Apart from exceedingly rare conditions, such as aneurysms of the atrial wall or dilatation of the atrial appendages, the significant atrial lesions are undue prominence of the valves of the embryonic venous sinus, juxtaposition of the atrial appendages, and division of the left atrium (cor triatriatum).

Atrial Septal Defect

Morphology

The term interatrial communication is used when referring to the group of lesions which produce the potential for shunting at the atrial level, and not necessarily shunting within the confines of the atrial septum itself. For example, the ostium primum atrial septal defect results in an interatrial communication, but is in reality, a deficiency of the atrioventricular septum and not one of the true atrial septum. In this section, true atrial septal defects, that is, defects within the confines of the fossa ovalis, along with the sinus venosus and coronary sinus defects are considered. Although the last two lesions permit unequivocal interatrial shunting, they are outside the confines of the atrial septum and should not be considered true atrial septal defects. Crucial to an understanding of the defects to be discussed in this section is knowledge of the anatomy of the normal atrial septum.

The normal atrial septum

There is a potential deficiency within the atrial septum of the normal heart because, during fetal life, the richly oxygenated placental blood returns to the right atrium through the umbilical vein. This blood needs to reach the left atrium in order to enter the left ventricle, the aorta and thence to reach, in particular, the brain. To do this, it must cross the atrial septum. The communication permitting this transfer is located opposite the orifice of the inferior vena cava and is so arranged that, during fetal life, the raised right atrial pressure promotes the required shunt from right to left atrial chambers. After birth, when left atrial pressure is higher than right, the anatomical arrangement permits mechanical closure of the communication. The septal communication, known as the foramen ovale, is arranged with a flange on the right atrial surface and a flap valve within the left atrium. The flange is often described as the ‘septum secundum’, but, in reality, is simply an infolding of the atrial roof. The greater part of the septum is made up of the floor of the fossa ovalis. During fetal life, this is a hinged flap with an upper margin which floats freely. This upper edge in most adult hearts is firmly fused to the right atrial flange, producing both mechanical and hemodynamic closure of the septum. In from one-quarter to one-third of normal hearts, however, the upper edge of the flap valve, or septum primum, is not fused with the flange. This arrangement, called a probe-patent foramen ovale, produces mechanical closure of the septum as long as the right atrial pressure is higher than the left. In its presence, if the pressure in the left atrium exceeds that in the right atrium, there will be the potential for interatrial shunting. The probe-patent atrial septum, nonetheless, is not usually considered as a septal defect.

Secundum atrial septal defect

Defects within the fossa ovalis are the only true types of atrial septal defects. They are the most common type of interatrial communication, and exist due either to a deficiency or perforation of the septum primum. If the limbus is normal and the septum primum does not appear deficient, then this lesion is referred to as a patent foramen ovale. The deficiency of the septum primum may vary from a few small fenestrations to complete absence, in which case the entire floor of the fossa ovalis is missing and may lead to a sufficiently large interatrial communication to warrant the term ‘common atrium’. In this circumstance the Eustachian valve of the inferior vena cava may be mistaken for the lower border of the defect, and surgical closure using this erroneously interpreted landmark will result in drainage of the inferior vena cava to the left atrium. A common atrium is seen much more commonly seen in the setting of an atrioventricular septal defect, particularly when there is coexisting atrial isomerism. When the septum primum is deficient or perforated, the potential for shunting across the septum will exist irrespective of the pressures in the atrial chambers. Unlike ventricular septal defects, true atrial septal defects are quite unlikely to close spontaneously.

Sinus venosus atrial septal defect

The sinus venosus defect occurs as a result of deficient tissue along the remnant of the right horn of the sinus venosus, which extends from the orifice of the superior vena cava to the orifice of the inferior vena cava. Hence, not only does the defect occur outside the true confines of the atrial septum, it can occur at any position between the superior vena cava and inferior vena cava. Sinus venosus defects require some other abnormal venous connection to create an extra-septal tunnel and provide a conduit between the right and left atria. That conduit is provided by biatrial connection of either the superior or inferior vena cava, often in association with anomalous connection of the right pulmonary veins. The superior sinus venosus defect, with anomalous connection of the superior vena cava, is by far the most common arrangement. This produces an interatrial communication cephalad to the limbus of the fossa ovalis, which may itself may be intact or exhibit a true atrial septal defect. Most commonly, the veins of the right and middle lobes drain into the superior vena cava or sinoatrial junction, although the entire venous drainage of the right lung may drain anomalously. The association of anomalous pulmonary and systemic venous connections often renders the defect difficult to close without producing either superior caval or pulmonary venous obstruction. Often, therefore, the surgical procedure will include placement of a gusset to enlarge the superior caval channel, which may place the artery to the sinus node at risk of injury. The inferior sinus venosus defect, which is much less common, is found at the mouth of the inferior vena cava, as a consequence of connection to this vein and, in part, to the left atrium. Like the superior sinus venosus defect, the inferior sinus venosus defect is often associated with anomalous connection of the right pulmonary veins.

Coronary sinus atrial septal defect

Coronary sinus defects occur as a result of a deficiency in the remnant of the left horn of the sinus venosus, which extends along the entire length of the coronary sinus. The opening of the coronary sinus is usually present in the usual location, however, the wall between the coronary sinus and the left atrium is variably deficient.

Unroofed Coronary sinus
Click on the image for a large picture
This wall may be fenestrated at various locations, or it may completely absent. Coronary sinus defects very commonly coexist with anomalous connection of a persistent left superior vena cava to the roof of the left atrium. The latter commonly occur in the absence of a crossing vein to the left superior vena cava, and the combination of a coronary sinus defect with a persistent left superior vena cava and no crossing vein results in a large right to left shunt at the coronary sinus level. This latter combination represents an extreme form of unroofing of the coronary sinus. Lesser forms of this lesion exist depending on the degree of fenestrations in the coronary sinus and the amount of blood carried in the persistent left superior vena cava. An interatrial communication at the site of the coronary sinus can rarely be found in the absence of a persistent left superior vena cava. The coronary veins themselves in this setting drain directly to the cavities of the atrial chambers as the coronary sinus, as such, is lacking. The unroofed coronary sinus syndrome can occur in otherwise normal hearts; however, it is frequently found in cases of atrial isomerism.

Pathophysiology & natural history

Essentially all clinically significant interatrial communications behave as nonrestrictive defects, defined as defects across which no pressure gradient exists. The direction and degree of shunting in these circumstances is then determined by the relative compliances of the left and right ventricles during diastole. In the neonatal and early infancy periods, right and left ventricular compliances are similar, resulting in little interatrial shunting. As right ventricular compliance increases, the degree of left-to-right shunting also increases. Although most infants are able to physiologically compensate for the extra volume-load without developing symptoms, a small number of infants with isolated interatrial communications symptoms of intractable congestive heart failure and failure to thrive.

Diagnosis

Symptoms are uncommon for isolated interatrial communications, the diagnosis often being suspected on auscultation of a murmur on routine examination, or rarely for episodes of transient cyanosis and paradoxical embolization. Chest radiography and electrocardiography are supportive, while two-dimensional echocardiography and color Doppler studies are usually diagnostic. Right-ventricular volume over-loading can also be detected, indicating the physiological significance of the lesion. Cardiac catheterization has limited utility in the diagnosis of interatrial communications, but can be useful to confirm the diagnosis, clarify the hemodynamics, and rule out associated defects.

In infants, the indication for closure of an interatrial communication is the presence of symptoms. These may include cardiac congestion, failure to thrive, paradoxical embolization, or transient cyanosis. A less clear indication is the presence of an asymptomatic interatrial communication with echocardiographic evidence of right-ventricular volume-overloading with or without catheterization evidence of a Qp:Qs of > 1.5:1. For the asymptomatic child, elective repair at age 4 or 5 years is widely recommended. This is based more on social issues than medical issues, and is held as a convenient time for elective repair with little harm done to the patient. There are currently no compelling reasons to alter this recommendation.

Management

Atrial septal defects are of historical importance in that they were the focus of some of the earliest attempts at surgical correction of intracardiac defects. Open repair using the atrial well technique was performed by Gross and coworkers in 1952. Repair under direct vision using moderate hypothermia and inflow occlusion was performed by Lewis and Taufic in 1953, and in 1954 Gibbons introduced cardiopulmonary bypass for closing an atrial septal defect.

Closure of atrial septal defect

Standard median sternotomy, pericardial cradle, and bicaval cannulation with a single aortic cannula is performed. The left ventricle is vented with a cannula placed through a stab incision in the right superior pulmonary vein. The cavae are snared, a cross-clamp is applied, and the heart arrested with an infusion of cold, dilute-blood cardioplegia (30 cc/kg). Myocardial temperature is routinely monitored by a metal probe placed into the intraventricular septum. The right atrium is opened, and the return of cardioplegia from the coronary sinus is ensured.

Juxtaposition of the atrial appendages

In the normal situation, the appendages of the atrial chambers are situated one on either side of the arterial pedicle. In some circumstances, usually in the presence of complex associated lesions but occasionally with simple lesions such as an ASD, both appendages are found on the same side of the arterial pedicle. In the individual with usual atrial arrangement, it is the right appendage which is most frequently juxtaposed in such a way that it comes to lie within the transverse sinus so that its tip is to the left of the great arteries. This arrangement is known as left juxtaposition. It is a harbinger of abnormal ventriculoarterial connections, usually discordant or double outlet right ventricle, and it is often found with the aorta in left-sided position. Left juxtaposition is also frequently found with tricuspid atresia. Whatever the associated lesions, the terminal groove. In some instances the appendage may be partially juxtaposed. Either partial or complete juxtaposition also distorts the internal architecture of the right atrium in such a way that the orifice of the appendage occupies the anticipated site of the fossa ovalis, the latter being squashed and deviated postero-inferiorly.

Much more rarely, the left atrial appendage may be distorted so that it comes to extend through the transverse sinus to assume a right-sided position. Such right juxtaposition tends to occur with much simpler lesions, such as ASD, but can also be found in the setting of complex lesions. Right juxtaposition can also be found with mirror-image atrial arrangement, when it is simply the mirror-image of the commoner left variant and is associated with similar lesions. It can also be seen with atrial isomerism when both appendages are of like morphology. The associated lesions then reflect the presence of isomerism.

Undue prominence of the valves of the venous sinus

It is well recognized that the right atrium is derived from two components. One is the primitive atrial segment of the heart tube, the other is the venous sinus (sinus venosus). Early in development, well-formed flaps or valves are seen between the two components. These valves are simply inflections of the adjacent walls of the two segments. For the larger part, the flaps regress during infancy, leaving only small folds which guard the entrances of the inferior vena cava and coronary sinus to the right atrium[944]. These structures are known as the Thebesian and Eustachian valves, respectively. In some hearts, more extensive remnants of the embryonic valves persist which are not always of functional significance. Usually they persist as fenestrated networks (Chiari nets) which extend from the terminal crest to be attached superiorly to the ‘septum spurium’. In rare circumstances, the valves can persist as more solid sheets and can then produce obstruction to flow through the right side of the heart. Very rarely, such structures can be discovered in otherwise normally connected hearts and produce obstruction to flow through the right heart chambers. Removal of the windsock-like lesion is then curative. More usually, the restrictive sheets are found in hearts in which there is already obstruction or atresia along the right-sided flow pathways. The typical setting for these lesions, which are then often described as ‘cor triatriatum dexter’, is in pulmonary atresia with an intact ventricular septum or in tricuspid atresia. Although it is often suggested that the prominence of these remnants of the valves of the embryonic venous sinus is causative of the associated lesions, it is just as likely that the valves themselves persist because of the presence of the obstructive lesions. The function of these valves during embryonic life is to direct the richly oxygenated inferior caval venous flow across the atrial septum and into the left-sided heart chambers. When, in extrauterine life, there is atresia along the right-sided pathways (pulmonary or tricuspid atresia), there is no incentive for the venous valves to regress.

Atrial Anomalies

Introduction