Atrial Septal Defect
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Direct answer
An atrial septal defect (ASD) is an opening between the atria that allows left-to-right shunting, and the secundum type is the commonest form. It typically causes a fixed, wide split second heart sound and an ejection systolic murmur at the pulmonary area, along with right ventricular enlargement. Many children are asymptomatic, and significant defects are closed by device or surgery when anatomy allows.
What you must remember
- Secundum ASD: the commonest type, lying in the fossa ovalis region of the septum, and suitable for device closure when rims are adequate.
- Primum ASD: a low defect with a cleft mitral valve and left axis deviation, strongly linked to atrioventricular septal defect and Down syndrome.
- Sinus venosus ASD: near the venous entrances, often with partial anomalous drainage of the right upper pulmonary vein.
- Fixed split S2: a wide split that does not change with respiration, reflecting constant right ventricular filling.
- ECG: right axis deviation and an incomplete right bundle branch block pattern (rSR' in V1) in secundum defects.
- Murmur: an ejection systolic murmur at the upper left sternal border from increased flow across the pulmonary valve.
- Closure criteria: closure is considered for a significant shunt, usually with right ventricular enlargement, and depends on defect size and surrounding rims.
A pathway from detection to closure
Most ASDs are found incidentally, so the work-up starts with a murmur or an abnormal ECG. The aim is to confirm the type and decide whether the shunt is large enough to need closure.
- Clinical clue: A fixed split second heart sound and a pulmonary ejection murmur raise suspicion, often in a well-looking child.
- ECG: Right axis deviation with rSR' in V1 favours secundum; left axis deviation points towards primum.
- Echocardiography: Echo confirms the type, measures the defect, checks the rims and looks for partial anomalous venous drainage.
- Shunt assessment: Right ventricular and atrial enlargement on echo indicate a significant shunt.
- Choose the route: Suitable secundum defects are closed by device, while primum and sinus venosus defects usually need surgical repair.
- After closure: Follow-up checks rhythm disturbances and residual shunt, and most children have a good outlook.
Where the exam tries to catch you
Most ASD questions test anatomy and the sound, so mix them up on purpose when you revise.
- Trap: a primum ASD is closed with a percutaneous device. Correct: primum defects usually lack adequate rims and often have a cleft mitral valve, so surgical repair is standard.
- Trap: a fixed split S2 is typical of a ventricular septal defect. Correct: the fixed, wide split reflects constant right ventricular filling and is characteristic of atrial septal defect.
- Trap: the ASD murmur is pansystolic. Correct: it is an ejection systolic murmur at the pulmonary area, unlike the pansystolic murmur of a VSD.
Frequently asked questions
Why is the second heart sound fixed in atrial septal defect?
The second heart sound is fixed in atrial septal defect because right ventricular filling and ejection are increased and do not change with respiration. The split stays wide during both inspiration and expiration, which is the classic bedside clue.
Which ASD type is associated with Down syndrome?
The primum atrial septal defect, which forms part of an atrioventricular septal defect, is strongly associated with Down syndrome. It usually comes with a cleft mitral valve and left axis deviation on ECG.
Which atrial septal defects can be closed with a device?
Secundum defects with adequate surrounding rims and a size within the device limits can be closed percutaneously. Primum and sinus venosus defects usually need surgical repair instead.
What ECG finding suggests a secundum atrial septal defect?
Right axis deviation with an incomplete right bundle branch block pattern, such as rSR' in V1, suggests a secundum atrial septal defect. Left axis deviation points instead towards a primum defect.