An atrial septal defect (ASD) is defined as an opening, or hole, in the interatrial septum. The defect within the oval fossa (ostium secundum ASD) is one of the most common congenital heart defects to occur as an isolated lesion. It occurs in 1 in every 1500 births and represents 6–10% of all congenital heart defects, being more frequent in females by a 2 : 1 ratio. Surgical closure was first performed in 1952 by F. John Lewis [1, 2] at the University of Minnesota using hypothermia and inflow occlusion. ASD was also the first congenital cardiac defect to be repaired using cardiopulmonary bypass. On May 6, 1953, John Gibbon [3] used the pump oxygenator that he had developed in his laboratory to close such a defect. It was also the first intracardiac lesion to be successfully treated using percutaneous transcatheter techniques [4]. In fact, with advances in technology and the evolution of transcatheter techniques, the majority of defects located within the oval fossa are now closed in the catheterization laboratory rather than in the operating room.

Anatomy and Pathology

In the normal heart, the atrial septum is closed. Its site is marked, when viewed from the right atrium, by the location of the oval fossa (Figure 15.1). The floor of the fossa (depression or hollow) is formed by the embryonic primary atrial septum, also known as the “septum primum,” which is a relatively thin flap. The flap extends superiorly and to the left of the rims of the fossa, so that in the normal heart it closes the oval foramen as a flap valve. In the normal arrangement, it is sufficiently large to overlap the rims of the fossa, which are largely made up of the infolded atrial walls, although frequently described as the “septum secundum.” The basal part of the flap is, however, buttressed against the septal vestibule by a true second septal component, which continues apically to form the surface of the triangle of Koch, making up the atrial component of the atrioventricular muscular sandwich. During fetal life, of course, the oval foramen is patent, since such patency is necessary for the shunting of oxygenated placental blood from the right to the left atrium when the lungs are not functioning. But, since the left atrial pressure exceeds right atrial pressure postnatally, there is physiologic closure of this opening subsequent to birth. The postero‐inferior rim separates the fossa from the orifice of the coronary sinus, which is guarded by the Thebesian valve, and from the orifice of the inferior caval vein, which is guarded by the Eustachian valve. Another important right atrial landmark is the terminal crest (crista terminalis). This is a thickened portion of the lateral right atrial wall, running from superior to inferior, and separating the smooth‐walled systemic venous sinus from the pectinated walls of the right atrial appendage. The terminal crest corresponds externally to the terminal groove, with the sinus node found cranially within the groove adjacent to the crest of the appendage.

In the majority of normal individuals, the flap valve of the oval fossa fuses with the cranial rim in postnatal life. In up to one‐third, nonetheless, the two components fail to fuse, producing probe patency of the oval foramen. This is of no functional significance as long as pressure in the left atrium remains higher than right atrial pressure. The probe patent foramen, therefore, should not be considered to represent an ASD, although on occasion it can permit interatrial shunting of blood. True anatomic interatrial communications are among the commonest congenital cardiac lesions, but not all are ASDs. It is those found within the confines of the oval fossa, and within the antero‐inferior buttress, that are the true septal defects.

Ostium Secundum Atrial Septal Defect

The so‐called ostium secundum defect is confined within the margins of the oval fossa (Figure 15.2). It exists because of a deficiency of the flap valve, which is derived from the primary atrial septum. Depending on the extent of the deficiency, the resulting defect can vary in size from a small pinhole to a large opening, with essentially no inferior rim. The floor of the fossa can also be fenestrated, thus producing multiple deficiencies in the floor.

Vestibular Atrial Septal Defect

This is a newly recognized type of ASD. It consists of a hole in the antero‐inferior buttress of the oval fossa [5, 6]. These defects often go unnoticed when small, and when large may be difficult to differentiate from the so‐called ostium secundum defect.

Sinus Venosus Interatrial Communication

These defects are found outside the confines of the oval fossa, and hence are interatrial communications, rather than true ASDs. They exist because of anomalous connection of one or more pulmonary veins to either the superior or inferior caval vein, with the anomalously attached pulmonary veins retaining their left atrial connection. Depending on the source of systemic drainage, therefore, the defects can be distinguished as being superior (Figure 15.3) or inferior. When the site of anomalous connection is close to the junction between the caval vein and the right atrium, the orifice of the afflicted caval vein can override the intact cranial or posterior rim of the oval fossa. More rarely, the pulmonary veins drain more distally, adjacent to the entrance of the azygos vein, and then the superior caval vein retains its exclusive connection to the right atrium [7].

Coronary Sinus Defect

The coronary sinus is the venous channel running within the left atrioventricular groove that serves as the confluence for the majority of the coronary veins draining the blood from the ventricular musculature. In the normal heart, it possesses its own walls as it runs through the atrioventricular groove prior to opening to the right atrium. In some individuals, however, windows exist between the cavity of the coronary sinus and the cavity of the left atrium. In these circumstances, the right atrial orifice of the sinus then functions as an interatrial communication. Should the oval fossa be intact, the shunting through the mouth of the sinus can be the only interatrial communication, which again is not a true ASD. Oftentimes, the deficiency of the walls between the coronary sinus and the left atrium is extreme, with only fenestrated remnants of the walls sometimes being present. The lesion usually then coexists with drainage of a persistent left superior caval vein to the roof of the left atrium, with this combination being described as unroofing of the coronary sinus. This finding should be used as a “tip‐off” to look for the persisting right atrial mouth of the coronary sinus functioning as an interatrial communication.

Atrioventricular Septal Defect with Exclusive Atrial Shunting (Ostium Primum Defect)

Yet another lesion producing an interatrial communication outside the confines of the atrial septum is the so‐called ostium primum, or partial atrioventricular septal defect. Its name as an “atrial septal defect” is incorrect, since in many instances it has nothing to do with a deficiency of the primary atrial septum [8]. We discuss this lesion in Chapter 17.

Common Atrium

Occasionally, the entire primary atrial septum, including the antero‐inferior buttress, will fail to develop. The phenotypic feature of this variant, therefore, will again be a common atrioventricular junction, and the lesion will be an atrioventricular rather than an atrial septal defect. The malformation is found most frequently in the setting of isomerism of the atrial appendages, or so‐called heterotaxy, particularly when there is right isomerism.

Pathophysiology and Natural History

The degree of left‐to‐right shunting at the atrial level is determined by the size of the ASD and the relative right and left ventricular compliance. Normally, the compliance of the left ventricle is less than that of the right ventricle, therefore there is a left‐to‐right shunt. This leads to right ventricular dilatation and excessive pulmonary blood flow. Patients with a large defect can have a Qp : Qs ratio of greater than 4 : 1. There are very rare anatomic variations reported where a large persistent Eustachian valve directs inferior caval blood venous flow through the defect into the left atrium. These patients would then present with cyanosis. Patients with an unroofed coronary sinus and a left superior caval vein may also present with cyanosis. Patients with a common atrium may have enough interatrial mixing of pulmonary and systemic venous return to cause a mild degree of cyanosis.

The natural history of ASDs has been determined by retrospective studies on patients who were not surgically repaired prior to the availability of surgical closure. These patients are known to be at risk of atrial arrhythmias (most commonly, atrial flutter and atrial fibrillation), right ventricular dysfunction, pulmonary hypertension, paradoxical emboli, and eventual congestive heart failure. Without surgical intervention, the mean age of death in patients with a significant ASD is 36 years [9]. In young children, ASDs larger than 8–10 mm in diameter are very unlikely to undergo spontaneous closure, whereas defects smaller than 4–5 mm are reported commonly to undergo such spontaneous closure [10, 11]. After the age of 4 years, however, spontaneous closure is very rare [12]. Bacterial endocarditis does not occur in patients with ostium secundum defects unless they have an associated cardiovascular lesion. Of note is that the existence of an isolated ASD is not an indication for prophylaxis of endocarditis, as per the American Heart Association guidelines [13].

Patients with an uncomplicated ASD typically do not have symptoms, and present only with an audible murmur. The murmur typically has two components, a fixed split second heart sound related to the prolonged time period for flow of blood through the right side of the heart and delay of the pulmonary valve closure, and a systolic murmur of relative (physiologic) pulmonary stenosis. Patients with a large defect may have a diastolic rumble secondary to relative (physiologic) tricuspid stenosis. The volume overload of the right ventricle and the pulmonary vascular bed is usually well tolerated for many years. There are only rare infants who develop congestive heart failure secondary to an uncomplicated ASD. The chance of developing obstructive pulmonary vascular disease is much less than for patients with ventricular septal defects or atrioventricular septal defects, although it can occur. In a review of 169 adult patients with an ASD, pulmonary hypertension, defined as mean pulmonary artery pressure >30 mmHg, occurred in up to one‐quarter of adults with an ASD, and elevated pulmonary vascular resistance occurred in up to one‐sixth [14]. Pulmonary hypertension was more common in adults with a sinus venosus defect (16% of 31 patients) as compared to those with defects in the oval fossa (4% of 138 patients). In these patients, the pulmonary vascular resistance may increase to a point where it is greater than the systemic vascular resistance. The intracardiac shunt will reverse, and instead of shunting left to right will shunt right to left. At that time, surgical closure will be contraindicated [15]. The most common cause of late mortality in ASD patients is congestive heart failure and arrhythmias, which increase in frequency as the patient ages, and in rough proportion to the shunt magnitude. In Murphy’s review of 123 patients undergoing repair of an ASD at the Mayo Clinic, the incidence of late atrial fibrillation was 4% if the ASD was closed before the age of 11, versus 55% if the ASD was closed after the age of 41 [16].

Because of the known eventual complications of atrial arrhythmias, right ventricular dysfunction, and pulmonary hypertension, resulting in a dramatic reduction in life expectancy from congestive heart failure, closure of ASD is recommended. This can now be accomplished with either surgical or transcatheter techniques. Closure of an uncomplicated defect is recommended in any patient with physical signs, or catheterization or echocardiographic evidence of a left‐to‐right shunt exceeding 1.5 : 1. In general, patients with a significant defect do not have a shunt less than 1.5 : 1 if they have physical findings of a significant systolic murmur and a fixed split second heart sound. In our series of 401 patients operated on between 1990 and 2018 at Ann & Robert H. Lurie Children’s Hospital of Chicago for defect in the oval fossa, the median age at closure was 3.9 years. Most centers prefer electively to close such defects before a child starts school.

Diagnosis

Physical examination typically reveals a fixed split second heart sound and a systolic murmur of relative physiologic pulmonary stenosis. There may also be a diastolic murmur or rumble of relative (physiologic) tricuspid stenosis. Chest x‐ray evaluation will show cardiomegaly. When there is significantly increased pulmonary blood flow, there may be bulging of the pulmonary trunk at the upper left cardiac border. The electrocardiogram will demonstrate right ventricular hypertrophy. Typically, there is an rSr’ or rsR’ pattern in lead V₁ and right axis deviation.

The most common and widely used technique to arrive definitively at the diagnosis is two‐dimensional echocardiography with color Doppler interrogation. The echocardiogram will visualize the location of the defect, estimate its size, and determine the location of the pulmonary veins. In our own series of patients, less than 10% underwent cardiac catheterization, and this percentage has dropped as the accuracy of echocardiographic techniques has improved. Cardiac catheterization is now only indicated when the diagnosis is uncertain, or associated anomalies are suspected. Cardiac catheterization will reveal, on blood gas analysis, an oxygen step‐up from the right atrium onward, with a pulmonary blood flow typically two to four times the systemic flow. Quite commonly, there can be a gradient of 10–30 mmHg across the pulmonary valve because of the increased flow of blood through the right side. Pulmonary arterial pressures are frequently normal unless pulmonary vascular obstructive disease has developed, usually only in patients over 20 years of age. Cardiac catheterization, computed tomographic (CT) scan, and magnetic resonance imaging (MRI) can be used to localize the pulmonary veins and rule out partial anomalous pulmonary venous connection if the veins are unable to be visualized with echocardiography.

Transcatheter Closure

It is estimated that in the USA approximately 80% of all secundum ASDs are successfully closed percutaneously [17]. There are currently two US Food and Drug Administration (FDA)‐approved device lines. The Amplatzer^® Septal Occluder (AGA Medical Corp., Golden Valley, MN, USA) is a self‐expandable, self‐centering, double‐disc device comprised of exposed nitinol wire mesh, available for defects ranging from 4 to 38 mm in diameter (Figure 15.4). The Gore Cardioform line (W.L. Gore and Associates, Flagstaff, AZ, USA) is a double‐disc occluder built on a nitinol wire frame, entirely covered with expandable polytetrafluoroethylene (ePTFE), and is available for defects ranging from 8 to 35 mm in diameter (Figure 15.5).

These devices are approved only for closure of vestibular or oval fossa defects in children more than 2 years of age, and greater than 12 kg. These devices are not indicated for closure of a sinus venosus defect with anomalous drainage of the right superior pulmonary vein, because of the potential for the creation of superior caval venous or right superior pulmonary venous stenosis. They are also contraindicated for closure of ostium primum defects, as the device would interfere with atrioventricular valve function.

Transcatheter ASD closures are almost always performed on an elective, outpatient basis, and typically require a single venous sheath. If using transesophageal echocardiography (TEE) guidance the patient will be under general anesthesia, but for larger children or adults the case can be performed with intracardiac echo, thus avoiding endotracheal intubation in exchange for placing an additional venous sheath. At Ann & Robert H. Lurie Children’s Hospital of Chicago we begin each case with a comprehensive hemodynamic survey to quantify the degree of atrial‐level shunting, to measure cardiac index and pulmonary vascular resistance, and to ensure that there are no additional intracardiac shunts. While the hemodynamic run is performed, the imaging specialist completes a comprehensive anatomic survey with a focus on defect size, 360‐degree assessment of defect rims, and total septal length.

With baseline data collected, the defect is sized using the “stop‐flow” technique. A compliant sizing balloon is advanced across the lesion and inflated under live color Doppler imaging. Once there is complete cessation of flow across the ASD inflation is stopped, and the minimum balloon waist is measured by echo and fluoroscopy. This is the diameter used to select device size, so any discrepancy between echo and fluoroscopic values must be investigated and resolved before proceeding with ASD closure.

With the device selected, the delivery catheter is advanced into the left atrium (Figure 15.5A). Using a combination of echo and fluoroscopic guidance, the left atrial disc is formed and then the entire system retracted until there is apposition against the atrial septum, with care made to avoid prolapse of the left atrial disc into the right atrium (Figure 15.5B). The right atrial disc is then deployed, with multimodality imaging again used to ensure there was 360‐degree capture of the defect rims (Figure 15.5C, D). Both the Gore and Amplatzer devices can be retrieved and redeployed multiple times, as needed to achieve appropriate position and rim capture. Before the device is released a repeat echo survey is performed to ensure there is no encroachment of the device on the atrioventricular valves, and no obstruction of the systemic or pulmonary venous return. There is almost always some degree of tension from the delivery system that resolves after final release, resulting in a slight shift in device position. A final echo survey is performed after release, but before removal of the venous sheath.

After care requires 4–6 hours of flat time and admission for overnight observation. A postcatheterization chest x‐ray and echocardiogram are performed before discharge, and patients are required to take a daily aspirin and subacute bacterial endocarditis (SBE) prophylaxis for six months. A repeat echocardiogram is performed six months from the time of implant, at which time there should be complete endothelialization of the device (Figure 15.5E, F). If the echocardiogram finds no evidence of a residual shunt, atrial septal aneurysm, or SBE, prophylaxis can then be discontinued.

Surgical Technique

Our preferred surgical technique for ASD closure is through a median sternotomy. Over the years the length of the incision has steadily shortened, and many centers now close ASDs without opening the entire sternum [18–20]. This allows for cosmetic closure, especially for females. An alternative technique is the right thoracotomy [21]. We have not used this approach because of the increased risk of air embolus and reports of injury to the right phrenic nerve [22]. The median sternotomy incision in particular permits repair of associated intracardiac lesions should they be discovered at the time of surgery. We currently use TEE in all cases to confirm the diagnosis, to assess the pulmonary veins, and to confirm complete closure at the end of the case. In most cases, the thymus can be divided in the midline, and resection is not necessary. Our preferred technique for closure has been to use an autologous pericardial patch. This patch is harvested at the beginning of the operation with stay sutures to retract the corners and placed in a sterile Petri dish filled with saline. We do not “tan” this patch with glutaraldehyde. Cardiopulmonary bypass is initiated with an aortic cannula and two venous cannulas. For an ostium secundum defect, the right atrial appendage can be cannulated first. This cannula is later advanced into the superior caval vein. The inferior caval vein should be cannulated at its junction with the right atrium, low enough so that if there is no inferior rim to the defect adequate exposure in this area can still be obtained. A different cannulation strategy is used for sinus venosus defects. The superior caval vein should be cannulated directly with a right‐angled cannula superior to the azygous vein. Our preference for closure has been to cool to 32 °C. We do not use a vent for either ostium secundum or sinus venosus defects, as prior to repair the left side is effectively decompressed through the defect itself.

Following the initiation of cardiopulmonary bypass and the placement of tapes around the superior and inferior caval veins, cold‐blood cardioplegia is administered through the ascending aorta. Once the cardioplegia is completed, the caval venous tapes are deployed and the right atrium is opened. Our oblique incision for ostium secundum defects is carried from the right atrial appendage in the direction of the cannula in the inferior caval vein (Figure 15.6) [7]. An attempt is made not to cross the terminal crest so as to preserve the preferential conduction pathway from the sinus to the atrioventricular node. For sinus venosus defects, the incision in the right atrium is different, and is carried from the tip of the right atrial appendage in the direction of the superior cavoatrial junction. The incision stops short of the terminal crest, and we do not cut across the cavoatrial junction, which is well exposed by superior retraction on the atrial wall.

Having opened the atrium, the opening in the atrial septum is easily visualized (Figure 15.7) [7]

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