What are the types of atrial septal defect?

There are four true types, named by location. Ostium secundum sits in the center of the septum at the fossa ovalis and is the most common. Ostium primum sits low, just above the AV valves, and is the partial form of an atrioventricular septal (endocardial cushion) defect. Sinus venosus sits at the SVC or IVC entry and frequently comes with an anomalous right pulmonary vein. The rarest is the unroofed coronary sinus defect. A patent foramen ovale is a flap that never sealed, not a true ASD.

Why does an ASD cause a wide, fixed split S2?

The left-to-right shunt adds volume to the right atrium and right ventricle, so more blood crosses the pulmonary valve and its closure sound (P2) is delayed, which widens the split. Because the right ventricle carries that extra volume throughout the respiratory cycle, P2 timing does not change with breathing, so the split is fixed. The systolic murmur at the left upper sternal border comes from increased flow across the pulmonary valve, not from blood crossing the defect, which is rarely if ever audible.

Which ASD is associated with Down syndrome and left axis deviation?

The ostium primum ASD. It is the partial form of an atrioventricular septal (endocardial cushion) defect, caused by failure of the septum primum to fuse with the endocardial cushions. It usually comes with a cleft mitral valve, so an apical holosystolic murmur of mitral regurgitation joins the fixed split S2. The ECG shows a superior (left) axis and often first-degree AV block. Primum defects do not close on their own and are repaired surgically, electively at about 1 to 3 years.

What is a sinus venosus ASD?

A sinus venosus defect sits in the posterior atrial septum where the superior vena cava, or less often the inferior vena cava, enters the right atrium. It is frequently associated with partial anomalous pulmonary venous return of the right upper or lower pulmonary vein to the right atrium or vena cava, which adds to the right-heart volume load. Transesophageal echo or cardiac MRI helps define the veins. A sinus venosus defect never closes on its own and needs surgery rather than a catheter device.

What is the difference between a PFO and an ASD?

A patent foramen ovale is the normal fetal flap between the atria that never sealed after birth, and it is present in about 1 in 4 adults. Because it opens only when right atrial pressure briefly exceeds left, it causes no murmur, no fixed split and no right-heart enlargement. An ASD is missing septal tissue with a persistent left-to-right shunt. Both can allow paradoxical embolism. PFO closure is reserved for selected patients under 60 after an embolic stroke of undetermined cause.

Which atrial septal defects can be closed with a catheter device?

Only ostium secundum defects, and only when there are adequate rims of septal tissue and enough distance from structures such as the aortic root, pulmonary veins and tricuspid annulus; Merck puts device closure as possible in about 85 to 90% of defects. Ostium primum, sinus venosus and coronary sinus defects need surgery. Moderate-to-large defects that enlarge the right heart are usually closed between ages 2 and 6, while small defects can be watched. Closure is contraindicated once Eisenmenger physiology develops.

ASD Types: Secundum vs Primum vs Sinus Venosus vs PFO

An atrial septal defect (ASD) is an opening in the wall between the atria that shunts blood left to right, volume-loads the right atrium and right ventricle, and produces the classic wide, fixed split S2 with a pulmonary flow murmur at the left upper sternal border. ASD types are named by location: ostium secundum (fossa ovalis, the most common), ostium primum (low septum beside the AV valves; an endocardial cushion defect linked to Down syndrome and a cleft mitral valve), sinus venosus (at the SVC or IVC entry, often with an anomalous right pulmonary vein) and the rare unroofed coronary sinus defect. A patent foramen ovale (PFO), present in about 1 in 4 adults, is a flap that never sealed rather than missing tissue: it causes no murmur or volume load, but it can let a venous clot reach the brain (paradoxical embolism). Among true ASDs, only secundum defects are routinely closed with a catheter device; primum, sinus venosus and coronary sinus defects are repaired surgically.

ASD types: the comparison table

	Ostium secundum	Ostium primum	Sinus venosus	Coronary sinus (unroofed)	PFO (look-alike)
Location	Center of the septum (fossa ovalis)	Low, anteroinferior septum, just above the AV valves	Posterior septum at the SVC entry (superior type) or IVC entry (inferior type)	Wall between the coronary sinus and left atrium	Flap-covered channel at the foramen ovale
Embryology	Excessive resorption of septum primum or deficient septum secundum	Septum primum fails to fuse with the endocardial cushions	Abnormal fusion of the embryologic sinus venosus with the atrium	Unroofing of the coronary sinus	Septum primum flap never seals against septum secundum after birth
How common	Most common type	Less common; the partial form of an AV septal defect	Less common	Rarest	About 1 in 4 adults
Associations	Usually isolated; syndromic forms include Holt-Oram (TBX5)	Down syndrome (strongest with the complete AV canal form); cleft mitral valve with mitral regurgitation	Partial anomalous pulmonary venous return (right upper, or with the inferior type right lower, pulmonary vein)	Persistent left SVC	Cryptogenic stroke; higher risk with a large PFO or atrial septal aneurysm
Murmur and S2	Wide, fixed split S2; ejection murmur at the LUSB; diastolic rumble at the LLSB if the shunt is large	Same, plus an apical holosystolic murmur of mitral regurgitation	Same as secundum; the anomalous veins add right-sided volume	Same as secundum	None; S2 splits normally
ECG axis clue	Right axis deviation, RVH, rSR′ in V1 (incomplete RBBB)	Left (superior) axis deviation, often first-degree AV block	Negative P wave in lead III (abnormal P-wave axis)	Nonspecific	Normal
Natural history	Defects under 3 mm usually close; many 3-8 mm defects close by age 3	Does not close spontaneously	Does not close spontaneously	Does not close spontaneously	Persists for life in about a quarter of people
Closure	Catheter device when septal rims are adequate (85-90% of defects); otherwise surgery	Surgery, electively at 1-3 years	Surgery	Surgery	Only selected patients under 60 after an embolic stroke of undetermined cause (device plus antiplatelet therapy)

Map the septum in 10 seconds

Center = SeCundum (fossa ovalis, where the foramen ovale was).
Bottom, by the valves = PRImum ("PRoximal to the valves"): the partial form of an AV septal (AV canal) defect, so look for a cleft mitral valve.
Back door, at the venae cavae = Sinus venosus: think "veins", the caval entry plus an anomalous right pulmonary vein.

Embryology: septum primum, septum secundum and the cushions

The septum primum grows toward the endocardial cushions; the shrinking gap beneath it is the ostium (foramen) primum.
Before that gap closes, perforations in the upper septum primum coalesce into a second opening, the ostium secundum.
The septum secundum then forms; the opening it leaves is the foramen ovale, covered by the septum primum, which acts as a flap valve.
After birth, left atrial pressure rises above right and holds the flap shut. If it never seals, the result is a PFO.

So a primum defect is a failure at step 1, a secundum defect at steps 2-3 and a PFO at step 4.

Physiology: why the S2 split is wide and fixed

Flow across a moderate or large ASD depends on the relative compliance of the two ventricles; the more compliant right ventricle fills preferentially, so the shunt runs left to right.
The extra volume passes through the right atrium, right ventricle and pulmonary valve, delaying P2: a wide split. Because right ventricular volume stays high throughout the respiratory cycle, P2 does not move with breathing: a fixed split.
The systolic ejection murmur at the LUSB is a flow murmur across the pulmonary valve; flow through the ASD itself is rarely, if ever, audible. A large shunt adds a low-pitched diastolic rumble at the lower sternal border from increased tricuspid flow.
Infants may lack these findings even with a large defect, and intracardiac pressures usually stay normal through childhood.
Know the other splits: physiologic (inspiration only), wide but still variable (RBBB, pulmonic stenosis), fixed (ASD), paradoxical (LBBB, aortic stenosis). More in the heart murmurs table.

ECG and imaging clues

Secundum (or any big shunt): right axis deviation, RVH and an rSR′ in V1 (right ventricular conduction delay). See RBBB vs LBBB.
Primum: a superior (left) axis, often with first-degree AV block. ASD plus left axis deviation = think AV canal.
Sinus venosus: a negative P wave in lead III is a clue.
Rising pulmonary pressure: the rSR′ disappears and V1 shows a tall R wave with a deeply inverted T wave.
Chest x-ray: right atrial and ventricular enlargement, a prominent main pulmonary artery and increased pulmonary vascular markings.
Echocardiography confirms the type, size and right-heart volume load. Transesophageal echo shows the anomalous veins of sinus venosus defects, and cardiac MRI helps with sinus venosus and coronary sinus defects. A PFO is shown with a bubble study: injected agitated saline crosses from right to left.

Natural history and complications

Childhood: most small or moderate ASDs cause no symptoms; a large shunt can slow weight gain. Many small central defects that close on their own are probably stretched PFOs.
Adulthood: exercise intolerance, exertional dyspnea, fatigue and palpitations from atrial arrhythmias (SVT, atrial flutter or fibrillation). Large unrepaired defects can cause pulmonary hypertension and RVH by the 30s or 40s.
Eisenmenger syndrome: an ASD is a pre-tricuspid shunt, so reversal with cyanosis comes late, usually in mid-to-late adulthood (most often after 40), whereas an unrepaired large VSD, PDA or more complex lesion can cause irreversible pulmonary vascular disease within the first few years of life. Once that happens, closure is contraindicated. Practice it with the Eisenmenger question.
Paradoxical embolism: even a mostly left-to-right shunt reverses transiently, so a venous clot can cross to the systemic side and cause a stroke, often in association with arrhythmias.

PFO: the look-alike that matters for stroke

Most people with a PFO never have problems and need no treatment.
There is no persistent shunt: exam, ECG and right-heart size are normal. The flap opens only intermittently, when right atrial pressure briefly exceeds left.
PFO is a frequent cause of cryptogenic stroke in younger patients, mainly through paradoxical embolism; a large PFO or an atrial septal aneurysm raises the risk.
Closure: transcatheter PFO closure plus antiplatelet therapy is recommended for patients under 60 with an embolic stroke of undetermined cause despite an extensive evaluation.

Closure options: device, surgery or neither

Situation	Approach
Small secundum ASD, no right-heart enlargement	Observe; echocardiography about every 3-5 years
Moderate-to-large secundum ASD (right-heart volume overload, or Qp:Qs ≥1.5:1)	Close, typically at 2-6 years: catheter device if anatomy allows, otherwise surgery
Primum ASD (partial AV septal defect)	Surgery at 1-3 years (a complete AV septal defect is repaired by 2-4 months)
Sinus venosus or coronary sinus defect	Surgery; not suitable for a device
Adult secundum ASD with pulmonary arterial hypertension	2025 ACC/AHA/HRS/ISACHD/SCAI guideline: closure is now recommended for many patients who still have a significant left-to-right shunt and PVR ≤2 Wood units or >2 to <5 Wood units
Eisenmenger physiology	Do not close; pulmonary vasodilators (a PDE-5 inhibitor or an endothelin receptor antagonist first)

Repair in childhood carries a perioperative mortality close to zero and near-normal long-term survival. Endocarditis prophylaxis is needed only for the first 6 months after repair, or longer if a residual defect sits next to the patch.

How USMLE tests ASD types

"Wide, fixed split S2" = ASD until proven otherwise; the murmur comes from the pulmonary valve, not the hole.
Down syndrome + left axis deviation + apical MR murmur = ostium primum / AV canal defect. More trisomy 21 findings in the Down syndrome breakdown.
An ASD with an anomalous right pulmonary vein = sinus venosus defect; get TEE or MRI. Don't confuse it with total anomalous return (TAPVR), a cyanotic newborn lesion.
Young adult with a stroke and no other cause = paradoxical embolism through a PFO or ASD.
Carpal bone or thumb anomalies + ASD = Holt-Oram syndrome (TBX5, autosomal dominant).
Cyanosis and clubbing in an adult with a known ASD = Eisenmenger; closing the defect is now harmful.

Practice question

A 3-year-old girl with Down syndrome is evaluated for poor weight gain. She has a wide, fixed split S2, a grade 2/6 midsystolic murmur at the left upper sternal border and a blowing holosystolic murmur at the apex. ECG shows left axis deviation and a prolonged PR interval. Echocardiography shows a left-to-right atrial shunt through a defect in the lowest part of the atrial septum, directly above the AV valves, a cleft mitral valve and an intact ventricular septum. Which developmental abnormality most likely caused this defect?

A. Excessive resorption of the septum primum B. Failure of the septum primum to fuse with the endocardial cushions C. Abnormal fusion of the sinus venosus with the right atrium D. Unroofing of the wall between the coronary sinus and left atrium E. Failure of the septum primum flap to seal against the septum secundum after birth

Answer: B. This is an ostium primum ASD, the partial form of an AV septal (endocardial cushion) defect: low position above the AV valves, a cleft mitral valve causing the apical murmur, a superior axis with first-degree AV block, and Down syndrome.

A: produces a secundum ASD in the fossa ovalis, with right axis deviation and an rSR′ in V1 but no cleft valve.
C: produces a sinus venosus defect at the SVC or IVC entry, typically with an anomalous right pulmonary vein.
D: produces a coronary sinus defect, the rarest type, often with a persistent left SVC.
E: describes a PFO, which causes no fixed split or murmur; its clinical relevance is paradoxical embolism.

Key takeaways

ASD = left-to-right atrial shunt → right-heart volume overload → wide, fixed split S2 and a pulmonary flow murmur at the LUSB.
Secundum (fossa ovalis): most common; right axis deviation, rSR′ in V1; device closure when the rims allow.
Primum (above the AV valves): endocardial cushion defect; Down syndrome, cleft mitral valve, left axis deviation; surgery.
Sinus venosus (caval entry): anomalous right pulmonary veins; surgery. Coronary sinus (unroofed): rare, persistent left SVC; surgery.
PFO: a flap in about 1 in 4 adults; normal exam; paradoxical embolism; closure only for selected patients under 60 after an embolic stroke.
Large unrepaired ASDs lead to atrial arrhythmias, pulmonary hypertension and, late, Eisenmenger physiology, which makes closure contraindicated.