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Atrial Septal Defect (ASD)

(Ostium Secundum Defect)

by Jeanne Marie Baffa, MD

An atrial septal defect (ASD) is an opening in the interatrial septum, causing a left-to-right shunt and volume overload of the right atrium and right ventricle. Children are rarely symptomatic, but long-term complications after age 20 yr include pulmonary hypertension, heart failure, and atrial arrhythmias. Adults and, rarely, adolescents may present with exercise intolerance, dyspnea, fatigue, and atrial arrhythmias. A soft midsystolic murmur at the upper left sternal border with wide and fixed splitting of the 2nd heart sound (S 2 ) is common. Diagnosis is by echocardiography. Treatment is transcatheter device closure or surgical repair.

ASDs account for about 6 to 10% of cases of congenital heart disease. Most cases are isolated and sporadic, but some are part of a genetic syndrome (eg, mutations of chromosome 5, Holt-Oram syndrome).

Classification

ASDs can be classified by location:

  • Ostium secundum: A defect in the fossa ovalis—in the center (or middle) part of the atrial septum

  • Sinus venosus: A defect in the posterior aspect of the septum, near the superior vena cava or inferior vena cava, and frequently associated with anomalous return of the right upper or lower pulmonary veins to the right atrium or vena cava

  • Ostium primum: A defect in the anteroinferior aspect of the septum, a form of atrioventricular septal (endocardial cushion) defect (see Atrioventricular Septal Defect)

Pathophysiology

To understand hemodynamic changes seen in ASD (and other anomalies), see Figure: Normal circulation with representative right and left cardiac pressures (in mm Hg). for normal hemodynamic data.

Normal circulation with representative right and left cardiac pressures (in mm Hg).

Representative right heart O 2 saturation =75%; representative left heart O 2 saturation = 95%. Atrial pressures are mean pressures.

AO = aorta; IVC = inferior vena cava; LA = left atrium; LV =left ventricle; PA = pulmonary artery; PV = pulmonary veins; RA = right atrium; RV =right ventricle; SVC = superior vena cava.

In ASD, shunting is left to right initially (see Figure: Atrial septal defect.). Some small ASDs, often just a stretched patent foramen ovale, close spontaneously during the first few years of life. Persistent moderate-to-large ASDs result in large shunts, leading to right atrial and right ventricular volume overload. If unrepaired, these large shunts may lead to pulmonary artery hypertension, elevated pulmonary vascular resistance, and right ventricular hypertrophy by the time people are in their 20s. Atrial arrhythmias, such as supraventricular tachycardia (SVT), atrial flutter, or atrial fibrillation may also occur later. Ultimately, the increase in the pulmonary artery pressure and vascular resistance may result in a bidirectional atrial shunt with cyanosis during adulthood (Eisenmenger reaction).

Atrial septal defect.

Pulmonary blood flow and RA and RV volume are increased. (N ote : Intracardiac pressures generally remain in the normal range throughout childhood.) With a large defect, RA and LA pressures are equal. AO = aorta; IVC = inferior vena cava; LA = left atrium; LV =left ventricle; PA = pulmonary artery; PV = pulmonary veins; RA = right atrium; RV =right ventricle; SVC = superior vena cava.

Symptoms and Signs

Most patients with small ASDs are asymptomatic. Larger shunts may cause exercise intolerance, dyspnea during exertion, fatigue, and atrial arrhythmias with palpitations. Passage of microemboli from the venous circulation across the ASD (paradoxical embolization), often associated with arrhythmias, may lead to cerebral or systemic thromboembolic events, such as stroke. Rarely, when an ASD is undiagnosed or untreated, Eisenmenger syndrome develops.

Auscultation typically reveals a grade 2 to 3/6 midsystolic (ejection systolic) murmur and a widely split, fixed S 2 at the upper left sternal border in children. A large left-to-right atrial shunt may produce a low-pitched diastolic murmur (due to increased tricuspid flow) at the lower sternal borders. These findings may be absent in infants, even those who have a large defect. A prominent right ventricular cardiac impulse, manifested as a parasternal heave or lift, may be present.

Diagnosis

  • Chest x-ray and ECG

  • Echocardiography

Diagnosis is suggested by cardiac examination, chest x-ray, and ECG and is confirmed by 2-dimensional echocardiography with color flow and Doppler studies.

With a significant shunt, ECG may show right axis deviation, right ventricular hypertrophy, or right bundle branch block (with rSR pattern in V 1 ). Chest x-ray shows cardiomegaly with dilation of the right atrium and right ventricle, a prominent main pulmonary artery segment, and increased pulmonary vascular markings.

Cardiac catheterization is not usually necessary unless transcatheter closure of the defect is planned.

Treatment

  • Observation, transcatheter closure, or surgical repair

Most small centrally located ASDs (< 3 mm) close spontaneously; many defects between 3 mm and 8 mm close spontaneously by age 3 yr. These defects probably represent a stretched patent foramen ovale rather than true secundum ASDs. Ostium primum ASDs and sinus venosus ASDs do not close spontaneously.

Asymptomatic children with a small shunt require only observation and periodic echocardiography. Although these children are theoretically at risk of paradoxical systemic embolization, this event is rare in childhood. Thus, it is not standard practice to close a small, hemodynamically insignificant defect.

Moderate to large ASDs (evidence of right ventricular volume overload on echocardiography) should be closed, typically between ages 2 yr and 6 yr. Repair may be considered earlier in children with chronic lung disease. Transcatheter closure with various devices (eg, Amplatzer ® or Gore HELEX ® septal occluder) is preferred when appropriate anatomic characteristics, such as adequate rims of septal tissue and distance from vital structures (eg, aortic root, pulmonary veins, tricuspid annulus), are present. Otherwise, surgical repair is indicated. Sinus venosus and ostium primum (atrioventricular septal type) defects are not amenable to device closure. If ASDs are repaired during childhood, perioperative mortality rate approaches 0, and long-term survival rates approach those of the general population.

Endocarditis prophylaxis is not needed preoperatively and is required only for the first 6 mo after repair or if there is a residual defect adjacent to a surgical patch.

Key Points

  • An atrial septal defect (ASD) is an opening in one of several parts of the interatrial septum, causing a left-to-right shunt.

  • Small atrial communications often close spontaneously, but larger ones do not, causing right atrial and ventricular overload and ultimately pulmonary artery hypertension, elevated pulmonary vascular resistance, and right ventricular hypertrophy; SVT, atrial flutter, or atrial fibrillation may also occur.

  • ASDs can allow emboli from the veins to enter the systemic circulation (paradoxical embolization), causing arterial occlusion (eg, stroke).

  • Auscultation typically reveals a grade 2 to 3/6 midsystolic murmur and a widely split, fixed S 2 ; these findings may be absent in infants.

  • Moderate to large ASDs should be closed, typically between ages 2 yr and 6 yr, using a transcatheter device when possible.

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