The interatrial septum is a thin partition separating the right and left atria. Its central region, the fossa ovalis, is the thinnest part and represents the area where the septum primum overlaps the foramen ovale of the septum secundum. Surrounding this is a muscular rim (the limbus of the fossa ovalis / annulus ovalis).

Important adjacent structures:

• Superiorly: SVC orifice
• Inferiorly: IVC orifice, coronary sinus ostium
• Anteriorly: Atrioventricular valves (tricuspid and mitral)
• Posteriorly: Pulmonary veins entering the left atrium

This is crucial for understanding ASD types. The notes and lecture slides describe this in detail ^[1][2]:

Normal atrial septum development ^[1][2]:

• (A) Septum primum grows caudally towards the endocardial cushion
• (B) Septum primum fuses with endocardial cushion, closing the ostium primum
• (C) Formation of ostium secundum within septum primum + Formation of septum secundum on the right atrial side of septum primum
• (D) Septum secundum covers the ostium secundum but does not completely divide the atria
• (E) Persistence of the orifice of septum secundum is referred to as the foramen ovale

Let me walk through this step by step:

Why does the foramen ovale exist in fetal life?

• In the fetus, the lungs are fluid-filled and non-functional → PVR is very high → very little blood flows through the pulmonary circuit
• Oxygenated blood from the placenta enters the RA via the IVC and umbilical vein/ductus venosus
• The Eustachian valve directs this oxygenated IVC blood preferentially across the foramen ovale into the LA → LV → systemic circulation (including the brain and coronary arteries)
• So the foramen ovale is a physiological R-to-L shunt essential for fetal survival

Why does it close at birth?

• First breath → lung expansion → dramatic fall in PVR → massive increase in pulmonary blood flow → increased pulmonary venous return to LA → LA pressure rises above RA pressure
• This pressure difference pushes the flap of septum primum against the rigid septum secundum, functionally closing the foramen ovale
• Over weeks to months, fibrosis seals the two septa together

• Secundum ASD: The only type amenable to percutaneous (transcatheter) device closure — because it is surrounded by adequate rims of septal tissue that the device can grip onto
• Primum ASD: Requires surgical repair and often concurrent repair of the cleft mitral valve; associated with left axis deviation on ECG (because the AV node is displaced posteroinferiorly by the endocardial cushion defect)
• Sinus venosus ASD: Requires surgical repair with baffling of anomalous pulmonary veins back to LA; not suitable for device closure

The direction and magnitude of shunting across an ASD depend on:

1. Size of the defect — a larger defect allows more flow
2. Relative compliance of the ventricles — this is the key determinant in ASD (unlike VSD where the pressure gradient is the main driver)
3. Relative resistance of the pulmonary and systemic vascular beds

Shunting occurs in ASD since RV compliance is greater than LV compliance ^[2]. The RV is a thin-walled, compliant chamber designed for low-pressure work. The LV is thick-walled and less compliant. Therefore, blood preferentially flows from LA → RA → RV (path of least resistance) rather than LA → LV.

This is fundamentally different from VSD, where shunting is driven by the pressure gradient between the ventricles.

Chronic L-to-R shunt throughout the cardiac cycle → RV volume overload ^[1][2]

Why RV volume overload and NOT LV overload?

• In ASD, the extra blood crosses at the atrial level and enters the RV → pulmonary arteries → lungs → pulmonary veins → LA → and then a proportion re-enters the RA through the ASD instead of crossing the mitral valve to the LV
• So the RV handles the excess volume load (both normal systemic venous return PLUS the shunted blood)
• This is different from VSD and PDA where the LV is volume-overloaded because excess blood returns from the lungs to the LA and then the LV

Large ASD = RV overload; Large VSD and PDA = LV overload; AVSD = both ventricles overloaded ^[2]

• Qp:Qs = ratio of pulmonary blood flow to systemic blood flow
• Normal = 1:1
• In ASD with significant L-to-R shunt: Qp:Qs ≥ 1.5:1 is haemodynamically significant
• Qp:Qs > 2:1 generally warrants closure
• Can be estimated by echocardiography (comparing stroke volumes across the pulmonary and aortic valves) or measured at cardiac catheterisation using the Fick principle

Usually well-tolerated until 20–40 years of age (presents with HF or atrial arrhythmias) ^[1][2]

Why is ASD so well-tolerated for decades?

1. The RV is naturally very compliant — it can accommodate a large volume overload for years without significant haemodynamic compromise
2. The pulmonary vascular bed has enormous reserve capacity
3. Pulmonary pressures remain low for a long time because the excess flow is at low pressure (unlike VSD where the high-pressure LV transmits systemic pressure to the pulmonary bed)

Timeline of natural history:

• Infancy–childhood: Usually asymptomatic; detected incidentally
• Adolescence–young adulthood: May develop exercise intolerance, fatigue
• 20–40 years: Symptoms of RV failure (peripheral oedema, hepatomegaly), atrial arrhythmias (AF, atrial flutter) due to chronic atrial stretching
• If untreated: Very rarely (~10%) progresses to Eisenmenger syndrome in elderly ^[1][2] — this is much less common and much later than in VSD/PDA

Spontaneous closure: > 80% if < 8mm ^[2]. Small secundum ASDs (< 5–6 mm) detected in infancy have a high spontaneous closure rate and can be observed.

• Chronic excessive pulmonary blood flow → progressive pulmonary vascular remodelling (medial hypertrophy, intimal fibrosis, plexiform lesions)
• PVR rises progressively → eventually exceeds SVR
• Shunt reverses to R-to-L → systemic cyanosis → Eisenmenger physiology
• This is rare and late in isolated ASD (unlike large VSD where it can occur in infancy/early childhood)
• Once Eisenmenger develops, the defect is inoperable — closure would remove the "pop-off valve" for the RV and cause acute RV failure

These are the closest mimics because they share the pathophysiology of pulmonary overcirculation.

Acyanotic CHD with L-to-R shunt includes ASD, VSD, AVSD, and PDA ^[1][2]

This is probably the most important differential in paediatric practice because innocent murmurs are extremely common (up to 50–80% of children have audible innocent murmurs at some point).

Key teaching point: If a child has a soft ESM at the pulmonary area but normal S2 splitting (varies with respiration), no parasternal heave, and a normal ECG → this is almost certainly an innocent murmur, NOT an ASD. The wide fixed splitting of S2 is the critical discriminator.

Partial anomalous drainage of right pulmonary vein into RA is associated with sinus venosus ASD ^[1][5]. However, PAPVR can also occur in isolation (without any ASD). Isolated PAPVR mimics ASD very closely:

• Same haemodynamics: extra oxygenated blood enters RA → RV volume overload
• Same signs: ESM at LUSB, wide split S2 (may not be truly "fixed"), parasternal heave
• Same ECG: RAD, incomplete RBBB
• Differentiation: Echocardiography may not identify all anomalous veins (especially if draining to SVC). CT angiography or MRI is often needed.

Since incomplete RBBB (rsR' in V1) due to RV dilation ^[5] is a classic ECG finding in ASD, other causes of incomplete RBBB should be considered:

• Normal variant (very common in children — up to 3–5% of healthy children have rsR' in V1)
• RV volume overload from any cause (TR, PAPVR, post-surgical)
• RV conduction delay (post-surgical — e.g., after TOF repair with right ventriculotomy)

The key is to correlate with clinical and echocardiographic findings — an isolated rsR' in V1 in an asymptomatic child with normal S2 splitting and no cardiomegaly is almost certainly benign.

If a child presents with infantile heart failure and an ASD is found, consider whether the ASD alone explains the severity, or whether there are additional lesions:

ASD alone is acyanotic. If a child has an ASD AND cyanosis, consider:

CXR is a readily available, low-radiation investigation that provides important indirect evidence of a haemodynamically significant ASD. In paediatrics, always use a PA erect film when possible (AP films in infants who cannot stand will overestimate cardiac size).

CXR: cardiomegaly, RA/RV dilation, enlarged pA, pulm plethora ^[5]

Small ASD: CXR may be completely normal ^[5]. This is important — a normal CXR does NOT exclude ASD.

Cardiac catheterisation is NOT routine for ASD diagnosis in children. Echocardiography provides sufficient information in the vast majority of cases. Catheterisation is reserved for specific indications:

Key catheterisation findings in ASD:

Cardiac MRI is an excellent supplementary investigation, particularly useful in older children and adolescents. Its main advantages over echo are:

Limitation: Requires prolonged scanning time (30–60 min), often needing sedation or general anaesthesia in young children (typically < 7–8 years).

CT angiography is less commonly used than cardiac MRI for ASD but has specific roles:

• Small ASD with no RV volume overload ^[2][5][11]
• Defect < 5–6 mm in infancy (very high spontaneous closure rate)
• Qp:Qs < 1.5:1
• No symptoms

Why can we safely observe? Because ASD is a low-pressure shunt with very slow progression. The RV tolerates volume overload well for years, and small defects have an excellent chance of closing spontaneously — unlike large VSDs that can cause irreversible pulmonary vascular disease in infancy.

In conditions like transposition of great arteries (TGA), HLHS, or pulmonary atresia with intact ventricular septum (PAIVS), the ASD is life-saving — it allows intercirculatory mixing.

Emergency IV PGE1 to maintain ductal patency. ± Transcatheter balloon atrial septostomy if ASD restrictive or intact ^[1]

• Balloon atrial septostomy (Rashkind procedure): An inflated balloon catheter is pulled across the foramen ovale/ASD to tear the interatrial septum, creating a large non-restrictive communication → improves mixing → buys time for definitive surgery
• In these situations, you MUST NOT close the ASD — it is an obligatory shunt

• Down syndrome children with ASD (or more commonly AVSD) may develop pulmonary vascular disease earlier and more aggressively than other children, partly due to associated upper airway obstruction causing chronic hypoxia ^[3]
• Earlier surgical intervention is recommended (by 3–6 months for complete AVSD)

• Isolated unrepaired ASD is generally well-tolerated in pregnancy, but there is increased risk of arrhythmias and paradoxical embolism
• Multidisciplinary management with obstetrician, paediatric cardiologist, and adult cardiologist ^[12] is essential
• Ideally, significant ASDs should be closed BEFORE pregnancy

Pathophysiology: Chronic L-to-R shunt → persistent RV volume overload → the RV dilates progressively → initially compensated (the compliant RV tolerates this for years) → eventually RV systolic function declines → RV failure develops → backward failure causes systemic venous congestion (elevated JVP, hepatomegaly, peripheral oedema, ascites); forward failure causes reduced cardiac output (fatigue, exercise intolerance).

Timeline: This is predominantly an adult complication — it rarely occurs in childhood with isolated ASD. Most patients develop symptomatic RV failure between 20 and 40 years of age if the ASD remains unrepaired ^[1][5].

Clinical features in older children/adolescents:

• Progressive exercise intolerance and fatigue (reduced CO on exertion)
• Peripheral oedema (gravitational, pitting)
• Hepatomegaly (RV back-pressure transmitted to hepatic veins → hepatic congestion)
• Elevated JVP
• Functional tricuspid regurgitation (TR) — from progressive RV and tricuspid annular dilation. This creates a PSM at the LLSB that was not present earlier, and further worsens RV volume overload (vicious cycle).

Why does RV failure take so long in ASD (vs VSD/PDA)? Because the RV is inherently compliant and designed for low-pressure work. The extra volume from the ASD shunt is at low pressure (atrial-level shunt, driven by compliance differences, not pressure gradients). The pulmonary vascular bed has enormous reserve capacity. This is fundamentally different from VSD/PDA where the high-pressure LV transmits systemic pressure directly to the pulmonary vasculature, causing rapid pulmonary vascular damage.

Pathophysiology: Chronic volume overload → RA and LA stretching and dilation → atrial wall fibrosis and electrical remodelling → abnormal conduction pathways and re-entrant circuits → atrial fibrillation (AF) and atrial flutter (AFlu).

Why does atrial stretching cause arrhythmias? The atrial myocytes are mechanically stretched beyond their optimal length → this activates mechanosensitive ion channels → alters the atrial refractory period → creates heterogeneous conduction velocity across the atria → sets up the substrate for re-entrant tachycardias. Additionally, fibrosis (scar tissue) from chronic stretch interrupts normal cell-to-cell electrical coupling, creating areas of slow conduction that further promote re-entry.

Timeline:

• Atrial arrhythmias are uncommon before age 20 in isolated ASD
• Incidence increases sharply after age 40 if unrepaired
• Importantly, even after late ASD closure (in adulthood), the risk of atrial arrhythmias remains elevated because structural atrial remodelling has already occurred — this is why we aim to close ASDs in childhood before remodelling becomes irreversible

Clinical relevance in adolescents:

• Palpitations, dizziness, reduced exercise tolerance
• AF with rapid ventricular response can precipitate acute HF in a marginally compensated RV
• AF increases thromboembolic risk (see paradoxical embolism below)

Eisenmenger syndrome: triad of (1) congenital L-to-R shunt (2) pulmonary arterial disease (3) cyanosis ^[5]

Incidence: up to 80% in large VSD and PDA in infancy/early childhood, < 10% in large ASD ^[5]

Pathophysiology: Chronic excessive pulmonary blood flow (Qp:Qs > 2:1 over many years) → progressive pulmonary arteriolar damage:

1. Medial hypertrophy of pulmonary arterioles (smooth muscle thickening → vasoconstriction)
2. Intimal fibrosis (endothelial proliferation → luminal narrowing)
3. Plexiform lesions (disorganised angiogenic proliferation — irreversible)
4. Necrotising arteritis (late, irreversible)

→ PVR rises progressively → eventually PVR ≥ SVR → shunt reverses to R-to-L → deoxygenated venous blood enters the systemic circulation → cyanosis = Eisenmenger syndrome.

Why is Eisenmenger rare and late in ASD?

• The shunt is low-pressure (atrial level, driven by compliance, not pressure)
• Pulmonary arteries receive high flow but at LOW pressure → slower vascular remodelling
• Contrast with large VSD: LV transmits systemic pressure directly to PA → rapid pulmonary vascular disease → Eisenmenger can occur in infancy/early childhood

Once Eisenmenger develops:

Complications of Eisenmenger ^[1][5]:

Prognosis: 30–40% 10-year mortality with mean age of death at 37 years if transplant not done ^[5]

Pathophysiology: A venous thrombus (e.g., from a DVT in the legs) travels through the systemic venous system to the RA → instead of being filtered by the pulmonary capillary bed, it crosses the ASD into the LA → enters the systemic arterial circulation → embolises to the brain (stroke), coronary arteries (MI), mesenteric arteries (bowel ischaemia), or limb arteries.

Why "paradoxical"? Normally, venous thrombi cause pulmonary embolism (PE). For a venous thrombus to cause a systemic arterial embolism, it must paradoxically cross from the venous to arterial side — this requires an intracardiac communication (ASD, PFO, or VSD with R-to-L shunting).

When does this occur?

• More likely with transient R-to-L shunting (e.g., Valsalva manoeuvre, coughing, straining) even in predominantly L-to-R shunts
• More commonly discussed with PFO in adults, but can occur with any interatrial communication
• Rare in children (because DVT is rare in childhood), but an important consideration in adolescents and young adults

General principle for isolated ASD:

Antibiotic prophylaxis NOT required in general for IE before dental or invasive procedures. Prophylaxis is generally NOT required for isolated secundum ASD ^[2]

Unlike VSD and PDA, isolated secundum ASD carries a very low risk of infective endocarditis. This is because IE requires a high-velocity turbulent jet impacting on an endothelial surface to create a nidus for bacterial adhesion — the low-velocity, low-pressure flow across an ASD rarely generates this. The ESM at the LUSB in ASD is a flow murmur across the pulmonary valve, not a turbulent jet across the defect itself.

Exceptions where IE risk is increased:

• Primum ASD with cleft mitral valve and MR — the MR jet creates turbulence and endothelial damage → IE risk at the mitral valve
• Associated mitral valve prolapse (MVP) — MVP is associated with secundum ASD ^[13]; MVP with MR carries a small IE risk
• Post-device closure (first 6 months) — prosthetic material before endothelialisation

MVP is associated with secundum ASD, Turner's syndrome, PDA, Wolff-Parkinson-White syndrome ^[13]

Pathophysiology: Chronic pulmonary overcirculation → pulmonary vascular engorgement → bronchiolar wall oedema and compression of small airways → impaired mucociliary clearance → increased susceptibility to lower respiratory tract infections (pneumonia, bronchiolitis).

Additionally, there may be airway compression from dilated pulmonary arteries — particularly the left main bronchus (compressed between the dilated left PA and the descending aorta).

Clinical relevance in paediatrics:

• Recurrent pneumonia or wheeze in a child with an unrepaired significant ASD should raise concern about the haemodynamic significance of the shunt
• This is more commonly a problem with large VSDs and AVSDs but can occur with very large ASDs

Pathophysiology:

• Increased metabolic demand from excessive pulmonary blood flow and increased cardiac work
• Difficulty feeding in infants (tachypnoea interferes with suck-swallow coordination)
• Recurrent infections add to catabolic stress

Paediatric significance:

• Rare with isolated ASD (more common in VSD, AVSD, PDA)
• If present, suggests a very large defect or associated lesions
• Plotting weight and height on growth charts is essential — weight drops before height and head circumference; increased height-to-weight ratio ^[1]