Normal Atrial Septum

An understanding of the morphology and classification of interatrial communications depends on knowledge of the extent of the normal atrial septum. This, in turn, requires an appreciation of the difference between partitions that separate the atrial cavities, which can be removed without transgressing on the pericardial cavity, as opposed to the removal of folds, which do produce a communication with the extracavitary space ( Fig. 29.1 ).

(A) Arrangement of the partitions between the atrial chambers as seen in frontal projection. (B) How the floor of the oval fossa can be removed without transgressing on extracardiac space. (C) Removal of the cranial rim of the fossa, however, creates a communication with the pericardial cavity, since the rim is an infolding between the atrial walls. We consider only the parts that can be removed without creating a communication with the extracardiac space to be true septal structures.

Thus, when one views the septal surface of the right atrium, 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 potentially interposed between the cavities of the right and left atria ( Fig. 29.2 , left ).

Septal surface of the right atrium. The yellow line shows the plane of section taken to produce the image at right, which shows that the anterior and posterior rims of the oval fossa are infoldings of the atrial walls. (An orthogonal section of the same heart is shown in Fig. 29.3 .) Note that the tendon of Todaro is the cranial continuation of the eustachian valve. It extends through the anteroinferior buttress of the fossa to insert into the central fibrous body. ICV, Inferior caval vein; SCV, superior caval vein.

When the heart is sectioned across the oval fossa, its anterior and posterior rims are seen as folds between the atrial walls (see Fig. 29.2 , right ). Sections taken through the atrial chambers in the frontal plane show that the superior rim of the fossa, often considered to represent the septum secundum, is similarly a fold (see Fig. 29.1 ). This fold is the area known to surgeons as the Waterston or Sondergaard groove. Inferiorly, the rim separates the fossa from the orifice of the coronary sinus, with the mouth of the coronary sinus itself separated from the mouth of the inferior caval vein by an additional fold, 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. 29.2 , left ). This anteroinferior muscular buttress, traversed by the tendon, is another true septal structure ( Fig. 29.3 ).

The heart shown in Fig. 29.2 has been sectioned again to show that the superior rim of the oval fossa is also a fold between the atrial walls. Note the walls of the coronary sinus in the left atrioventricular groove. The star shows its mouth in the right atrium. There is an additional fold between the walls of the sinus and the inferior caval vein, which is known as the sinus septum. ICV, Inferior caval vein; SCV, superior caval vein.

Thus only the floor of the oval fossa and the anteroinferior muscular buttress constitute true interatrial septal structures, which can be removed without exiting the cavities of the heart (see Fig. 29.1 ).

Types of Interatrial Communication

Recognition of the extent of the normal septum underscores the fact that not all interatrial communications are septal deficiencies. Of the channels that permit interatrial shunting, those within the confines of the oval fossa, along with the much rarer deficiencies of the anteroinferior muscular buttress, are true septal defects. The ostium primum defects, along with the sinus venosus defects and those found at the site of the mouth of the coronary sinus, are interatrial communications, but they lie outside the confines of the atrial septum ( Fig. 29.4 ).

Location of the various lesions that permit interatrial shunting. The oval fossa and vestibular defects are true atrial septal deficiencies. The remaining lesions are properly described as interatrial communications rather than atrial septal defects.

Defects Within the Oval Fossa

These lesions, which are true septal defects, are by far the commonest type of interatrial communication. Although most frequently termed secundum defects , this term is justified only because the defect represents persistence of the second interatrial communication to be formed during cardiac development. Should they be found postnatally, it is because of deficiencies of the floor of the fossa, which is derived from the primary atrial septum. If development proceeds normally, the floor of the fossa is usually of sufficient dimension to overlap the infolded superior rim, often described as the septum secundum . The floor and rim, however, do not always fuse with one another, and failure of such fusion produces the probe-patent oval foramen ( Fig. 29.5 ).

Normal heart sectioned along its short axis. The flap valve of the oval fossa, derived from the primary atrial septum, has failed to fuse with the superior rim, even though it is of sufficient dimensions to close the septal deficiency. This constitutes persistent patency of the oval fossa.

Even if the flap valve fails to fuse with the superior interatrial fold, there will be no interatrial shunting as long as the left atrial pressure is higher than right, which is the normal postnatal situation. Nonetheless, probe patency is known to be responsible for paradoxic embolism and can be an important finding in those undertaking deep sea diving. The suggested association with migraine, however, has still to be proven. The simplest true defect within the oval fossa is found when the flap valve is minimally deficient, so that it fails to entirely overlap the left atrial margin of the rim of the fossa. With increasing deficiency of the flap valve or in presence of a perforated valve, the defect becomes larger ( Fig. 29.6 , left ). In extreme cases there may be no floor to the fossa (see Fig. 29.6 , right ).

Examples of the lesions that can be found within the confines of the oval fossa. Left, Shown in a flap valve that is not only of insufficient size to overlap the margins of the fossa but is also perforate posteroinferiorly and anteroinferiorly. Right, Example in which the flap valve is deficient over the full extent of the fossa in combination with a perimembranous inlet ventricular septal defect (VSD). Note that the defect extends to the mouth of the inferior caval vein.

When the deficiency is marked, the hole can extend toward the mouth of the inferior caval vein, which may straddle the persisting rim to open in part to the left atrium (see Fig. 29.6 , right ). Although these defects may exist in isolation and do not disturb the location of the conduction tissues, they often occur in combination with many other congenital cardiac malformations, as shown at right in Fig. 29.6 .

Vestibular Defect

The heart shown in Fig. 29.7 , in addition to the multiple perforations in the floor of the oval fossa, has an additional defect in the anteroinferior buttress. This lesion is the second type of true atrial septal defect.

The right atrium has been opened to show the perforated floor of the oval fossa. In addition, there is a relatively large defect within the anteroinferior buttress of the fossa. This is a vestibular defect.

Sinus Venosus Defects

The essence of the sinus venosus defects is that they exist outside the confines of the oval fossa. Found most frequently in the mouth of the superior caval vein, they can also be found at the orifice of the inferior caval vein or draining to the midpoint of the systemic venous sinus. 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 ( Fig. 29.8 , left ).

Features of the sinus venosus defect. (A) The usual variant, in which the orifices of the superior caval vein (SCV) overrides the rim of the oval fossa, itself deficient in this heart. A probe has been placed through the tube of extracavitary tissue enclosed with the superior rim (see Fig. 29.9 ). (B) The more unusual variant, in which the defect is within the orifice of the SCV. In both instances, the key feature are the anomalous connections of the right pulmonary veins.

In some instances there is no overriding of the orifice of the superior caval vein (see Fig. 29.8 , right ). Despite the normal attachment of the caval vein to the right atrium, the anomalous attachment of the right pulmonary veins still creates the interatrial communication outside the confines of the intact atrial septum. Therefore the phenotypic feature of the lesions is the anomalous attachment of one or more pulmonary veins to a systemic vein, with the pulmonary veins retaining their left atrial connection ( Fig. 29.9 ).

Phenotypic feature of the sinus venosus defect, namely the anomalous attachment of a right pulmonary vein to a systemic venous channel, in this example to the superior caval vein (SCV), while the pulmonary vein retains its left atrial connection. The presence of the extraseptal communication thus turns the superior interatrial fold into a tube containing fibroadipose tissue (also see Fig. 29.8 ).

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. 29.8 , left ). 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 cavoatrial 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 jeopardize the sinus node. Inferior sinus venosus defects are far less common. They too are associated with an anomalous attachment of the right pulmonary veins, but in these cases the anomalous attachment of the right inferior pulmonary vein creates the extracardiac conduit. Closure of such a defect should not jeopardize either the sinus or the atrioventricular node. Inferior sinus venosus defects must, nonetheless, be distinguished from defects within the oval fossa that extend to the mouth of the inferior caval vein (see Fig. 29.6 , right ). If the middle right pulmonary vein is anomalously connected to the right atrium while retaining its left atrial connection, the sinus venosus defect can drain to the middle part of the smooth-walled systemic venous component of the right atrium ( Fig. 29.10 ).

In this heart, the middle and inferior right pulmonary veins are anomalously connected to the systemic venous component of the right atrium while retaining their left atrial connections. The sinus venosus defect is located midway between the orifices of the superior and inferior caval veins. There is also a persistent left superior caval vein draining to the right atrium through the enlarged mouth of the coronary sinus.

Coronary Sinus Defects

In the absence of the walls that usually separate the sinus from the left atrium, the right atrial orifice of the sinus becomes an interatrial communication ( Fig. 29.11 ).

Heart with a persistent left superior caval vein (SCV) draining to the roof of the left atrium. The walls that would normally separate the cavity of the sinus from the left atrium are absent. In this setting, the right atrial orifice of the sinus becomes an interatrial communication.

Most often a persistent left superior caval vein is also connected to the left atrial roof. 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. The opening of the coronary sinus is usually large. Its anterior margin encroaches on the triangle of Koch, approximating the area of the atrioventricular node. Therefore care must be taken when the defect is closed. Unless previously diagnosed, a coronary sinus defect may be difficult to recognize as an interatrial communication in the operating room, appearing simply to be the mouth of the coronary sinus itself. Such defects are significant in the setting of hypoplastic left heart syndrome with intact atrial septum, since they provide an overflow for the obstructed left atrial chamber.

Morphogenesis

During normal development, the first sign of atrial septation is the growth of the muscular primary septum from the roof of the atrial component of the primary heart tube. As it grows down to divide the primary atrium, it carries on its leading edge a cap of mesenchymal tissue ( Fig. 29.12 , left ).

Initial stages of atrial septation. (A) Initial growth of the primary septum from the atrial roof, with the mesenchymal cap on its leading edge. At this early stage, there is as yet no formation of the pulmonary vein, but the margins of the dorsal mesocardium protrude into the atrial cavity as the pulmonary ridges. As the primary septum grows toward the atrioventricular (AV) cushions, with the primary foramen formed between the cushions and the mesenchymal cap, the pulmonary vein canalizes to open into the atrial cavity through the site of the dorsal mesocardium. (B) By this stage, the origin of the septum from the atrial roof is beginning to break down to form the site of the secondary foramen.

The space between the cap and the atrioventricular endocardial cushions, which divide the atrioventricular canal into the eventual mitral and tricuspid valvar orifices, is the primary atrial foramen. Prior to closure of the primary foramen, the pulmonary vein canalizes within the pharyngeal mesenchyme behind the developing heart, opening to the atrial cavity between the ridges formed at the site of the dorsal mesocardium, which is the initial connection between the heart tube and the pharyngeal mesenchyme (see Fig. 29.12 , right ). The fusion of the cap with the endocardial cushions thus obliterates the primary atrial foramen. The site of closure is reinforced by the growth of additional tissues through the right-sided ridge of the two pulmonary ridges, thus producing the vestibular spine ( Fig. 29.13 , left ). By the time the primary foramen has closed, the cranial origin of the primary septum has broken down to form the secondary atrial foramen (see Fig. 29.12 , right , and Fig. 29.13 , left ).

Final stages of atrial septation. (A) the growth of the vestibular spine reinforces the site of closure of the primary foramen, at the same time committing the orifice of the pulmonary vein to the left atrium. The vestibular spine and the mesenchymal cap muscularize to form the anteroinferior buttress of the atrial septum. Infolding of the atrial roof, concomitant with the transfer of the incorporation of the pulmonary veins to the left atrium, produces the superior rim of the oval fossa. (B) This infolding usually overlaps the upper edge of the primary atrial septum, being of sufficient dimensions to close the oval foramen when left atrial pressure exceeds right atrial pressure subsequent to birth.

In the final stages of septation, the upper edge of the remaining primary atrial septum is usually overlapped by the infolding of the atrial roof between the superior caval and pulmonary veins. This fold forms the superior rim of the oval fossa, often described as the septum secundum (see Fig. 29.13 , right ).

Failure of septation of the common atrioventricular junction due to lack of growth of the vestibular spine prevents the lower edge of the primary septum from fusing with the ventricular septum. This results in the ostium primum defect, which is an atrioventricular septal defect with a common atrioventricular junction. This lesion is described further in Chapter 32 . Therefore maldevelopment of the primary atrial septum itself, after it has fused with the ventricular septum and the endocardial cushions, results in defects within the oval fossa. This can be due either to deficiency of its superior edge, so that it no longer overlaps the infolded superior rim, or to 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. 29.5 ). 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 spontaneous closure of some defects within the oval fossa during the neonatal period. Sinus venosus defects are explained on the basis of anomalous connection of the right pulmonary veins to a systemic venous channel, with the pulmonary veins retaining their left atrial connections (see Fig. 29.10 ). The coronary sinus defect is explained on the basis of resorption of the dual walls that usually separate the lumen of the coronary sinus itself from the cavity of the left atrium (see Fig. 29.11 ). The rare vestibular defect (see Fig. 29.7 ) is due to improper fusion or muscularization of the components forming the anteroinferior buttress of the atrial septum; this process involves both the vestibular spine and the mesenchymal cap carried on the leading edge of the atrial septum.

Associated Anomalies

Effect of Size of the Defect

Most isolated interatrial communications that are recognized clinically are essentially nonrestrictive, and flow across them will depend on very subtle pressure transients between the atria, generated, at least in part, by the function of the ventricles. At birth, right and left ventricular compliance is 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, and right ventricular hypertrophy regresses. Hypertrophy per se does not worsen compliance, but the normal response to a volume load is for the atrium and ventricle to dilate progressively, and this is the norm no matter what the type of interatrial communication. Progressive dilation may be subclinical for years, and the pulmonary vascular resistance stays low in childhood and early adult life, but patients are susceptible to an increase in resistance if the flow of blood to the lungs remains high for decades. If the right ventricle fails, Eisenmenger syndrome may result, with reversal of the atrial shunt to a predominant right-to-left pattern. A right-to-left atrial shunt may also be promoted by tricuspid stenosis or severe tricuspid valvar regurgitation.

In some cases the interatrial communication may result from incompetence of the valve of the oval foramen rather than from a true deficiency of the atrial septum. This incompetence can lead to considerable left-to-right shunting (even if the defect is small) 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, and atrial shunting may improve with treatment of the underlying problem. A small amount of left-to-right shunting is usual in the newborn and young infant and defects less than 6 mm at this age are likely not to be important later in life; however, probe patency of the oval fossa is present in up to one-third of these individuals as adults.

Cardiac Response to the Interatrial Communication

In childhood, the usual hemodynamic characteristics of an uncomplicated interatrial communication are a large net left-to-right shunt and normal pulmonary arterial pressure. Left-to-right flow across the defect is phasic and occurs predominantly in late ventricular systole and early diastole. Numerous studies have documented that most patients with a typical defect within the oval fossa also have a small right-to-left shunt. This small shunt is not detectable by oximetry, so there is no systemic desaturation, but it can be demonstrated by indicator dilution techniques, contrast echocardiography, and Doppler studies.

The contribution of the pulmonary venous return from each lung to the total left-to-right shunt is unequal. In a typical large defect, 80% of the pulmonary venous return from the right lung shunts left to right. This is in contrast to between 20% and 40% of the pulmonary venous return from the left lung. In a sinus venosus defect, the anomalously connected right pulmonary veins provide most of the shunted blood. Interestingly, this preferential shunting from the right lung does not occur to any great extent with an ostium primum interatrial communication.

As discussed, a large left-to-right shunt at the atrial level leads to enlargement of both the right atrium and the right ventricle. The left atrial and left ventricular dimensions are usually normal in childhood, and systemic cardiac output is almost always normal in children. In contrast, left ventricular end-diastolic volume may be less than normal, and 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. Numerous studies in adults have shown significant left ventricular dysfunction, which may persist even after surgical correction.

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. The incidence of pulmonary hypertension in patients younger than 20 years is no more than 5% in most studies but increases to 20% of those aged from 20 to 40 years and is found in half of the patients older than 40 years. However, severe elevation of resistance and the Eisenmenger reaction are unusual and are decreasing in frequency as surveillance techniques improve. The changes in the pulmonary vascular bed at this stage are similar no matter what the cause of pulmonary vascular disease may be, including a predominance of intimal fibrosis and endothelial proliferation, albeit with less medial muscular hypertrophy than is seen in patients with ventricular septal defects. The Eisenmenger reaction is not a uniform response in older age, and such a response is somewhat idiosyncratic. However, a progressive rise in pulmonary artery pressure with worsening cardiopulmonary function is the norm (presumably due to increased shunting as left ventricular compliance worsens with age and right ventricular function worsens as a result), thus providing the rationale for surgery in childhood. Nonetheless, congestive heart failure rarely occurs before the fourth or fifth decade. Rarely an isolated defect may cause congestive heart failure in infancy; early surgery may be indicated in such cases, although a search for other precipitating factors is a crucial part of the evaluation of these infants. 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 atria and occur most commonly in patients older than 40 years of age. As with the other complications associated with interatrial communications, atrial arrhythmias rarely occur in childhood. Nonetheless, electrophysiologic studies have demonstrated a high incidence of subclinical dysfunction of the sinus node, along with conduction disturbances, in children prior to operative intervention.