To understand AVSD, you must understand what the endocardial cushions do in normal development:

1. Week 4–5: The primitive heart tube loops and begins to form four chambers. The atrioventricular canal is initially a single channel connecting the common atrium to the common ventricle.
2. Endocardial cushion formation: Mesenchymal swellings (superior and inferior endocardial cushions) develop in the AV canal. These are derived from cardiac jelly and neural crest–derived mesenchyme that undergoes epithelial-to-mesenchymal transition (EMT).
3. Fusion of cushions (Weeks 5–8): The superior and inferior cushions fuse in the midline, dividing the single AV canal into right and left AV orifices (future tricuspid and mitral valve orifices).
4. Contributions of fused cushions:
   • Lower (primum) atrial septum: The septum primum grows downward to meet the fused cushions, closing the ostium primum
   • Inlet ventricular septum: The superior portion of the muscular ventricular septum grows upward to meet the fused cushions
   • Septal leaflets of AV valves: The cushion tissue differentiates into the septal leaflet of the tricuspid valve and the anterior (septal-facing) leaflet of the mitral valve

Failure of endocardial cushion fusion → the structures that the cushions normally form are deficient:

• Primum ASD: The septum primum cannot anchor to the fused cushions (because they haven't fused), leaving an opening in the lowest part of the atrial septum (ostium primum defect)
• Inlet VSD: The ventricular septum cannot grow up to meet fused cushions, leaving a defect in the inlet (posterior-superior) portion of the ventricular septum
• AV valve abnormalities: Instead of two separate AV valves, there is a single common AV valve (in complete forms) or two AV valves with a cleft in the anterior mitral leaflet (in partial forms)

A distinctive anatomical feature of AVSD is the "goose-neck" deformity of the left ventricular outflow tract (LVOT) on angiography/echo:

• Normally, the aortic valve sits wedged between the mitral and tricuspid valves
• In AVSD, the AV valves are displaced inferiorly and the LVOT is elongated and narrowed, giving a characteristic elongated appearance
• This is due to the abnormal relationship between the aortic valve and the common AV valve — there is loss of the normal aortic-mitral fibrous continuity

AVSD can be classified into partial and complete ^[1]:

^[1][2]

There is also an intermediate (transitional) AVSD form:

• Two separate AV valve orifices (like partial)
• BUT a restrictive inlet VSD is present (unlike partial)
• Behaves somewhere between partial and complete

Additionally, AVSD may be described as balanced vs unbalanced:

• Balanced: Both ventricles are of adequate size to support biventricular repair
• Unbalanced: One ventricle is hypoplastic (too small for biventricular repair) → may require univentricular palliation ^[1]

The haemodynamic consequences depend on (a) which components are present and (b) their size:

Haemodynamic consequences ^[1]:

1. RV volume overload due to L-to-R shunt (primum ASD) and TR

   • Blood shunts from LA → RA through the primum ASD (because LAP > RAP after birth once PVR drops)
   • The RV receives extra volume from this shunt
   • If the common AV valve or tricuspid component is regurgitant, more volume is dumped into the RA/RV

2. LV volume overload due to L-to-R shunt (VSD) and MR

   • In complete AVSD, blood shunts from LV → RV through the inlet VSD
   • Increased pulmonary venous return comes back to the LA/LV, causing LV volume overload
   • MR (through the cleft mitral valve or common AV valve) further volume-loads the LA and LV

3. Pulmonary hypertension (pHT), especially in the presence of a large VSD component and Down syndrome associated with upper airway obstruction (chronic hypoxia)

   • Large VSD → high-pressure, high-volume L-to-R shunt → direct transmission of LV systolic pressure to the pulmonary vasculature
   • Over time, pulmonary vascular remodelling (medial hypertrophy → intimal fibrosis → plexiform lesions) → irreversible pHT
   • Down syndrome accelerates pHT for two reasons:
     • (a) Upper airway obstruction (macroglossia, hypotonia, adenotonsillar hypertrophy) → chronic alveolar hypoxia → hypoxic pulmonary vasoconstriction
     • (b) Possible inherent pulmonary vascular abnormality in Trisomy 21

4. RV pressure overload due to pHT

   • As PVR rises, RV afterload increases → RV hypertrophy → eventually RV failure
   • If PVR exceeds SVR → reversal of shunt (R-to-L) → Eisenmenger syndrome (cyanosis)

Always look for features of Down syndrome (the most common association):

• Facial features: epicanthic folds, flat nasal bridge, protruding tongue (macroglossia), low-set ears, upslanting palpebral fissures
• Limbs: single palmar crease (simian crease), sandal gap toes, short 5th finger with clinodactyly
• Hypotonia
• Intellectual disability
• Brushfield spots (speckled iris)

Also consider features of heterotaxy syndromes (especially if AVSD is associated with other complex cardiac or abdominal anomalies):

• Asplenia or polysplenia
• Malrotation of the gut
• Bilateral right- or left-sidedness of lungs/bronchi

This is the typical presentation of complete AVSD with a large VSD component ^[1]. The PVR has dropped, unmasking the large L-to-R shunt. Other conditions that present with infantile heart failure around this age share the same pathophysiological mechanism — a significant L-to-R shunt that becomes haemodynamically apparent once PVR falls.

Partial AVSD is relatively asymptomatic, presenting with a murmur ^[1]. The haemodynamics resemble a primum ASD with MR. The differential here focuses on other causes of murmurs in well children.

40–50% of complete AVSDs are associated with Down syndrome ^[1]. Down syndrome has cardiac defects in ~45% of patients ^[5]. The differential of cardiac lesions in Down syndrome specifically is:

• Both present with HF at 1–2 months
• Both have PSM and signs of volume overload
• Key differentiator: AVSD has LAD on ECG and wide fixed split S2 (ASD component). VSD has normal or right axis. Echo shows the primum ASD and AV valve abnormality in AVSD.

• Both have wide fixed split S2
• Both cause RV volume overload
• Key differentiator: Secundum ASD has right axis deviation and RBBB. AVSD (primum) has left axis deviation and 1st degree AV block ^[1][2]. Also, AVSD has MR (from cleft MV) which secundum ASD does not.

• Both have a PSM at the apex and can cause LV volume overload
• Key differentiator: AVSD will also have the ASD component (wide fixed split S2), LAD on ECG, and biventricular enlargement. Isolated MR has a normal atrial septum on echo.

• Both can present with early HF and biventricular overload
• Key differentiator: Truncus has a single S2 (single semilunar valve), systolic ejection click, and potential cyanosis. Truncus is associated with DiGeorge syndrome (22q11.2 deletion) ^[5], while AVSD is associated with Down syndrome ^[1].

• AOS past paper highlight: displaced apex + signs of HF + murmur at apex and LLSB → could be cardiomyopathy with secondary MR from dilatation, especially in an older child ^[7]. VSD less likely to cause right heart failure at older ages ^[7].
• Key differentiator: Echo resolves this — cardiomyopathy shows globally impaired LV function without structural septal defects. AVSD shows septal defects and AV valve abnormalities with potentially preserved systolic function (until late).

AVSD can be detected on fetal echocardiography, typically triggered by:

• Abnormal 4-chamber view on the routine 18–20 week anomaly scan — the 4-chamber view is the most sensitive screening view for AVSD because the primum ASD, inlet VSD, and common AV valve are all visible in this single plane
• Known maternal risk factors: Maternal diabetes, family history of CHD, abnormal first-trimester screening suggesting Trisomy 21
• Confirmed or suspected fetal Trisomy 21 (e.g., after positive NIPT or abnormal nuchal translucency)

When fetal AVSD is detected:

1. Karyotyping (amniocentesis or CVS) should be offered, given the 40–50% association with Trisomy 21 ^[1]
2. Detailed fetal echocardiography to classify the type and assess for associated anomalies
3. Multidisciplinary counselling (fetal medicine, paediatric cardiology, genetics, neonatology)
4. Plan delivery at a tertiary centre with paediatric cardiac surgery capability

For neonates/infants presenting postnatally:

Step 1 — Clinical assessment:

• History: feeding difficulty, tachypnoea, diaphoresis, failure to thrive, recurrent chest infections
• Examination: dysmorphic features (Down syndrome?), signs of HF, auscultation findings
• SpO₂ screening: All neonates in Hong Kong undergo pulse oximetry screening (pre- and post-ductal) as part of newborn screening. AVSD is typically acyanotic (SpO₂ > 95%) unless Eisenmenger physiology has developed or there is an associated cyanotic lesion

Step 2 — ECG and CXR (first-line investigations, always done together)

Step 3 — Echocardiography (definitive diagnostic investigation)

Step 4 — Additional investigations as needed (for surgical planning or assessment of associated conditions)

ECG is the initial investigation for ALL potential cardiac conditions ^[12]. In AVSD, the ECG findings are highly characteristic and often provide the strongest clue before echocardiography.

How to determine axis on ECG — a quick refresher:

• Lead I positive, Lead II positive → Normal axis (0° to +90°) ^[9]
• Lead I positive, Lead II negative → Left axis deviation ^[9]
• Lead I negative, Lead II positive → Right axis deviation
• Lead I negative, Lead II negative → Extreme axis ("northwest" axis)

For AVSD: You expect Lead I positive, Lead aVF negative (axis more negative than −30°).

Exam pearl: The combination of LAD + 1st degree AVB + biventricular hypertrophy on ECG in an infant = AVSD until proven otherwise ^[1][2].

CXR findings in AVSD ^[1]:

• Cardiomegaly: Due to biventricular dilatation + biatrial enlargement
  • Paediatric note: Cardiomegaly is defined as CTR ≥ 0.5 in children/adults, but ≥ 0.6 in infants ^[1]. Also, the thymus in infants and young children can simulate cardiomegaly, creating diagnostic difficulty ^[1] — the "sail sign" of a normal thymus can widen the mediastinal silhouette and mimic true cardiomegaly
• RA/RV dilatation: Prominent right heart border
• LA/LV dilatation: Prominent left atrial appendage (3rd mogul sign on the left heart border, below the PA shadow) ^[1]; LV enlargement: apex extends laterally and points downward ^[1]
• Pulmonary plethora: Increased pulmonary vascular markings, extending to the periphery of the lung fields — this is the hallmark of L-to-R shunt physiology (increased pulmonary blood flow)
  • L-to-R shunt (e.g., VSD/AVSD): pulmonary plethora + cardiomegaly (volume overload) ^[1]
  • This is in contrast to pulmonary oligaemia (decreased markings) seen in conditions with reduced pulmonary blood flow (e.g., TOF, pulmonary atresia)
• Enlarged pulmonary artery segment: The main PA and proximal pulmonary arteries are prominent

CXR in Partial AVSD (less severe):

• May show only mild cardiomegaly with mild pulmonary plethora
• Can be near-normal if the shunt is small

CXR in Complete AVSD with pHT:

• Marked cardiomegaly
• Pruning of peripheral pulmonary vessels (dilated proximal PA with attenuated distal vessels) — suggests established pulmonary hypertension
• Pulmonary plethora may actually decrease as PVR rises (less L-to-R shunting)

Echo is diagnostic and evaluates size ^[1]. Echocardiography is the gold standard for diagnosing AVSD and is the single most important investigation. It provides all the anatomical and haemodynamic information needed for diagnosis and surgical planning.

Echocardiographic Views Used:

• Apical 4-chamber view: The most important view — shows the primum ASD, inlet VSD, and AV valve morphology in a single plane. This is also the view used for fetal screening (the 4-chamber view on the anomaly scan).
• Parasternal long axis (PSLA): Shows the "goose-neck" LVOT deformity, AV valve morphology, and LV size/function
• Parasternal short axis (PSSA): Shows the cleft in the anterior mitral leaflet (in partial AVSD) at the level of the AV valves. Also assesses papillary muscle anatomy.
• Subcostal view: Excellent for visualising the atrial and ventricular septa in infants (because the subcostal window is particularly good in infants due to their thin body habitus and relatively large liver providing an acoustic window)

Key Echo Findings — What to Look For and Report:

Transthoracic echocardiography (TTE) is almost always sufficient in infants and children because of their excellent acoustic windows (thin chest wall, little subcutaneous fat). Transesophageal echocardiography (TEE) is rarely needed preoperatively but may be used intraoperatively to assess the quality of repair (checking for residual ASD/VSD and residual AV valve regurgitation immediately after the surgeon comes off bypass).

• Pre-ductal (right hand) and post-ductal (foot) SpO₂ should be measured in all neonates
• In uncomplicated AVSD, SpO₂ should be > 95% (it is an acyanotic lesion with L-to-R shunting)
• If SpO₂ is low ( < 95% or > 3% difference between pre- and post-ductal):
  • Consider associated cyanotic lesion (e.g., AVSD + TOF in Down syndrome)
  • Consider Eisenmenger physiology (very late presentation — implies shunt reversal)
  • Consider pulmonary pathology superimposed (pneumonia on a plethoric lung)

Cardiac catheterisation is not routinely required for the diagnosis of AVSD in infants, as echocardiography provides all the necessary anatomical and haemodynamic information. However, it may be indicated in specific situations:

Key haemodynamic measurements on catheterisation:

• Qp:Qs ratio (oxygen saturation step-up method)
• PA pressure (systolic, diastolic, mean)
• PVR (in Wood units): PVR = (mean PAP − PCWP) / Qp
  • PVR < 6–8 Wood units·m² generally considered operable
  • PVR > 8–10 Wood units·m² with poor vasodilator response → likely inoperable (Eisenmenger)

• Not a first-line investigation for AVSD
• May be used in older children or adolescents with complex anatomy or for late assessment of ventricular function and AV valve regurgitation when echo windows are suboptimal
• Excellent for quantifying ventricular volumes, ejection fraction, and regurgitant fraction
• Can assess for subaortic stenosis post-repair

While laboratory tests do not diagnose AVSD per se, they are important in the overall assessment of the child:

This deserves special mention because prenatal diagnosis of AVSD is increasingly common and affects counselling and management:

• Routine anomaly scan (18–20 weeks): The 4-chamber view is part of the standard screening. AVSD is one of the easiest CHD lesions to detect prenatally because the 4-chamber view is directly disrupted (absence of the AV septum, common AV valve)
• Detection rate: AVSD has one of the highest prenatal detection rates among CHDs (~60–70% in experienced centres)
• If detected:
  • Offer karyotyping (amniocentesis/CVS) — 40–50% will have Trisomy 21 ^[1]
  • Detailed fetal echocardiography by paediatric cardiologist to classify type, assess balance, and look for associated anomalies
  • Genetic counselling and family-centred discussion about prognosis, expected postnatal course, and surgical options
  • Plan delivery at a centre with neonatal intensive care and paediatric cardiac surgery

1. Reduce pulmonary congestion (treat the "backward failure" — fluid in the lungs)
2. Optimise cardiac output (treat the "forward failure" — poor perfusion)
3. Support growth and nutrition (overcome the catabolic state of heart failure)
4. Prevent infection (minimise the risk of respiratory infections in a plethoric lung)

This is often underappreciated but is critically important:

• Infants with AVSD and HF have dramatically increased caloric requirements (up to 130–150 kcal/kg/day, vs the normal 100–120 kcal/kg/day) because of:
  • Increased work of breathing
  • Increased myocardial oxygen consumption
  • Chronic sympathetic activation
• Yet they cannot feed adequately because tachypnoea disrupts suck-swallow-breathe coordination
• Strategies:
  • High-calorie formula: Concentrate formula or add caloric supplements (e.g., MCT oil, glucose polymers) to increase caloric density without increasing volume
  • Nasogastric (NG) tube feeding: If the infant cannot take adequate volumes orally, continuous or bolus NG feeds ensure adequate caloric intake. This is a very common intervention on paediatric cardiology wards.
  • Fluid restriction: Typically 120–150 mL/kg/day (less than the standard ~150–180 mL/kg/day) to reduce volume overload — compensated by increasing caloric density
  • Growth monitoring: Plot weight, length, and head circumference on growth charts at every visit. In congestive HF, weight drops before height and HC ^[13]

O₂ should be used with caution in large L-to-R shunts: ↑PAO₂ → pulmonary vasodilation → ↓PVR → ↑shunting ^[13]. This is a critical paediatric-specific concept:

• Supplemental oxygen is a pulmonary vasodilator → it reduces PVR
• In a child with a large L-to-R shunt (AVSD, VSD, PDA), reducing PVR increases the shunt → worsens pulmonary overcirculation and heart failure
• Therefore, oxygen should not be given routinely to infants with acyanotic L-to-R shunts unless there is genuine hypoxaemia from an additional cause (e.g., superimposed pneumonia)
• Target SpO₂ > 92–94% — there is no benefit to achieving supranormal saturations

Exceptions: competent AV valves with small ASD/VSD → expectant ^[1].

• Age group: Infants with complete AVSD — typically onset at 1–2 months (or earlier if severe MR) ^[1]
• Mechanism: PVR drops postnatally → progressive L-to-R shunting → biventricular volume overload → pulmonary overcirculation → pulmonary oedema (backward failure) + poor systemic perfusion (forward failure)
• Pathophysiology from first principles: The heart is pumping blood in a futile cycle — a proportion of LV output goes through the VSD to the PA and back to the LA/LV via the pulmonary veins, never reaching the systemic circulation. The heart has to work harder to maintain systemic cardiac output. Eventually, compensatory mechanisms (tachycardia, sympathetic activation, ventricular dilatation via Frank-Starling) are exhausted → overt heart failure.
• Manifestations in infants: Tachypnoea, feeding difficulty, diaphoresis during feeds, failure to thrive, hepatomegaly (the paediatric equivalent of peripheral oedema — infants do not get ankle swelling because of their horizontal posture and proportionally lower venous hydrostatic pressure in the lower limbs) ^[1][13]

• Mechanism: Caloric expenditure dramatically exceeds intake:
  • ↑ Metabolic demand: increased work of breathing + increased myocardial oxygen consumption + chronic sympathetic activation
  • ↓ Caloric intake: tachypnoea disrupts the suck-swallow-breathe coordination, making feeding exhausting for the infant → reduced volume of each feed
• Pattern: In congestive HF, weight drops before height, and height drops before head circumference ^[13] — this pattern is seen because the body prioritises brain growth. Weight is the first parameter to be affected because it is the most sensitive to caloric balance.
• Clinical significance: FTT is one of the indications for surgical repair. Catch-up growth is expected after successful surgery.

This is the most feared and most important complication of untreated AVSD because it determines operability.

• Mechanism: Chronic exposure of the pulmonary vasculature to high flow (from the shunt) and high pressure (transmitted systemic pressure through the unrestrictive VSD) → progressive pulmonary vascular remodelling:

  • Stage 1 (reversible): Medial hypertrophy of pulmonary arterioles → increased PVR but still responsive to vasodilators → still operable
  • Stage 2 (partially reversible): Intimal fibrosis + reduction in vessel number
  • Stage 3 (irreversible): Plexiform lesions, necrotising arteritis, fibrinoid necrosis → fixed, irreversible pHT → inoperable

• Eisenmenger syndrome: Defined as the triad of (1) congenital L-to-R shunt, (2) pulmonary arterial vascular disease, and (3) cyanosis ^[15]. When PVR exceeds SVR, the shunt reverses direction → deoxygenated blood enters the systemic circulation → cyanosis.

Eisenmenger syndrome: refer to triad of (1) congenital L-to-R shunt (2) pulmonary arterial disease (3) cyanosis ^[15].

• Why AVSD is high-risk for Eisenmenger: Complete AVSD has a large, unrestrictive VSD that transmits full systemic pressure to the PA from birth. This is a higher-risk scenario for pHT than a secundum ASD (which is a low-pressure shunt). Eisenmenger develops in up to 80% of large unrepaired VSD and PDA in infancy/early childhood, < 10% in large ASD ^[15].
• Down syndrome accelerates pHT because of upper airway obstruction → chronic hypoxia → hypoxic pulmonary vasoconstriction, compounding the haemodynamic insult ^[1]. Irreversible pHT can develop as early as 6–12 months in Down syndrome children with complete AVSD — this is why surgery is usually performed by 3 months ^[1].

Clinical features of Eisenmenger syndrome in childhood/adolescence ^[15]:

Once Eisenmenger syndrome is established, surgery to close the defect is CONTRAINDICATED — removing the "pop-off" VSD/ASD would cause acute RV failure because the RV can no longer unload into the LV. Management is palliative: pulmonary vasodilators (sildenafil, bosentan, epoprostenol), supportive care, and in highly selected cases, heart-lung transplantation ^[15].

• Risk: AVSD creates turbulent blood flow across the defect and across the abnormal AV valves → endothelial damage → deposition of fibrin-platelet thrombi → a nidus for bacterial seeding during transient bacteraemia (e.g., from dental procedures, skin breaks)
• Why IE is relevant in AVSD: The abnormal AV valve (cleft mitral valve or common AV valve) is particularly susceptible because of the jet lesion created by regurgitant flow hitting the atrial or ventricular endocardium
• Organism: In paediatric IE, the most common organisms are Streptococcus viridans (from the oral cavity) and Staphylococcus aureus ^[16]
• Prevention: IE prophylaxis is indicated for unrepaired CHD and for repaired CHD with residual defects adjacent to prosthetic material ^[16]. Prophylaxis consists of amoxicillin 50 mg/kg (max 2g) PO as a single dose 30–60 minutes before dental procedures.
• Complications of IE ^[16]:
  • Heart failure (valve destruction)
  • Stroke (septic emboli from vegetations > 1cm)
  • Mycotic aneurysm (persistent bacteraemia weakens vessel walls)
  • Glomerulonephritis (immune complex deposition)
  • Splenic abscess / embolic phenomena (Osler nodes, Janeway lesions, Roth spots)

• Mechanism: Pulmonary plethora → interstitial oedema → impaired mucociliary clearance → "wet" lungs provide an ideal environment for bacterial proliferation
• Clinical relevance: Recurrent pneumonia or bronchiolitis in an infant with AVSD is both a consequence of the overcirculation and an indication for surgical repair (if not already planned)
• Paediatric specifics: In children with Down syndrome, there is additional susceptibility to respiratory infections due to immune dysregulation (T-cell dysfunction, relatively low IgG subclasses) + upper airway obstruction (macroglossia, hypotonia)

• Mechanism: The cleft mitral valve or common AV valve is structurally abnormal. Over time, the regurgitant jet causes further endocardial damage and valve deformation → progressive MR/TR → worsening volume overload → accelerated HF
• Clinical relevance: Severe MR can cause earlier presentation of HF (before PVR has fully dropped) and is an important determinant of the urgency of surgical repair

• Incidence: ~10–15% of patients require reoperation for MR during long-term follow-up
• Mechanism: The AV valve tissue in AVSD is inherently dysplastic (it originates from deficient endocardial cushions). Even after meticulous cleft closure and valve repair, the tissue may:
  • Dehisce (suture line gives way) → recurrent cleft → recurrent MR
  • Progressively degenerate → increasing MR over years
  • Develop fibrosis/thickening → stenosis or mixed stenosis-regurgitation
• Monitoring: Serial echocardiography (annual or biannual) assessing MR severity, LV size, and LV function
• Management: If MR becomes haemodynamically significant (progressive LV dilatation, declining LV function, symptoms):
  • Re-repair (preferred if technically feasible — preserves the native valve)
  • Mitral valve replacement (MVR): Mechanical valve (requires lifelong anticoagulation with warfarin — significant issue in growing children, as mechanical valves do not grow with the child and will eventually need upsizing) or bioprosthetic valve (limited lifespan, will need re-replacement, but avoids anticoagulation)

• Incidence: ~2–5% of patients develop permanent CHB after AVSD repair
• Mechanism: In AVSD, the AV node is displaced posteriorly and inferiorly (away from its normal position at the apex of Koch's triangle). The His bundle takes an abnormal course along the posterior-inferior rim of the VSD. During surgical patch closure, sutures placed too close to the conduction system can damage the AV node or His bundle → AV block.
• Timing:
  • Immediate post-op: Transient heart block is common (swelling, oedema around the conduction tissue). Most cases resolve within 7–10 days. Managed with temporary epicardial pacing wires placed during surgery.
  • Persistent: If CHB persists > 7–10 days post-op, it is likely permanent → requires permanent pacemaker (PPM) implantation
  • Late-onset: CHB can develop months to years after repair due to progressive fibrosis around the conduction tissue
• ECG appearance of CHB: Regular bradycardia, PP interval normal, P waves more than QRS complexes, no fixed relationship between P and QRS → 3rd degree AV block ^[9]
• Pacemaker specifics in paediatrics: Epicardial leads are used in infants and small children (transvenous leads are reserved for older children/adolescents with adequate vein size). The generator is placed in the abdomen (not the pectoral region as in adults) because the child's chest is too small.

• Incidence: ~5–10% of patients, can develop at any time from months to years post-repair
• Mechanism: AVSD inherently has an abnormal LVOT anatomy — the "goose-neck" deformity means the LVOT is elongated and narrow. After surgery:
  • Subaortic ridge/membrane: Fibrous tissue can grow on the ventricular septal crest below the aortic valve, creating a discrete subaortic stenosis
  • Accessory AV valve tissue: Tissue from the repaired AV valve may protrude into the LVOT
  • Abnormal septal geometry: The orientation of the patch closure may alter LVOT geometry
• Clinical significance: Progressive LVOTO creates LV pressure overload → LVH → eventually LV failure and syncope. Also creates a substrate for aortic regurgitation (the subaortic jet damages the aortic valve cusps).
• Monitoring: Serial echo with Doppler assessment of LVOT gradient. Peak gradient > 50 mmHg at rest, or progressive LVH, indicates need for intervention.
• Management: Surgical resection of the subaortic membrane/ridge. May recur (subaortic stenosis in AVSD has a higher recurrence rate than isolated subaortic stenosis).

• Incidence: Small residual defects are relatively common; haemodynamically significant residual defects are uncommon in experienced centres
• Mechanism: Patch dehiscence (suture line leak), inadequate patch sizing, or missed additional defects
• Monitoring: Echo with colour Doppler to detect residual shunts
• Management:
  • Small residual shunts: Usually well-tolerated. Observe. May close spontaneously if muscular VSD.
  • Large residual shunts: Reoperation or (in selected cases) transcatheter closure of residual defect

• Types:
  • Atrial arrhythmias (atrial flutter, atrial fibrillation, intra-atrial re-entrant tachycardia — also called "incisional tachycardia"): Due to atrial scarring from the surgical incision + chronic atrial dilatation from prior volume overload. The scar tissue creates circuits for re-entrant arrhythmias.
  • Ventricular arrhythmias: Less common but more dangerous. Can occur if there is residual LV dysfunction or significant ventricular scarring.
  • Sinus node dysfunction: Can occur if the sinus node or its artery is damaged during surgery (especially if the repair involves extensive atrial manipulation).
• Monitoring: Holter monitoring (24-hour ambulatory ECG) at intervals during follow-up
• Management: Antiarrhythmic drugs, catheter ablation for refractory atrial tachycardias, pacemaker for sinus node dysfunction, ICD for ventricular arrhythmias with high sudden cardiac death risk

• When: If surgery is performed too late (after irreversible pulmonary vascular changes have begun)
• More common in: Down syndrome children operated after 6 months of age
• Management: Pulmonary vasodilator therapy (sildenafil — PDE-5 inhibitor, bosentan — endothelin receptor antagonist, inhaled iloprost/epoprostenol — prostacyclin analogues). In severe cases, consider heart-lung transplant.
• Prevention: This is precisely why surgery is recommended by 3 months of age ^[1] — early repair before irreversible vascular changes occur.

• Risk period: Highest in the first 6 months after repair (prosthetic material not yet endothelialised). Also elevated lifelong if residual defects persist adjacent to prosthetic material.
• IE prophylaxis recommendations:
  • First 6 months post-repair: Prophylaxis before dental procedures is recommended (prosthetic patch material)
  • After 6 months with no residual defect: Prophylaxis generally NOT required
  • After 6 months with residual defect adjacent to patch material: Lifelong prophylaxis ^[16]