Atrial Septal Defect (ASD)

Types of ASD

• Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 504

Pathophysiology

• Pulmonary blood flow may be 2-8x normal (Qp/Qs can exceed 3:1 in large defects)
• Right heart volume overload - right atrial and right ventricular dilation and hypertrophy
• Increased flow through the pulmonary valve causes a flow murmur
• Over decades, sustained right-sided pressure and volume overload can lead to pulmonary arterial hypertension - less common in ASDs than VSDs, but possible
• Paradoxical right-to-left shunting can occur transiently during increased intrathoracic pressure (Valsalva, coughing), risking paradoxical embolism and stroke
• In advanced disease: Eisenmenger syndrome (reversal of shunt to right-to-left, cyanosis) - though this is far less common than with VSDs
• Sabiston Textbook of Surgery, p. 2564; Robbins, p. 504-505

Clinical Presentation

• Dyspnea on exertion - the most common presenting symptom
• Fatigue and exercise intolerance
• Palpitations, atrial fibrillation/flutter - due to atrial dilation and volume overload
• Syncope or stroke - from atrial arrhythmias or paradoxical embolism
• Signs of right heart failure: peripheral edema, elevated JVP, hepatomegaly in late disease
• More than 70% of patients are functionally impaired by the 5th decade if untreated
• Goldman-Cecil Medicine, p. 980

Physical Examination

• Goldman-Cecil Medicine, p. 984

Investigations

• ECG: Incomplete right bundle branch block (rSR' pattern in V1) is characteristic; right axis deviation; prolonged PR interval, atrial fibrillation/flutter with advanced disease
• Chest X-ray: Pulmonary vascular plethora (bilateral increased lung markings), dilated main pulmonary artery and branches, right atrial and right ventricular enlargement
• Echocardiography (TTE/TEE): Definitive diagnostic tool - identifies the ASD location and size, quantifies the shunt ratio (Qp/Qs), estimates pulmonary artery pressures, and assesses RV function. Note: sinus venosus ASDs can be missed on TTE and require TEE or MRI
• Cardiac catheterization: Not routinely needed, but used when pulmonary hypertension needs quantification
• Goldman-Cecil Medicine, p. 985-986

Management

Indications for Closure

• Qp/Qs ratio ≥ 1.5:1 with evidence of right heart volume overload
• Paradoxical embolism
• Symptomatic patients at any age

1. Catheter-Based (Device) Closure

• Adequate septal rim (≥5 mm on most borders)
• Suitable defect anatomy (secundum type)
• Defect diameter typically ≤38 mm

2. Surgical Closure (Open Heart Surgery)

• Primum and sinus venosus ASDs (not amenable to device closure)
• Inadequate septal rim for device anchorage
• Associated anomalies requiring surgical correction (anomalous pulmonary veins, AV valve repair)
• Failed device closure

• Direct suture closure - for small defects
• Patch closure - autologous pericardium or prosthetic material (Dacron/Gore-Tex) for larger defects

• Superior type: intracardiac patch baffle, or the Warden procedure (SVC translocation) when anomalous veins drain high
• Inferior type (scimitar syndrome): patch baffle in the IVC, potentially requiring hypothermic circulatory arrest
• Sabiston Textbook of Surgery, p. 2564-2397

3. Medical Management

• There is no medical therapy that closes the defect
• Antiplatelet therapy (aspirin) may be prescribed post-device closure for 3-6 months
• Rate/rhythm control and anticoagulation for atrial arrhythmias
• Diuretics and heart failure therapy if right heart failure is present

Summary Table

ATRIAL SEPTAL DEFECT (ASD)

Detailed MBBS Finals Exam Answer

DEFINITION

EMBRYOLOGY & CLASSIFICATION

1. The septum primum grows down as a crescent-shaped membrane to partition the atria; the anterior gap beneath it is the ostium primum (allows fetal right-to-left flow)
2. Before the ostium primum closes, perforations develop posteriorly forming the ostium secundum
3. A second ingrowth - the septum secundum - grows from the right side to cover the ostium secundum, leaving a residual channel: the foramen ovale
4. In fetal life, pulmonary pressure > systemic pressure, so the foramen ovale stays open (right-to-left)
5. At birth, lung expansion drops pulmonary vascular resistance (PVR), left atrial pressure exceeds right atrial pressure, and the foramen ovale closes

Types of ASD (Classified by Location)

• Harrison's Principles of Internal Medicine 22E, p. 2114; Robbins, p. 504

Exam tip: Primum ASD + mitral cleft + VSD = Atrioventricular Septal Defect (AVSD) / Endocardial cushion defect - seen in Down syndrome (trisomy 21).

Do NOT confuse ASD with Patent Foramen Ovale (PFO): PFO is persistence of the flap valve of the fossa ovalis without true septal deficiency. It causes no right heart dilation and is present in ~25% of adults. It is detected by bubble contrast echo (agitated saline) with Valsalva. - Harrison's, p. 2114

PATHOPHYSIOLOGY

Direction of Shunt

Hemodynamic Consequences

Left atrium → (through ASD) → Right atrium
         ↓
  Increased RV volume load
         ↓
  Increased pulmonary blood flow (Qp/Qs may be 2-8x normal, up to >3:1)
         ↓
  Right heart dilation (RA + RV enlargement)
         ↓
  Dilated pulmonary artery trunk and branches

• Robbins, p. 505; Sabiston Textbook of Surgery, p. 2564

Progression over Time

Exam tip: Eisenmenger is less common in ASDs than in VSDs or PDAs because the lower-pressure atrial shunt causes less vascular trauma to pulmonary arterioles.

Paradoxical Embolism

CLINICAL FEATURES

In Children

• Recurrent respiratory tract infections (due to increased pulmonary blood flow)
• Mild exercise intolerance

In Adults (symptoms typically emerge after the 3rd decade)

• Dyspnea on exertion (most common presenting symptom)
• Fatigue and reduced exercise tolerance
• Palpitations - from supraventricular arrhythmias (SVT, atrial flutter, AF)
• Syncope - from arrhythmias or reduced cardiac output
• Stroke - from paradoxical embolism or AF
• Signs of right heart failure in advanced disease: peripheral edema, elevated JVP, hepatomegaly

Physical Examination Findings

• Goldman-Cecil Medicine, p. 984; Braunwald's, p. 878

INVESTIGATIONS

1. ECG

• Incomplete right bundle branch block (rSR' pattern in V1) - most characteristic finding
• Right axis deviation
• Right atrial enlargement (tall peaked P waves)
• Prolonged PR interval (1st degree heart block)
• Atrial fibrillation / flutter in older patients
• In primum ASD: left axis deviation (due to involvement of the AV node region)

Exam tip: Left axis deviation on ECG + ASD = Primum ASD (down syndrome link). Incomplete RBBB = Secundum ASD.

2. Chest X-Ray

• Pulmonary plethora - increased bilateral vascular markings (increased pulmonary blood flow)
• Dilated main pulmonary artery and its branches
• Right atrial and right ventricular enlargement
• Normal or small aortic knuckle (relatively reduced systemic flow)
• In Eisenmenger: oligaemic lung fields peripherally + dilated central pulmonary arteries ("pruning")

3. Echocardiography (Definitive Investigation)

• TTE (transthoracic echo): Identifies primum and secundum ASDs, measures defect size, shows right heart dilation. Doppler estimates Qp/Qs and pulmonary artery pressure
• TEE (transoesophageal echo): Required for sinus venosus ASDs (often missed on TTE) and for pre-procedural sizing before device closure. Also used intra-operatively during device deployment
• Bubble contrast echo (agitated saline): Detects right-to-left shunting (bubbles in LA/LV = positive); needed to detect PFO during Valsalva
• Findings: drop-out of interatrial septum signal, RV volume overload (RV dilation, paradoxical septal motion), LA-to-RA color Doppler jet

4. Cardiac MRI

• Excellent for sinus venosus defects, anomalous pulmonary venous anatomy, and pre-surgical planning
• Accurately quantifies Qp/Qs

5. Cardiac Catheterization

• Not routinely needed for diagnosis
• Indicated when: pulmonary hypertension suspected, conflicting echo results, pre-operative assessment of PVR
• Finding: step-up in oxygen saturation at the right atrial level (due to mixing of oxygenated left atrial blood)
• Qp/Qs calculated from oxygen saturations

6. Pulse Oximetry

• At rest and on exercise - to detect right-to-left shunting (desaturation) in patients with suspected Eisenmenger syndrome - Braunwald's, p. 878

MANAGEMENT

General Principles

• Closure is recommended when Qp/Qs ≥ 1.5:1 with RV volume overload, regardless of symptoms
• Optimal timing: before school age (4-5 years); adult repair still beneficial if done before irreversible PH develops
• Contraindication to closure: Eisenmenger syndrome (fixed severe pulmonary hypertension with resting right-to-left shunt)

• 60% of ASD interventions worldwide are now catheter-based - Sabiston, p. 2565

1. Catheter-Based (Percutaneous) Device Closure

• Transfemoral venous access; catheter guided across the defect
• Amplatzer Septal Occluder (most widely used) - a self-centering, double-disc nitinol mesh device deployed under TEE + fluoroscopic guidance
• Device endothelialises within 3-6 months

• Atrial arrhythmias
• Complete heart block (rare)
• Device thrombus (aspirin prescribed for 3-6 months)
• Device erosion (~1 in 1000 cases) - can cause haemopericardium; any chest pain post-procedure warrants urgent echo - Braunwald's, p. 878-879

2. Surgical Closure (Open Heart Surgery)

• Primum ASD (always requires surgery; device not possible)
• Sinus venosus ASD (always requires surgery; cannot be catheter-closed)
• Coronary sinus defect
• Secundum ASD with inadequate rims for device anchorage
• Very large secundum defects
• Associated anomalies requiring surgical repair (AV valve repair, PAPVR repair)
• Failed or complicated device closure

1. General anaesthesia; median sternotomy (or right mini-thoracotomy for minimally invasive approach)
2. Cardiopulmonary bypass established (aortic + venous cannulation)
3. Aortic cross-clamp + cardioplegia for myocardial protection
4. Right atriotomy - direct visualisation of the defect
5. Closure:
   • Direct suture closure - for small defects (primary closure with continuous suture)
   • Patch closure - autologous pericardium or synthetic patch (Dacron / Gore-Tex) for larger defects
6. De-airing of the left atrium before closure
7. Termination of bypass; closure

FIGURE 113.10 - Surgical closure for ASD. Sabiston Textbook of Surgery

• Intracardiac patch baffle redirecting anomalous pulmonary veins to LA
• Warden procedure: SVC is divided, the cardiac end oversewn, and the cephalic SVC end is anastomosed to the right atrial appendage; a patch baffles the anomalous veins to the LA

• Complex repair; may require patch baffle within the intrahepatic IVC
• May need hypothermic circulatory arrest - Sabiston, p. 2396

• Atrial arrhythmias may persist (especially AF) - surgery does not reliably reduce arrhythmia burden in older adults - Braunwald's, p. 878
• Follow-up TTE at 1 week, 1 month, then annually for at least 5 years (device closure)
• Patients with elevated PVR at closure need ongoing monitoring as PAH can progress

3. Medical Management

• No pharmacological therapy closes the defect
• Antiplatelet therapy (aspirin 75-150 mg/day) for 3-6 months post-device closure (prophylaxis against device thrombus)
• Rate/rhythm control + anticoagulation (warfarin or NOAC) for atrial fibrillation
• Diuretics + ACE inhibitors for right heart failure symptoms
• Pulmonary arterial hypertension (PAH) therapy (e.g., sildenafil, bosentan, prostacyclins) for Eisenmenger syndrome - treats symptoms but does not reverse the shunt
• Endocarditis prophylaxis: NOT routinely required for isolated secundum ASD (low risk); required for 6 months post-device/surgical repair

COMPLICATIONS OF UNTREATED ASD

QUICK COMPARISON: ASD vs PFO

SUMMARY - KEY EXAM POINTS

Quick Navigation

• VSD: loudest murmur in small defects (maladie de Roger)
• TOF: murmur from RVOT stenosis, NOT the VSD; squatting relieves tet spells
• DORV: Taussig-Bing (subpulmonic VSD) is physiologically TGA
• TGA: the only condition where cyanosis does NOT respond to oxygen; single loud S2; egg-on-a-string CXR
• AVSD: left/superior axis deviation on ECG + Down syndrome = pathognomonic combination

What's New / Expanded in This Version

• Full operative steps: right atriotomy vs. ventriculotomy vs. pulmonary arteriotomy by defect type
• Conduction anatomy with exact suture placement rules (anterosuperior safe, posteroinferior must go into RV muscle)
• Supracristal VSD: aortic cusp repair at same sitting
• Swiss-cheese VSD: PA banding, oversized patch, hybrid device closure
• Amplatzer device closure: indications and heart block risk

• Historical context: Blalock-Taussig 1944; classic vs. modified BT shunt with diagrams
• Full transatrial-transpulmonary repair steps
• Transannular patch decision criteria (Z-score <-2.5; PA/Ao ratio <0.5; RV/LV ratio >0.7)
• Anomalous LAD crossing RVOT: use RV-PA conduit (Hancock/homograft)
• TOF with pulmonary atresia + MAPCAs: unifocalisation surgery
• Late PVR: transcatheter Melody/Sapien valve criteria (RVEDVi >160 mL/m²)

• Full repair strategy by each VSD subtype
• Taussig-Bing without PS: ASO + VSD closure
• Taussig-Bing with PS: Rastelli, REV, or Yasui procedure
• Non-committed VSD: Rastelli-type tunnel + conduit; PA banding timing
• Conduit timing: delay until 2-3 years if extracardiac conduit needed (allows larger size)

• Full step-by-step ASO: great vessel transection → coronary transfer (trapdoor technique, Yacoub A-E configurations) → Lecompte manoeuvre → anastomoses → pericardial patch reconstruction
• DHCA in neonates
• LV retraining protocol (PA band + BT shunt)
• TGA-VSD with severe PS: Rastelli, REV, Nikaidoh procedures
• Mustard/Senning: full explanation of physiologic vs. anatomic correction and why they failed (systemic RV)

• Rastelli A/B/C classification with surgical implications
• Full two-patch technique step-by-step: AV valve division → cleft closure → VSD patch (with conduction system protection) → AV valve-to-patch suturing → ASD patch
• Diagram of AV node position at posteroinferior junction
• Pulmonary hypertensive crisis management post-op