Atrial Septal Defect in Adults
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Direct answer
Atrial septal defect is an interatrial communication causing left-to-right shunting and right ventricular volume overload, which can lead to pulmonary hypertension and atrial arrhythmias if untreated. Classic findings are a fixed wide split second heart sound, a pulmonary ejection murmur and an RSR' pattern in V1. Ostium secundum is the commonest type, and closure is indicated for a pulmonary to systemic flow ratio of 1.5 or more or after paradoxical embolism.
What you must remember
- Fixed split S2: a wide split that does not vary with respiration, because pulmonary valve closure is delayed by right ventricular volume overload.
- Murmurs: a pulmonary ejection systolic murmur from increased flow, and a mid-diastolic flow murmur at the lower left sternal edge from increased tricuspid flow.
- Secundum versus primum: secundum lies in the fossa ovalis with right or normal axis; primum lies low in the septum with a mitral cleft and left axis deviation.
- ECG: incomplete right bundle branch block with RSR' in V1 is typical, with right ventricular enlargement in larger shunts.
- Sinus venosus: the superior type is associated with anomalous drainage of right upper pulmonary veins and is often missed on transthoracic echo.
- Closure: transcatheter device for suitable secundum defects; surgery for primum, sinus venosus and unsuitable rims.
- Complications: atrial fibrillation and flutter, paradoxical embolism and pulmonary hypertension in later life.
- Contraindication: irreversible pulmonary vascular disease with shunt reversal, where closure is harmful.
A split second sound that never changes
A 42-year-old woman with palpitations and fatigue has a soft ejection murmur at the upper left sternal edge, a wide split of the second sound that does not vary with breathing, and an ECG showing incomplete right bundle branch block with an RSR' in V1 and a right axis.
- Read the split. A physiological split widens on inspiration and narrows on expiration, whereas paradoxical splitting narrows with inspiration and occurs with left bundle branch block or aortic stenosis.
- Name the defect. A fixed split implies an atrial communication, and that single finding leads to the diagnosis.
- Image the heart. Echo with colour Doppler and a bubble study confirms the defect, most often the secundum type in the fossa ovalis, and the right ventricle is measured for size and function.
- Use transoesophageal echo if needed. It helps assess sinus venosus defects or rim adequacy when transthoracic images are unclear.
- Quantify the shunt. A shunt ratio of 1.5 or more with right ventricular dilatation supports closure, usually by catheter device when the rims are adequate.
- Weigh the pulmonary pressures. Age and pulmonary pressure matter, because decades of left-to-right flow can make pulmonary vascular disease irreversible, so the aim is to close the defect before the pulmonary circulation is damaged.
Where the examiner tests the split
Three places the examiner probes are the split, the axis and the decision to close.
- Trap: treating every wide split as physiological. Correct: the fixed split of an atrial septal defect does not change with breathing, and that feature is the diagnostic clue.
- Trap: ignoring the ECG axis in a split S2. Correct: secundum defects typically show right or normal axis, whereas primum defects show left axis deviation, so a left axis hints at an atrioventricular canal defect rather than a simple secundum lesion.
- Trap: insisting on closure when pulmonary hypertension is irreversible. Correct: closure is contraindicated there because the right heart cannot tolerate the change.
- Trap: dismissing a septal defect found after a stroke or leg thrombosis as incidental. Correct: suspect paradoxical embolism, which is itself a reason to consider closure.
Frequently asked questions
What is a fixed split second heart sound?
A fixed split second heart sound is a wide split that does not change with respiration. It is the classic auscultatory finding of atrial septal defect, caused by delayed pulmonary valve closure.
How do ostium secundum and ostium primum defects differ on ECG?
Ostium secundum usually shows right or normal axis with incomplete right bundle branch block. Ostium primum shows left axis deviation because it is associated with an atrioventricular canal defect.
When is closure of an atrial septal defect indicated in adults?
Closure is indicated when the pulmonary to systemic flow ratio is 1.5 or more with right heart volume overload, or after paradoxical embolism. It is only appropriate when pulmonary vascular resistance is not irreversibly raised.
What is the risk of an untreated atrial septal defect in middle age?
Untreated atrial septal defects can lead to atrial fibrillation or flutter, paradoxical embolism and pulmonary hypertension. These risks rise with age, which is why timely closure matters.
Which investigation is used for sinus venosus defects?
Transoesophageal echocardiography or cardiac MRI is used for sinus venosus defects, because transthoracic echo often misses them. Anomalous pulmonary venous drainage is identified on these studies.