Atrial septal defects (ASDs) are a group of anatomic abnormalities that result in shunting of blood between the atria. ASD is the second most common congenital heart lesion after ventricular septal defect and occurs in 5 to 6 infants out of 1000 live births. Evaluation is by echocardiography and closure by transcatheter device or surgery because both share outstanding long-term outcomes.
Key wordsSecundum ASD, Sinus Venosus ASD, Ostium Primum, Unroofed Coronary Sinus, Partial AVSD
Atrial septal defects (ASDs) are a group of anatomic abnormalities that result in shunting of blood between the atria ( Fig. 47.1 ). ASD is the second most common congenital heart lesion after ventricular septal defect and occurs in 5 to 6 infants out of 1000 live births. The most common ASD, representing approximately 80% of ASDs, is the secundum ASD, which is essentially a hole in (or deficiency of) the septal tissue in the region of the fossa ovalis. The ostium primum ASD (10% of ASDs), a hole in the septum near the atrioventricular (AV) valves, is typically associated with a cleft in the left AV valve. It is more appropriately referred to as a partial atrioventricular septal defect (AVSD), and a partial AVSD is the least extreme defect among the spectrum of AVSDs because it only creates shunting at the atrial level. The sinus venosus ASD is a communication above the level of the atrial septum due to override of the embryologic sinus venosus. Sinus venosus ASD almost always occurs in the superior portion of the heart and typically involves partial anomalous pulmonary venous (PAPVR) drainage of the right upper and middle lobes. Very rarely a sinus venosus defect may be situated in the inferoposterior portion of the atrial septum, this time overriding the inferior vena caval orifice. Finally, unroofed coronary sinus (CS), the least common of these four types of ASD, is an opening in the inner wall or “roof” of the CS within the left atrium (LA), thus shunting the blood in or out of the CS and then draining into the right atrium (RA) through the ostium of the CS.
This chapter will describe each of these ASDs, including their embryology, evaluation, natural history, intervention, and outcomes.
Embryology and Anatomy
Secundum Atrial Septal Defect and Patent Foramen Ovale
During the fourth week of embryonic development the septum primum forms at the superior portion of the common atria and grows inferiorly toward the endocardial cushion. The inferior opening is called ostium primum, which closes with the fusion of endocardial cushions. The ostium secundum forms in the superior portion of the atrium during the fifth and sixth embryonic weeks. Anterior superior infolding of common atria to the right of septum primum results in closure of ostium secundum and formation of fossa ovalis. This muscular portion of the septum is termed the septum secundum ( Fig. 47.2 ). The rim of the thickened muscular portion is called the limbus . Most of this structure is not a true “wall” between the left and right atria but rather opposition of two separate atrial walls. If a surgeon were to cut into the limbus superiorly or medially to enter the LA, that incision would likely create a hole outside the heart. The area of the oval fossa and the muscular septum immediate anterior and inferior is the only “true” septal wall. The oval fossa is covered by a thin-walled tissue called the septum primum. The septum primum is anchored at the inferior edge and not at the superior margin of the limbus, thus forming a flap valve. This flap valve in gestational life allows the oxygen-rich blood from the placenta via the ductus venosus and inferior vena cava (IVC) to flow into the left atria ( Fig. 47.3A ). This left atrial blood then travels through the left ventricle (LV) and ascending aorta, allowing the maximal oxygenated blood to be delivered to the brain. Immediately after a neonate begins spontaneous breathing, flow of blood increases to the lungs and drains into the LA. This increases the left atrial pressure and pushes the septum primum against the septum secundum, closing the oval fossa (see Fig. 47.3B ). In the majority of people the septum primum fuses closed; however, in up to 25% of people the septum primum is fully covering the oval fossa but can be opened in the conditions when right atrial pressure increases, the so-called patent foramen ovale (PFO) (see Fig. 47.2 ). The significance of the PFO is described later. Any deficiency in the normal septum primum, whether it is too small to fully cover the oval fossa, contains one or more holes (“fenestrations”), or is absent, leads to persistent communication between the atria at the level of the oval fossa. This is the secundum ASD and understandably is also referred to as an oval fossa ASD ( Fig. 47.4A-D ).
Ostium Primum Atrial Septal Defect
The septum primum originally develops as a ridge of muscular tissue that grows toward the endocardial cushion tissue of the developing AV valves. It eventually fuses with the cushion and closes the primordial hole, the ostium primum, between the AV valves and the septum primum. Arrest of complete formation of the endocardial cushion and the septum primum leaves a spectrum of AVSDs. The AV septum is that portion of the septum that in the normal heart actually separates the LV from the RA. This is due to the higher attachment of the normal mitral valve to the septum compared to the attachment of the tricuspid valve ( Fig. 47.5 )—a feature that is helpful in distinguishing the morphologic right from the morphologic left ventricle. In AVSDs the absence of this AV septum can be identified by the attachment of the mitral and tricuspid valves at the same level (see Fig. 47.5 ). When the ventricular portion of the endocardial cushion is intact, the resulting septal defect is only at the atrial level (see Fig. 47.5 ). The vast majority are associated with a persistent cleft or “zone of apposition” in the anterior leaflet of the left AV valve (see Fig. 47.4E ). Current nomenclature regards this lesion more commonly as a partial AVSD.
Sinus Venosus Atrial Septal Defect
The right horn of the sinus venosus is embryologically connected to the primitive atria and eventually forms the inflow of the superior vena cava (SVC) where it enters into the RA. When this confluence is malpositioned toward the LA, it forms an override of the left and right atria, thus creating a connection, the sinus venosus ASD, that lies superior to the true atrial septum (see Fig. 47.4F ; Fig. 47.6 ). Because of this leftward orientation of the sinus venosus, the right upper and right middle lobe pulmonary veins get incorporated with the SVC in the majority of sinus venosus ASDs, creating PAPVR return ( Fig. 47.7 ). This creates two levels of left-to-right shunt, from LA to RA and from pulmonary vein to RA. A less common defect involves the ostia of IVC and right inferior pulmonary vein and is called inferior sinus venosus ASD.
Unroofed Coronary Sinus
The embryologic origin of the CS is the left horn of the sinus venosus that merges with the RA around the time of atrial septation. The regression of the roof of this structure happens after that point of development, explaining why the ostium of the CS remains in the typical location despite the opening into the LA. Many of the unroofed CSs are associated with a persistent left superior vena cava (LSVC). For this reason, shunting can be right to left from the LSVC to the LA, as well as left to right from the LA into the CS and into the RA through the normal ostium.
Secundum type of ASDs can close spontaneously, but other types do not. A report by Campbell noted that mean age of death was 37.5 ± 4.5 years with 75% dying by the age of 50 years and 90% dying by the age of 60 years. The primary pathophysiologic result of any isolated ASD is left-to-right shunting. The degree of shunting is related to both the size of the actual defect and the relative difference in the compliance of the right ventricle (RV) compared to the LV. Compliance is defined as the “distensibility” of a ventricular chamber, with stiffer chambers being less compliant. Physiologically, compliance is measured as ΔP/ΔV, where P is pressure and V is volume. In compliant chambers, large increases in volume result in chamber distention (much like a well-inflated balloon can easily increase in size when air is blown into it) with minimal increases in pressure. In noncompliant chambers (much like a small noninflated balloon), wall stiffness resists distention with volume infusion, and instead the intracavitary pressure increases as volume is added. Noncompliance, or wall stiffness, is a hallmark of “diastolic dysfunction” (as is often seen in some forms of congenital heart disease—like tetralogy of Fallot), and in these patients the noncompliance of the RV compared with the LV can result in right-to-left shunting across an ASD. Shortly after a baby is born, the RV is stiff and less compliant than the LV, and it is common for newborns to be mildly cyanotic from right-to-left shunting across the foramen ovale. As the lungs fill with air and the RV begins to pump against lower pulmonary resistance, the RV compliance improves, the atrial level shunt reverts to left to right, and this typically will close the foramen ovale by pushing the top of the septum primum against the top of the septum secundum (see Fig. 47.3 ). Although the direction of the shunt across an ASD is determined primarily by the differential compliance of the downstream chambers (RV and LV; with the RV generally being more compliant than the LV), other factors can also influence this downstream “compliance,” such as severe stenosis or hypoplasia of the inlet (tricuspid or mitral) valve. The direction of the atrial shunt can also be influenced by “streaming” of blood from the IVC that might be directed along the atrial septum and across a PFO or secundum ASD. Additionally, because shunting between atria occurs at low pressure, the development of pulmonary vascular disease is a late process if it occurs at all. Even in the sixth and seventh decade of life, many ASDs will continue to present without fixed pulmonary hypertension. The enlargement of the right atrium and ventricle, however, do cause late atrial arrhythmias, tricuspid regurgitation, and RV dysfunction. These changes are slow to occur, and the precise window for surgical intervention remains poorly defined.
Presentation and Evaluation
The typical presentation of a child with an ASD is that of a completely asymptomatic child with a murmur noted on routine examination. Some children present with shortness of breath, easy fatigue, or other vague symptoms. In a child with profound symptoms or congestive heart failure, other causes, including concomitant cardiac defects, must be excluded. Older children and adults may present with symptoms of mild fatigue and exercise intolerance, which gradually worsen with the age. Palpitations (due to atrial arrhythmias) are a common complaint. Patients who develop Eisenmenger syndrome (reversal of the left-to-right shunt secondary to pulmonary hypertension) present with cyanosis and syncope, although this is an uncommon occurrence from isolated ASD. Due to the low velocity/low pressure, shunting at the ASD per se does not produce a murmur. However, the volume overload in the RV produces a pulmonary systolic ejection murmur heard at the left sternal border due to relative pulmonary stenosis. The long ejection time of the RV due to this volume overload also creates the fixed split second heart sound associated with significant ASDs from delayed closure of the pulmonary valve. A large ASD may also have a diastolic murmur of relative tricuspid stenosis as a result of increased flow across the AV valve. A chest x-ray examination may demonstrate right heart enlargement, most notably a right atrial bulge. An electrocardiogram (ECG) can show P-wave changes and large R waves consistent with RA and RV enlargement. Other ECG findings common in ASDs include incomplete right bundle branch block and RSR′ in the V 1 lead. This latter finding is associated with an increased likelihood of needing closure of the ASD.
The fundamental diagnostic tool is the echocardiogram. Transthoracic echocardiography can quickly, painlessly, noninvasively, and with relative cost efficiency diagnose an ASD and quantify the degree of right atrial and ventricular enlargement ( Fig. 47.8 ). Perhaps more critically, the size and margins of the defect can be assessed, which helps determine the preferred mode of intervention, surgical versus transcatheter device. In older patients with questionable pulmonary hypertension the relative left-to-right versus right-to-left shunting, the estimation of RV pressure calculated by tricuspid regurgitation velocity, and any degree of RV dysfunction can also be determined with echocardiography. In the typical pediatric patient with an uncomplicated ASD, no other diagnostic studies are warranted outside of preprocedural laboratory studies, including an electrocardiogram. However, in older patients a history of palpitations requires an electrophysiologic evaluation with a minimum of 24-hour Holter monitoring. Any further concerns for arrhythmias may warrant an electrophysiologic study to determine the need for catheter-based or surgical antiarrhythmia ablations. The role of cardiac catheterization for diagnosis of this condition is extremely limited in the pediatric population. In the older patient with a large ASD and any question of elevated pulmonary pressures a cardiac catheterization can ascertain if closure of the ASD will prompt risk of RV dysfunction following closure. Cardiac magnetic resonance (CMR) imaging is often used to define the levels of PAPVR into the SVC in sinus venosus ASDs when not clearly defined on echocardiogram. CMR is also valuable in older patients with long-standing ASDs to assess RV size and function. Gated computed tomographic imaging is an emerging and potentially useful alternative to CMR, particularly for patients with complex ASDs.