Which of the following acyanotic congenital heart defects is characterized by a left-to-right shunt at the atrial level and is the most common type of atrial septal defect?

Explanation

## Atrial Septal Defects — Classification and Epidemiology **Key Point:** Ostium secundum ASD accounts for approximately 70–75% of all atrial septal defects and is the most common type. ### Anatomical Location Ostium secundum ASD occurs in the region of the **fossa ovalis**, resulting from inadequate growth of the septum secundum or excessive resorption of the septum primum. ### Hemodynamic Features - **Left-to-right shunt** at the atrial level - Shunt occurs because left atrial pressure > right atrial pressure throughout the cardiac cycle - Results in increased pulmonary blood flow (Qp:Qs ratio) - Right atrium and right ventricle dilate due to volume overload ### Clinical Significance - Often asymptomatic in childhood - May present with dyspnea, fatigue, or recurrent respiratory infections in older children - Atrial arrhythmias (particularly atrial fibrillation) common in adolescence and adulthood - Risk of pulmonary hypertension if untreated over decades ### Comparison with Other ASDs | Type | Frequency | Location | Associated Findings | |------|-----------|----------|---------------------| | **Ostium Secundum** | 70–75% | Fossa ovalis | Isolated; may have MVP | | Ostium Primum | 15–20% | Endocardial cushion defect | Part of AV canal defect; cleft mitral valve | | Sinus Venosus | 10–15% | Near SVC or IVC entry | Partial anomalous pulmonary venous return | | Unroofed Coronary Sinus | <1% | Coronary sinus | Rare; may have LSVC to CS | **High-Yield:** Ostium secundum ASD is the **most common type** and the most frequently tested variant in NEET PG. **Clinical Pearl:** Secundum ASDs may close spontaneously if the defect is small (<8 mm); larger defects require surgical or catheter-based closure.