easy fatigue, or frank congestive heart failure. A few remain free of disabling symptoms throughout life.
Physical Examination
The general physical examination is usually normal, although there is a tendency toward a slender physique ( Fig. 25-19 ). Associated non-cardiac abnormalities are uncommon in individuals with a defect within the oval fossa or a sinus venosus defect. 8 Skeletal anomalies of the forearm and hand do occur occasionally, and this syndrome may be inherited. 10 Non-cardiac anomalies much more commonly accompany ostium primum defects. Notable examples include Down’s syndrome 47 and the visceral anomalies typically present with isomerism of the atrial appendages. 48
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