Define Atrial Septal Defect. Describe its types, clinical features, investigations and management.

Definition

• An Atrial Septal Defect (ASD) is an opening in the interatrial septum causing an acyanotic, left-to-right shunt at the atrial level.
• It is a common CHD, more frequent in girls.

Types

• Ostium secundum — the commonest (in the region of the fossa ovalis).
• Ostium primum — low defect, part of AV-septal defects (associated with Down syndrome).
• Sinus venosus — near the entry of the SVC/IVC (with anomalous pulmonary venous drainage).

Haemodynamics & Clinical Features

• Left-to-right shunt → right heart volume overload and increased pulmonary flow.
• Often asymptomatic in childhood (a large shunt may cause exertional dyspnoea, recurrent chest infections, failure to thrive).
• Signs — a wide, fixed splitting of the second heart sound (the hallmark) with an ejection systolic murmur in the pulmonary area (increased flow across the pulmonary valve).

Investigations

• ECG — right axis deviation and incomplete right bundle branch block (RBBB); left-axis in ostium primum.
• Chest X-ray — cardiomegaly with pulmonary plethora; echocardiography is diagnostic.

Management & Complications

• Small ASDs may close spontaneously; a significant shunt is closed by a device (catheter) or surgery, usually in early childhood.
• If untreated over decades → pulmonary hypertension, atrial arrhythmias, and paradoxical embolism.

Paradoxical Embolism & Late Risks

• An ASD allows a venous clot to cross to the systemic circulation (paradoxical embolism → stroke).
• Long-standing large shunts cause pulmonary hypertension and atrial arrhythmias in adulthood.

Timing of Closure

A haemodynamically significant ASD (right heart enlargement) is usually closed electively in early childhood (around 3–5 years) — most often by a transcatheter device for secundum defects — to prevent long-term pulmonary hypertension and arrhythmia.

Fixed splitting of S2 is the diagnostic auscultatory sign.