What does ASD (Atrial Septal Defect) mean?

Atrial Septal Defect (ASD): Definition and Clinical Overview

ASD is a persistent communication (hole) between the two upper chambers of the heart (the atria), representing one of the most common congenital heart defects diagnosed in adults. 1

Anatomic Types

ASD encompasses several distinct anatomic subtypes based on location within the atrial septum:

• Secundum ASD (75% of cases): Located in the region of the fossa ovalis, the central portion of the atrial septum 1, 2
• Primum ASD (15-20% of cases): Positioned inferiorly near the crux of the heart, representing a variant of atrioventricular septal defect with nearly always an associated cleft in the anterior mitral valve leaflet 1, 2
• Sinus venosus ASD (5-10% of cases): Located superiorly near the superior vena cava entry or inferiorly near the inferior vena cava entry, frequently associated with partial anomalous pulmonary venous drainage 1, 2
• Coronary sinus septal defect (<1% of cases): An uncommon defect causing shunting through the ostium of the coronary sinus 1, 2

Pathophysiology

The fundamental consequence of ASD is left-to-right shunting of blood, causing right ventricular volume overload and pulmonary overcirculation. 1, 3

The hemodynamic effects include:

• Blood flows from the higher pressure left atrium to the lower pressure right atrium 3
• This creates excessive blood flow through the right heart chambers and pulmonary circulation 1, 3
• The right ventricle dilates to accommodate the increased volume load 1
• Long-standing volume overload leads to atrial arrhythmias (atrial flutter, atrial fibrillation, sick sinus syndrome) 1, 2
• Flow-related pulmonary arterial hypertension may develop, though much later than with high-pressure shunts like ventricular septal defects 1

Clinical Manifestations

Large atrial shunts produce symptoms from excess pulmonary blood flow and right-sided heart failure, including frequent pulmonary infections, fatigue, exercise intolerance, and palpitations. 1

Physical examination findings include:

• Fixed splitting of the second heart sound 3
• Systolic pulmonary flow murmur 3
• Mid-diastolic rumble across the tricuspid valve with large shunts due to increased flow 3
• Right axis deviation and right bundle-branch block on ECG (particularly with secundum ASD) 1

Adult comorbidities such as ischemic heart disease, hypertension, or diabetes can reduce left ventricular compliance, increasing left atrial pressure and worsening the left-to-right shunt. 1

Diagnostic Approach

ASD should be diagnosed by imaging techniques with demonstration of shunting across the defect and evidence of right ventricular volume overload. 1

• Transthoracic echocardiography is the initial diagnostic modality 2
• Transesophageal echocardiography provides superior visualization, particularly for sinus venosus defects (visible by transthoracic imaging in only 25% of cases) 2
• Saline contrast echocardiography is more sensitive than Doppler for detecting low-velocity right-to-left shunts 2, 4
• Patients with unexplained right ventricular volume overload should be referred to an Adult Congenital Heart Disease center for further diagnostic studies to rule out obscure ASD, partial anomalous venous connection, or coronary sinus septal defect. 1

Critical Distinction: When Right-to-Left Shunting Develops

Right-to-left shunting occurs when right atrial pressure exceeds left atrial pressure, typically in the setting of severe pulmonary hypertension (Eisenmenger syndrome), and represents an absolute contraindication to ASD closure. 2, 4

This reversal of shunt direction:

• Causes hypoxemia unresponsive to supplemental oxygen 2
• Indicates irreversible pulmonary vascular disease 4
• Closure is contraindicated when pulmonary artery systolic pressure exceeds two-thirds of systemic pressure, pulmonary vascular resistance exceeds two-thirds of systemic vascular resistance, or a net right-to-left shunt is present. 4