Atrial Septal Defect

Introduction
History of the Procedure

F. John Lewis successfully performed the first open heart closure of an atrial septal defect (ASD) on September 5, 1952, in Minneapolis, Minnesota. He used direct vision with hypothermia and inflow occlusion. Subsequent development of the cardiopulmonary bypass machine widely expanded the treatment of this and other congenital heart diseases more complex than ASD and spurred rapid expansion in the field of thoracic surgery.

In terms of establishing normal anatomy and eliminating sequelae, early repair of ASDs may well be the most successful application of open heart surgery of any congenital heart lesion. For nearly 50 years, surgical closure of ASDs has been almost exclusively accomplished with direct suture or patch repair on cardiopulmonary bypass. In recent years, use of a variety of transcatheter occlusion devices inserted through the defect in the septum has become common. For some patients, these devices have become an alternative to surgical procedures for the repair of the ostium secundum ASD. Surgical innovators have simultaneously developed minimally invasive techniques involving small incisions and even robotically assisted thoracoscopic approaches.
Problem

ASD is one of the most commonly recognized congenital cardiac anomalies in adults, but it is rarely diagnosed and even less commonly results in disability in infants. It is characterized by a defect in the interatrial septum that allows pulmonary venous return to pass from the left to the right atrium, resulting in right atrial and right ventricular chamber dilation, the extent of which depends on the size of the shunt. Patients, especially those with small or isolated defects, are usually asymptomatic through the first 3 decades of life, though more than 70% become impaired by the fifth decade. Early surgical closure of most types of ASD is recommended.
Frequency

ASD accounts for 10% of all congenital heart disease and for 22-40% of congenital heart disease in adults.

Ostium secundum defect is the most common type and accounts for 60-70% of all cases, for approximately 7% of all congenital cardiac defects and for 30-40% of all congenital heart disease in patients over age 40.

Ostium primum type accounts for 15-20% of all ASDs.

Sinus venosus type ASDs are seen in 5-15% of all patients.

Sex: ASD occurs with a female-to-male ratio of approximately 2:1.

Age: Patients with ASD are usually asymptomatic through infancy and childhood. Symptoms become more common with advancing age. By the age of 40 years, 90% of untreated patients have symptoms of exertional dyspnea, fatigue, palpitation, or sustained arrhythmia.
Etiology

ASD is a congenital cardiac disorder caused by the spontaneous malformation of the atrial septum.
Ostium secundum ASDs: incomplete adhesion between the original flap of the valve of the foramen ovale and the septum secundum after birth causes the probe-patent foramen ovale. 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. About 10-20% of patients have associated mitral valve prolapse.
Ostium primum ASDs: These defects are caused by incomplete fusion of septum primum with 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. The defect is often large.
Sinus venosus ASDs: Abnormal fusion between the embryologic sinus venosus and the atrium causes these defects. In most cases, the defect lies superior, high in the atrial septum near the entry of superior vena cava, and it is associated with partial 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 defect: 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. The diagnosis can be made by injecting contrast agent into left upper extremity; coronary sinus opacification precedes right atrial opacification.
Other ASDs
Certain ASDs may occur on a familial basis. Holt-Oram syndrome is characterized by an autosomal dominant pattern of inheritance, deformities of the upper limbs (most often, absent or hypoplastic radii), and ECG abnormalities, such as right bundle-branch block or first-degree AV block. A single gene defect with a penetrance of nearly 100% is the apparent cause of Holt-Oram syndrome. Approximately 40% of cases are due to new mutations; the rest are inherited from a parent.
Another example is the syndrome of familial ASD with prolonged AV conduction. This syndrome is an autosomal dominant trait with a high degree of penetrance but no associated skeletal abnormalities.
Both Holt-Oram syndrome and familial ASD with prolonged AV conduction affect nearly 50% of first-degree relatives of the patient.
Pathophysiology

The magnitude of the left-to-right shunt depends on defect size and relative compliance of the ventricles and the relative resistance in both pulmonary and systemic circulation. In patients with small ASDs, left atrial pressure may exceed right atrial pressure by several millimeters of mercury, whereas mean atrial pressures are nearly identical if the defect is large. Left-to-right shunting occurs predominantly in late ventricular systole and in early diastole, with some augmentation during atrial contraction. The shunt results in diastolic overload of the right ventricle and increased pulmonary blood flow.

Resistance in the pulmonary vascular is commonly normal or low in older infants or children with ASD, and the volume load is usually well tolerated though pulmonary blood flow may be 2-5 times more than systemic blood flow. A transient and small right-to-left shunt that occurs with the onset of left ventricular contraction, especially during respiratory periods of decreasing intrathoracic pressure, is common in patients with ostium secundum ASD, even in absence of pulmonary hypertension.

Pregnancy can increase shunt volume and lead to congestive heart failure (CHF). Pulmonary artery pressure usually remains normal and well tolerated.

A chronic left-to-right shunt fixes pulmonary hypertension and eventually reverses the direction of the shunt, resulting in Eisenmenger syndrome.
Presentation

History:
The malformation often goes unnoticed for decades because symptoms may be absent and because physical signs are subtle.
Even isolated defects of moderate-to-large size do not generally cause symptoms in infancy and childhood. Occasional cases of CHF and recurrent pneumonia are seen in infancy. Most children are asymptomatic, though some may have easy fatigability and exertional dyspnea. They may be somewhat underdeveloped and prone to respiratory infections. 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 ECGs.
Symptoms usually take 30-40 years to develop. They are mainly consequences of pulmonary hypertension, atrial tachyarrhythmias, and, sometimes, associated mitral valve disease. Virtually all patients with ostium secundum ASD who survive beyond the 6th decade are symptomatic.
Clinical deterioration in older patients occurs by means of several mechanisms.
First, an age-related decrease in left ventricular distensibility 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 4th decade and precipitate right ventricular failure.
Third, most symptomatic adults older than 40 years of age have mild-to-moderate pulmonary hypertension in the presence of a persistent large left-to-right shunt; therefore, the aging right ventricle is burdened by pressure and volume overload.
Another mechanism for symptoms is related to clinically significant mitral regurgitation that is seen in about 15% of patients. Its incidence, extent and degree of dysfunction increases with age. These abnormalities have been attributed mainly to the effects of left ventricular cavity deformity on mitral apparatus.
Most common presenting symptom is dyspnea and easy fatigability. Other symptoms include palpitations, syncope, and CHF.
The development of palpitations related to atrial arrhythmias is the most common symptom in adults.

Physical:
The findings depend on the hemodynamic consequences of the left-to-right shunt, which in turn depends on the size of the defect, the diastolic properties of both ventricles, and the relative impedance in pulmonary and systemic circulation.
Blood flow across the ASD does not cause a murmur at the site of the shunt because no substantial pressure gradient is present between the atria.
The patient often has a hyperdynamic right ventricular impulse due to increased diastolic filling and a 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.
ASDs with moderate-to-large left-to-right shunts produce a pulmonary outflow murmur that begins shortly after the S1, peaks in early-to-mid systole and that ends before the S2. An associated thrill indicates a large shunt or pulmonic stenosis.
S2 is widely split and fixed because of greatly 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.
Increased right ventricular stroke volume across the pulmonary outflow tract and valve creates a crescendo-decrescendo midsystolic (ejection) murmur. This murmur is usually grade 2 or 3 and is heard in the second interspace at the 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.
In patients with an ostium primum defect and an associated cleft mitral valve, an apical pansystolic murmur of mitral regurgitation may be present. This murmur can be heard along the left sternal border as the jet is directed into the right atrium through the low ASD. Mitral regurgitation murmur can also be heard if valve prolapse is present.
In patients who develop pulmonary 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.
In case of shunt reversal (Eisenmenger syndrome), cyanosis and clubbing may become evident.
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 midsystolic (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.
Indications

The decision to repair any kind of 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 hypertension. Elective closure is advised for all ASDs with echocardiographic evidence of right ventricular overload or with a clinically significant shunt (pulmonary vascular resistance [Qp]–to–systemic vascular resistance [Qs] ratio >1.5). Lack of symptoms is not a contraindication for repair. In patients with interatrial septal aneurysm and secundum ASD, spontaneous closure may occur, and patients may be followed up relatively conservatively for a period before repair is advised.

For both children and adults, surgical mortality rates for uncomplicated secundum ASDs are approximately 1-3%. Because of the risk of paradoxical embolization, closure may be recommended, even for patients with small shunts in whom the incidence of CHF, pulmonary hypertension, and arrhythmias is low. 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 benefit of catheter closure of small secundum defects remains to be determined.

Long-term prevention of death and complications is best achieved when the ASD is closed before the age of 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.
Contraindications

Closure of an ASD is not recommended in patients who have severe pulmonary hypertension or severe pulmonary vascular disease (Qp-Qs ratio 0.7 or above) without a clinically significant shunt or in patients who have a reversed shunt with at-rest arterial oxygen saturations of <90% with little or no residual left-to-right shunt. In addition to the high surgical mortality and morbidity risk, closure of the defect may worsen the prognosis. Whether patients whose condition is diagnosed well in the sixth decade of life benefit from surgical closure remains of controversial.

http://emedicine.medscape.com/article/162914-overview