A 24-year-old woman attends a cardiology clinic because of intermittent awareness of her heartbeat. She has been otherwise healthy with no prior medical problems. Cardiovascular examination reveals a widely separated second heart sound that does not vary with breathing and a grade 2–3/6 crescendo–decrescendo systolic murmur best heard at the left upper parasternal area, which becomes louder during inspiration. Her ECG demonstrates an incomplete right bundle branch block.
What is the most likely underlying diagnosis?
- Secundum atrial septal defect
- Severe tricuspid regurgitation
- Isolated pulmonary valve stenosis
- Small ventricular septal defect
- Patent ductus arteriosus
Answer: Secundum atrial septal defect
Detailed discussion
The key diagnostic clue in this scenario is the fixed, wide splitting of the second heart sound (S2). In normal physiology, S2 splitting varies with respiration due to changes in venous return and right ventricular ejection time. In an atrial septal defect (ASD), there is continuous left-to-right shunting at the atrial level, leading to chronic right ventricular volume overload. This causes delayed right ventricular systole irrespective of respiratory phase, producing a split S2 that is wide and fixed.
The systolic murmur heard in ASD is not due to flow across the septal defect itself, but rather results from increased blood flow across the pulmonary valve, producing a “relative pulmonary stenosis” murmur. This explains why the murmur is ejection systolic, best heard at the upper left sternal border, and why it increases during inspiration (a typical feature of right-sided murmurs due to augmented venous return).
ECG findings commonly include right bundle branch block (often incomplete) and right axis deviation, reflecting right ventricular dilation and conduction delay. Among ASDs, secundum ASD is the most common type and is classically associated with RBBB on ECG.
Symptoms are often mild or absent in early adulthood, but patients may present with palpitations due to atrial arrhythmias, particularly atrial fibrillation or flutter, which become more common with age. Long-standing untreated ASD can lead to pulmonary hypertension, right heart failure, and eventually Eisenmenger syndrome, although this typically occurs later in life.
Pulmonary stenosis may produce an ejection systolic murmur at the same site and may increase with inspiration, but it is usually associated with an ejection click and a respiratory-variable split S2, not a fixed split. Ventricular septal defects produce a harsh pansystolic murmur at the lower left sternal edge, while patent ductus arteriosus causes a continuous machinery murmur. Tricuspid regurgitation gives a pansystolic murmur that increases with inspiration but does not cause fixed splitting of S2.
From an MRCP perspective, always remember: fixed split S2 = ASD until proven otherwise.
Exam cheat sheet
- Fixed, wide split S2 → atrial septal defect
- Murmur in ASD = flow across pulmonary valve, not the defect
- Murmur: ejection systolic, upper left sternal edge
- Right-sided murmurs ↑ with inspiration
- ECG: incomplete RBBB ± right axis deviation
- Most common type in adults: secundum ASD
- Complications: AF, pulmonary hypertension, Eisenmenger syndrome
Flash cards
Q: What causes fixed splitting of S2 in ASD?
A: Constant left-to-right shunt causing delayed RV emptying independent of respiration
Q: Why is the murmur in ASD systolic?
A: Increased flow across the pulmonary valve
Q: Best auscultation site for ASD murmur?
A: Upper left sternal edge
Q: Typical ECG finding in secundum ASD?
A: Incomplete right bundle branch block
Q: Does the murmur in ASD arise from the septal defect?
A: No, it arises from pulmonary valve flow
Q: Which heart sounds are characteristic of ASD?
A: Fixed, widely split S2
Q: Why do right-sided murmurs increase with inspiration?
A: Increased venous return to the right heart
Q: Most common adult presentation of ASD?
A: Dyspnoea or palpitations
Q: Long-term complication of untreated ASD?
A: Pulmonary hypertension and Eisenmenger syndrome
Q: Most common ASD type?
A: Secundum ASD
MCQs
- A fixed split second heart sound is most characteristic of which condition?
A. Pulmonary stenosis
B. ASD
C. VSD
D. Mitral regurgitation
E. PDA
Answer: B - The systolic murmur in ASD is best explained by:
A. Turbulent flow across the atrial septum
B. Mitral valve prolapse
C. Increased pulmonary valve flow
D. Tricuspid regurgitation
E. Pulmonary regurgitation
Answer: C - Which ECG abnormality is most typical in secundum ASD?
A. Left bundle branch block
B. Complete heart block
C. Incomplete RBBB
D. Left axis deviation
E. ST elevation
Answer: C - Which murmur increases with inspiration?
A. Mitral regurgitation
B. Aortic stenosis
C. Pulmonary stenosis
D. Aortic regurgitation
E. Mitral stenosis
Answer: C - Which feature best differentiates ASD from pulmonary stenosis?
A. Systolic murmur
B. Murmur location
C. Fixed splitting of S2
D. Right axis deviation
E. Dyspnoea
Answer: C - The most common type of ASD in adults is:
A. Primum
B. Sinus venosus
C. Secundum
D. Coronary sinus
E. Post-infarction
Answer: C - Which complication becomes more common with age in ASD?
A. Ventricular tachycardia
B. Atrial fibrillation
C. Complete heart block
D. Infective endocarditis
E. Acute pulmonary oedema
Answer: B - Which murmur is continuous throughout systole and diastole?
A. ASD
B. VSD
C. PDA
D. MR
E. PS
Answer: C - In ASD, the pulmonary valve murmur is described as:
A. Pansystolic
B. Late systolic
C. Ejection systolic
D. Early diastolic
E. Mid-diastolic
Answer: C - What happens to S2 splitting in ASD during inspiration?
A. Narrows
B. Widens
C. Becomes fixed
D. Disappears
E. Reverses
Answer: C
Summary for quick exam revision
Atrial septal defect classically presents with a fixed, widely split second heart sound due to constant left-to-right shunting and delayed right ventricular emptying independent of respiration. The systolic murmur heard in ASD is an ejection systolic murmur at the upper left sternal edge caused by increased flow across the pulmonary valve rather than flow through the defect itself. This murmur increases with inspiration, consistent with a right-sided lesion. ECG often shows incomplete right bundle branch block and right axis deviation due to right ventricular volume overload. Secundum ASD is the most common type encountered in adults. Patients may be asymptomatic or present with palpitations from atrial arrhythmias. Over time, untreated ASD can lead to pulmonary hypertension, right heart failure, and Eisenmenger syndrome. For MRCP, fixed splitting of S2 is the single most important bedside clue pointing toward ASD.