Ostium Secundum Atrial Septal Defect

Ostium secundum atrial septal defect is a deficiency of the septum at the fossa ovalis, allowing blood to pass between the atria. It is the commonest atrial septal defect, forming about 75% of the total, and usually causes a left-to-right shunt with right heart volume overload.

It occurs in about 1 in 1500 live births and affects women about twice as often as men. It is the commonest congenital heart defect first diagnosed in adults. Familial cases occur (GATA4, TBX5, NKX2-5 variants).

Many defects under 3 mm close in the first 18 months, and some defects of 3 to 8 mm close in early childhood, while defects over 8 mm seldom close. Untreated large defects cause right heart dilatation, atrial arrhythmia, paradoxical embolism and, in 5 to 10% of adults, pulmonary arterial hypertension.

Pathophysiology.

The shunt direction depends on relative ventricular compliance. The right ventricle is more compliant than the left, so blood flows left to right during diastole, producing right atrial and right ventricular volume overload and increased pulmonary flow.

Chronic overload dilates the right atrium and annulus, which causes tricuspid regurgitation and atrial arrhythmia. Late rise in pulmonary vascular resistance can reverse the shunt (Eisenmenger syndrome), which is uncommon with isolated atrial defects.

With age, left ventricular stiffness increases (hypertension, coronary disease), and closure can raise left atrial pressure and cause pulmonary oedema. Testing for this before closure in older patients is important.

Children are usually asymptomatic. Adults develop exertional dyspnoea, fatigue, palpitations, recurrent chest infections, and, later, right heart failure, atrial fibrillation or flutter, and embolic stroke. Cyanosis appears late with shunt reversal.

Signs are a wide, fixed split second heart sound, a soft systolic pulmonary flow murmur, a tricuspid mid-diastolic flow murmur in large shunts and a right ventricular heave.

• ECG: right axis deviation, incomplete right bundle branch block (rSR' in V1), and left axis deviation suggests a primum or sinus venosus lesion instead. Atrial fibrillation or flutter in older adults.
• Chest radiograph: cardiomegaly, enlarged pulmonary arteries and increased vascular markings.
• Transthoracic echocardiography: defect size, rims (aortic, superior, inferior, posterior; at least 5 mm of rim is needed for device closure except at the aortic rim), right heart dilatation, septal flattening, tricuspid regurgitation and pulmonary pressure. Transoesophageal or intracardiac echocardiography for device sizing. Three dimensional echocardiography defines shape.
• Cardiac MRI for shunt quantification and right ventricular volumes where echocardiography is limited; CT or MRI for anomalous pulmonary veins.
• Catheterisation when pulmonary hypertension is suspected, or in older adults before closure. Record pulmonary vascular resistance and, in patients over 40 or with left ventricular stiffness, pulmonary capillary wedge pressure during temporary balloon occlusion of the defect (a rise to over 15 mmHg favours a fenestrated device or medical therapy first).

Differential diagnosis.

• Sinus venosus defect and partial anomalous pulmonary venous return.
• Ostium primum defect and unroofed coronary sinus.
• Patent foramen ovale with right-to-left shunt.
• Pulmonary hypertension of other cause, and right ventricular cardiomyopathy.

Indications for closure. Right ventricular volume overload with a pulmonary to systemic flow ratio of 1.5 or more, regardless of symptoms, when pulmonary vascular resistance is under 3 Wood units. Resistance of 3 to 5 Wood units: closure only when a net left-to-right shunt persists and pulmonary pressure falls with treatment, by specialist decision. Do not close when resistance is over 5 Wood units without a net left-to-right shunt, or with Eisenmenger syndrome. Paradoxical embolism may be an indication for closure even with a small shunt.

Closure.

• Percutaneous device closure is the first choice when anatomy is suitable: stretched defect diameter of up to about 38 mm and adequate rims, usually weight over 15 kg. Before device placement, give cefazolin 1 g intravenously (children 25 mg/kg, maximum 1 g) and heparin 100 units/kg intravenous bolus (activated clotting time over 200 seconds). Nickel allergy needs assessment because devices contain nickel.
• Surgical closure (direct suture or patch) is chosen for large defects with deficient rims, associated anomalies needing repair, or failed device attempts. Operative mortality is under 1%.
• Complications of device closure are device erosion (about 0.1%, presenting with chest pain or tamponade, usually in the first months and needs emergency echocardiography), atrial arrhythmia, atrioventricular block, thrombus, embolisation and residual shunt.
• Closure at any age improves symptoms and exercise capacity. Closure before age 25 largely prevents later atrial arrhythmia, and in older adults atrial fibrillation often persists after closure.

Medication.

• After device closure: aspirin 75 to 100 mg orally once daily for 6 months (children 3 to 5 mg/kg daily); clopidogrel 75 mg once daily for 1 to 6 months is added by some centres. After surgical closure: aspirin 75 to 100 mg daily for 3 to 6 months.
• Right heart failure or after closure with stiff left ventricle: furosemide 20 to 40 mg orally once or twice daily (titrate to weight; electrolytes and creatinine within 1 week of changes).
• Atrial fibrillation or flutter: anticoagulate at a low threshold given paradoxical embolism risk. Apixaban 5 mg twice daily (2.5 mg twice daily if two of age 80 or over, weight 60 kg or under, creatinine 133 micromol/L or over) or rivaroxaban 20 mg once daily with the evening meal (15 mg when creatinine clearance is 15 to 49 mL/min). Rate control with metoprolol succinate 25 to 200 mg daily (maximum 400 mg daily). Catheter ablation is effective for flutter.
• Endocarditis prophylaxis is not needed for the unrepaired defect. For 6 months after device or patch closure (lifelong when previous endocarditis or a residual shunt next to prosthetic material): amoxicillin 2 g orally (children 50 mg/kg, maximum 2 g) 30 to 60 minutes before dental procedures involving gingival manipulation; with penicillin allergy azithromycin 500 mg (children 15 mg/kg, maximum 500 mg) or doxycycline 100 mg (adults).
• Pulmonary arterial hypertension: ambrisentan 5 mg once daily, increased to 10 mg after 4 weeks (maximum 10 mg), with tadalafil 40 mg once daily (start 20 mg in mild to moderate renal or hepatic impairment; avoid when creatinine clearance is under 30 mL/min), then repeat catheterisation before any decision to close. Both are avoided or contraindicated in pregnancy, tadalafil with nitrates and ambrisentan in moderate to severe hepatic impairment; haemoglobin and liver tests at baseline and when indicated.

Monitoring. Echocardiography at 24 hours, 1 month, 6 months and 12 months after device closure, then every 1 to 3 years. ECG and ambulatory ECG yearly. Clinical review for arrhythmia and symptoms of residual shunt.

Referral and escalation. Refer every confirmed defect with right heart enlargement, stroke or embolic event, or suspected pulmonary hypertension to a congenital heart service. Urgent referral for chest pain or collapse after device closure, new arrhythmia or right heart failure.

Special groups.

• Pregnancy: well tolerated in the absence of pulmonary hypertension. Paradoxical embolism risk requires avoidance of air in lines and enoxaparin 40 mg subcutaneously once daily during immobility and after delivery (20 mg when creatinine clearance is under 30 mL/min). Pulmonary hypertension makes pregnancy unsafe.
• Children: closure usually at age 3 to 5 years when the right ventricle is dilated; earlier for symptoms or failure to thrive.
• Older adults: test for left ventricular stiffness before closure, and use a fenestrated device or treat first when wedge pressure rises.
• Region specific: late presentation with pulmonary hypertension or atrial fibrillation is common where screening is limited, so confirm haemodynamics before closure.