Atrial septal defect (ASD) is one of the more commonly recognized congenital cardiac anomalies presenting in adulthood. Atrial septal defect is characterized by a defect in the interatrial septum allowing pulmonary venous return from the left atrium to pass directly to the right atrium. Depending on the size of the defect, size of the shunt, and associated anomalies, this can result in a spectrum of disease from no significant cardiac sequelae to right-sided volume overload, pulmonary arterial hypertension, and even atrial arrhythmias.
With the routine use of echocardiography, the incidence of atrial septal defect is increased compared to earlier incidence studies using catheterization, surgery, or autopsy for diagnosis. The subtle physical examination findings and often minimal symptoms during the first 2-3 decades contribute to a delay in diagnosis until adulthood, the majority (more than 70%) of which is detected by the fifth decade. However, earlier intervention of most types of atrial septal defect is recommended.
The 3 major types of atrial septal defect (ASD) account for 10% of all congenital heart disease and as much as 20-40% of congenital heart disease presenting in adulthood. The most common types of ASD include the following:
- Ostium secundum: The most common type of ASD accounting for 75% of all ASD cases, representing approximately 7% of all congenital cardiac defects and 30-40% of all congenital heart disease in patients older than 40 years.
- Ostium primum: The second most common type of ASD accounts for 15-20% of all ASDs. Primum ASD is a form of atrioventricular septal defect and is commonly associated with mitral valve abnormalities.
- Sinus venosus: The least common of the three, sinus venosus (SV) ASD is seen in 5-10% of all ASDs. The defect is located along the superior aspect of the atrial septum. Anomalous connection of the right-sided pulmonary veins is common and should be expected. Alternate imaging is generally required.
Sex: ASD occurs with a female-to-male ratio of approximately 2:1.
Age: Patients with ASD can be asymptomatic through infancy and childhood, though the timing of clinical presentation depends on the degree of left-to-right shunt. Symptoms become more common with advancing age. By the age of 40 years, 90% of untreated patients have symptoms of exertional dyspnea, fatigue, palpitation, sustained arrhythmia, or even evidence of heart failure.
Atrial septal defect (ASD) is a congenital cardiac disorder caused by the spontaneous malformation of the interatrial septum.
- Ostium secundum ASD: This type of ASD results from incomplete adhesion between the flap valve associated with the foramen ovale and the septum secundum after birth. The patent foramen ovale usually results from abnormal resorption of the septum primum during the formation of the foramen secundum. Resorption in abnormal locations causes a fenestrated or netlike septum primum. Excessive resorption of the septum primum results in a short septum primum that does not close the foramen ovale. An abnormally large foramen ovale can occur as a result of defective development of the septum secundum. The normal septum primum does not close this type of abnormal foramen ovale at birth. A combination of excessive resorption of the septum primum and a large foramen ovale produces a large ostium secundum ASD.
- Ostium primum ASD: These defects are caused by incomplete fusion of septum primum with the endocardial cushion. The defect lies immediately adjacent to the atrioventricular (AV) valves, either of which may be deformed and incompetent. In most cases, only the anterior or septal leaflet of the mitral valve is displaced, and it is commonly cleft. The tricuspid valve is usually not involved.
- Sinus venosus ASD: Abnormal fusion between the embryologic sinus venosus and the atrium causes these defects. In most cases, the defect lies superior in the atrial septum near the entry of superior vena cava. Often there is associated anomalous drainage of the right superior pulmonary vein. The relatively uncommon inferior type is associated with partial anomalous drainage of the right inferior pulmonary vein. Anomalous drainage can be into the right atrium, the superior vena cava, or the inferior vena cava.
- Coronary sinus ASD: Coronary sinus defect is characterized by unroofed coronary sinus and persistent left superior vena cava that drains into the left atrium. A dilated coronary sinus often suggests this defect. This can result is desaturation due to right-to-left shunt into the left atrium. The diagnosis can be made by injecting contrast agent into left upper extremity; coronary sinus opacification precedes right atrial opacification.
Atrial septal defect (ASD) may occur on a familial basis. Holt-Oram syndrome characterized by an autosomal dominant pattern of inheritance and deformities of the upper limbs (most often, absent or hypoplastic radii) has been attributed to a single gene defect in TBX5.1 The penetrance is nearly 100% for Holt-Oram syndrome. Approximately 40% of Holt-Oram cases are due to new mutations.
Ellis van Creveld syndrome is an autosomal recessive disorder associated with skeletal dysplasia characterized by short limbs, short ribs, postaxial polydactyly, dysplastic nails and teeth, and a common atrium, occurring in 60% of affected individuals.2
Mutations in the cardiac transcription factor NKX2.5 have been attributed to the syndrome familial ASD associated with progressive atrioventricular block.3 This syndrome is an autosomal dominant trait with a high degree of penetrance but no associated skeletal abnormalities.
The magnitude of the left-to-right shunt across the ASD depends on the defect size, the relative compliance of the ventricles, and the relative resistance in both the pulmonary and systemic circulation. With small ASD, left atrial pressure may exceed right atrial pressure by several millimeters of mercury, whereas with large ASD, mean atrial pressures are nearly identical. Shunting across the interatrial septum is usually left-to-right and occurs predominantly in late ventricular systole and early diastole. Likely some augmentation occurs during atrial contraction. Note, however, that a transient and small right-to-left shunt can occur, especially during respiratory periods of decreasing intrathoracic pressure, even in the absence of pulmonary arterial hypertension.
The chronic left-to-right shunt results in increased pulmonary blood flow and diastolic overload of the right ventricle. Resistance in the pulmonary vascular bed is commonly normal in children with ASD, and the volume load is usually well tolerated even though pulmonary blood flow may be more than 2 times systemic blood flow. Altered ventricular compliance with age can result in an increased left-to-right shunt contributing to symptoms. The chronic significant left-to-right shunt can alter the pulmonary vascular resistance leading to pulmonary arterial hypertension, even reversal of shunt and Eisenmenger syndrome.
Because of an increase in plasma volume during pregnancy, shunt volume can increase, leading to symptoms. Pulmonary artery pressure usually remains normal.
- The atrial septal defect (ASD) malformation can go undiagnosed for decades due to subtle physical examination findings and a lack of symptoms.
- Even isolated defects of moderate-to-large size may not cause symptoms in childhood. However, some may have symptoms of easy fatigability, recurrent respiratory infections, or exertional dyspnea. In childhood, the diagnosis is often considered after a heart murmur is detected on routine physical examination or after an abnormal finding is observed on chest radiographs or electrocardiogram (ECG).
- If undetected in childhood, symptoms can develop gradually over decades and are largely the result of changing compliance with age, pulmonary arterial hypertension, atrial arrhythmias, and, sometimes, those associated with mitral valve disease in a primum ASD. Virtually all patients with ASD who survive beyond the sixth decade are symptomatic.
- Clinical deterioration in older patients occurs by means of several mechanisms.
- First, an age-related decrease in left ventricular compliance augments the left-to-right shunt.
- Second, atrial arrhythmias, especially atrial fibrillation, but also atrial flutter or paroxysmal atrial tachycardia, increase in frequency after the fourth decade and can precipitate right ventricular failure.
- Third, most symptomatic adults older than 40 years have mild-to-moderate pulmonary arterial hypertension in the presence of a persistent large left-to-right shunt; therefore, the aging right ventricle is burdened by both pressure and volume overload.
- Another mechanism for symptoms particularly associated with primum ASD is related to clinically significant mitral regurgitation. Its incidence, extent, and degree of dysfunction increases with age. Mitral valve insufficiency leads to further increase in left atrial pressure and a higher degree of left-to-right shunt.
- Overall, the most common presenting symptoms include dyspnea, easy fatigability, palpitations, sustained atrial arrhythmia, syncope, stroke, and/or heart failure.
- In adults, one of the most common symptoms is the development of palpitations related to atrial arrhythmias.
- The findings on physical examination depend on the degree of left-to-right shunt and its hemodynamic consequences, which, in turn, depends on the size of the defect, the diastolic properties of both ventricles, and the relative resistance of the pulmonary and systemic circulations.
- The patient can have a hyperdynamic right ventricular impulse due to increased diastolic filling and large stroke volume.
- Palpable pulsation of the pulmonary artery and an ejection click can be detected because of a dilated pulmonary artery.
- S1 is typically split, and the second component may be increased in intensity, reflecting forceful right ventricular contraction and delayed closure of the tricuspid leaflets.
- S2 is often widely split and fixed because of reduced respiratory variation due to delayed pulmonic valve closure (seen only if pulmonary artery pressure is normal and pulmonary vascular resistance is low). This characteristic abnormality is found in almost all patients with large left-to-right shunts.
- Blood flow across the ASD does not cause a murmur at the site of the shunt because no substantial pressure gradient exists between the atria. However, ASD with moderate-to-large left-to-right shunts result in increased right ventricular stroke volume across the pulmonary outflow tract creating a crescendo-decrescendo systolic ejection murmur. This murmur is heard in the second intercostal space at the upper left sternal border.
- Patients with large left-to-right shunts often have a rumbling middiastolic murmur at the lower left sternal border because of increased flow across the tricuspid valve.
- Auscultatory findings of the ASD may resemble those of mild valvular or infundibular pulmonic stenosis and idiopathic dilatation of the pulmonary artery. These disorders all manifest as a systolic ejection murmur, but they differ from the ASD by movement of the S2 with respiration, a pulmonary ejection click, or the absence of a tricuspid flow murmur.
- In patients with an ostium primum defect and an associated cleft of the mitral valve, an apical systolic regurgitant murmur of mitral regurgitation may be present.
- In patients who develop pulmonary arterial hypertension and right ventricular hypertrophy, a right ventricular S4 may be present. In such cases, the midsystolic pulmonic murmur is softer and shorter, the tricuspid flow murmur is not present, the splitting of S2 is narrowed with accentuated pulmonic component, and murmur of pulmonic regurgitation may become apparent.
- ASD is an acyanotic lesion. Thus, the patient should be normally saturated. In the rare case of severe pulmonary arterial hypertension, atrial shunt reversal (Eisenmenger syndrome) may occur, leading to cyanosis and clubbing.
The decision to repair any kind of atrial septal defect (ASD) is based on clinical and echocardiographic information, including the size and location of the ASD, the magnitude and hemodynamic impact of the left-to-right shunt, and the presence and degree of pulmonary arterial hypertension. In general, elective closure is advised for all ASDs with evidence of right ventricular overload or with a clinically significant shunt (pulmonary flow [Qp]–to–systemic flow [Qs] ratio >1.5). Lack of symptoms is not a contraindication for repair.
In childhood, spontaneous closure of secundum ASD may occur. However, in adulthood, spontaneous closure is unlikely. Patients may be monitored relatively conservatively for a period before intervention is advised. Considerations and even contraindications to consider no intervention include small size of the defect and shunt, severe pulmonary arterial hypertension, diagnosis during pregnancy (intervention can be deferred until after), severe left or right ventricular dysfunction. Guidelines for the management of adults with congenital heart disease have been recently updated.4
For both children and adults, surgical mortality rates for uncomplicated secundum ASD are approximately 1-3%. Because of the lifetime risk associated with ASD, as outlined including paradoxical embolization, there should be ongoing evaluation and review of the indication and risks for closure, even for patients with small shunts. However, such closure remains controversial because patients with small defects generally have a good prognosis, and the risk of cardiopulmonary bypass may not be warranted. The widespread use of catheter closure of secundum ASD with lower mortality and without cardiopulmonary bypass has raised the question regarding the need to close even small defects.
Long-term prevention of death and complications is best achieved when the ASD is closed before age 25 years and when the systolic pressure in the main pulmonary artery is less than 40 mm Hg. Even in elderly patients with large shunts, surgical closure can be performed at low risk and with good results in reducing symptoms.
Closure of an atrial septal defect (ASD) is not recommended in patients with a clinically insignificant shunt (Qp-Qs ratio 0.7 or below) and in those who have severe pulmonary arterial hypertension or irreversible pulmonary vascular occlusive disease who have a reversed shunt with at-rest arterial oxygen saturations of less than 90%. In addition to the high surgical mortality and morbidity risk, closure of a defect in the latter situation may worsen the prognosis. Whether the patient whose condition is diagnosed well in the sixth decade of life would benefit from surgical closure remains controversial.