Question
A 30-year-old woman is evaluated after an incidental abnormal ECG. She has no symptoms. On auscultation, there is a gentle systolic murmur at the upper left sternal border. The second heart sound is widely split and remains unchanged during inspiration and expiration. Her ECG shows a right bundle branch block with rightward axis.
What is the most probable diagnosis?
a. Congenital bicuspid aortic valve
b. Ostium primum type interatrial defect
c. Ostium secundum type interatrial defect
d. Pulmonary valve narrowing
e. Muscular ventricular septal defect
Answer
Ostium secundum atrial septal defect
Detailed Explanation
This is a classic MRCP-style “spot diagnosis” question.
1. Key Clinical Clue → Fixed split S2
- The second heart sound (S2) is widely split and does not vary with respiration
- This is pathognomonic for atrial septal defect (ASD)
- Mechanism:
- Constant left-to-right shunt
- Continuous right ventricular volume overload
- Delayed pulmonary valve closure (P2)
- Hence, fixed splitting
2. Murmur
- Ejection systolic murmur at upper left sternal edge
- Not due to the defect itself
- Due to increased flow across pulmonary valve
3. ECG findings → crucial for subtype
- Right bundle branch block (RBBB)
- Right axis deviation (RAD)
👉 This combination strongly points to:
- Ostium secundum ASD (most common type)
4. Differentiation from ostium primum ASD
| Feature | Ostium secundum | Ostium primum |
|---|---|---|
| Frequency | Most common | Less common |
| Axis deviation | Right axis deviation | Left axis deviation |
| ECG | RBBB | RBBB + LAD + prolonged PR |
| Associations | Holt-Oram syndrome | AV valve defects |
5. Why other options are wrong
- Bicuspid aortic valve
- Murmur at upper right sternal edge
- No fixed split S2
- Pulmonary stenosis
- Wide split S2 but varies with respiration
- Not fixed
- VSD
- Pansystolic murmur at lower left sternal edge
- No fixed split S2
Cheat Sheet (High-Yield)
- Fixed split S2 = ASD until proven otherwise
- ASD murmur = flow murmur (pulmonary valve)
- ECG patterns:
- Secundum → RBBB + RAD
- Primum → RBBB + LAD + long PR
- Most common ASD = Ostium secundum
- Complication → paradoxical embolism → stroke
Flash Cards
Q1: What heart sound abnormality is classic for ASD?
A: Fixed splitting of S2
Explanation: Due to constant RV volume overload delaying P2
Q2: ECG finding in ostium secundum ASD?
A: RBBB + right axis deviation
Explanation: Reflects right ventricular volume overload
Q3: ECG finding in ostium primum ASD?
A: RBBB + left axis deviation + prolonged PR
Explanation: Involvement near AV node
Q4: Cause of murmur in ASD?
A: Increased flow across pulmonary valve
Explanation: Not due to septal defect itself
Q5: Most common ASD type?
A: Ostium secundum
Explanation: Accounts for ~70% of cases
MCQs (Challenging Level)
1. A patient has fixed splitting of S2. What is the underlying physiological mechanism?
a. Delayed aortic valve closure due to LV hypertrophy
b. Constant right ventricular volume overload
c. Intermittent pulmonary hypertension
d. Early closure of mitral valve
Answer: b
Explanation: Continuous L→R shunt → persistent RV overload → delayed P2 independent of respiration.
2. Which ECG finding best differentiates ostium primum from secundum ASD?
a. Presence of RBBB
b. Right ventricular hypertrophy
c. Left axis deviation
d. Atrial fibrillation
Answer: c
Explanation: LAD is characteristic of primum ASD due to AV septal involvement.
3. Which of the following is FALSE regarding ASD?
a. Fixed split S2 is characteristic
b. Murmur is due to increased pulmonary flow
c. Secundum ASD shows LAD
d. Paradoxical embolism can occur
Answer: c
Explanation: Secundum ASD shows RAD, not LAD.
4. A murmur at the upper left sternal edge in ASD is due to:
a. Turbulence across atrial septum
b. Pulmonary valve flow increase
c. Tricuspid regurgitation
d. Aortic stenosis
Answer: b
Explanation: ASD itself is silent; murmur arises from increased pulmonary flow.
5. Which congenital syndrome is associated with ostium secundum ASD?
a. Marfan syndrome
b. Holt-Oram syndrome
c. Turner syndrome
d. Noonan syndrome
Answer: b
Explanation: Holt-Oram → ASD + limb abnormalities.
6. Which condition produces wide splitting of S2 that varies with respiration?
a. ASD
b. Pulmonary stenosis
c. Eisenmenger syndrome
d. Mitral stenosis
Answer: b
Explanation: In PS, delayed P2 varies with inspiration → not fixed.
Summary for Quick Exam Revision
Atrial septal defect is a key congenital lesion presenting in adulthood, with fixed splitting of the second heart sound as the most diagnostic clinical sign, caused by constant right ventricular volume overload from a left-to-right shunt. The murmur heard is not from the defect itself but from increased flow across the pulmonary valve, producing an ejection systolic murmur at the upper left sternal border. ECG findings are crucial for subtype differentiation: ostium secundum ASD shows right bundle branch block with right axis deviation, while ostium primum ASD shows RBBB with left axis deviation and often prolonged PR interval due to AV node involvement. Secundum ASD is the most common type and may be associated with Holt-Oram syndrome. Important complications include paradoxical embolism leading to stroke. Differentials like pulmonary stenosis may mimic wide splitting but lack the hallmark fixed nature, while VSD and bicuspid aortic valve have distinct murmur locations and no fixed S2 splitting.