During a routine cardiac examination of a 6-year-old child, a soft systolic ejection murmur is heard at the left upper sternal border with a fixed split second heart sound. Which is the most common location for an atrial septal defect (ASD)?

Explanation

## Anatomical Classification and Frequency of ASD **Key Point:** Ostium secundum ASD is the most common type, accounting for 70–75% of all ASDs. It results from inadequate growth of the septum secundum or excessive resorption of the septum primum in the region of the fossa ovalis. ## ASD Types and Epidemiology | ASD Type | Frequency (%) | Embryologic Defect | Associated Features | |----------|---------------|-------------------|---------------------| | Ostium secundum | 70–75 | Deficiency of septum secundum or primum | Most common; may have associated MVP | | Ostium primum | 15–20 | Endocardial cushion defect | Part of AVSD spectrum; mitral regurgitation common | | Sinus venosus | 10–15 | Abnormal development of sinus venosus | Often associated with partial PAPVR | | Unroofed coronary sinus | <1 | Deficiency of wall between CS and LA | Rare; may have LSVC | **High-Yield:** Ostium secundum ASD is associated with: - Maternal lithium exposure in utero (Ebstein anomaly association) - Holt–Oram syndrome (skeletal abnormalities + ASD) - Down syndrome (though more commonly associated with ostium primum) **Mnemonic:** **SINUS** = Sinus venosus (associated with PAPVR) **SECUNDUM** = Septal deficiency (most common, 70–75%) **PRIMUM** = Part of endocardial cushion defects (15–20%) ## Clinical Presentation - **Fixed split S2**: The hallmark finding; does not vary with respiration because increased RV stroke volume is constant (due to left-to-right shunt). - **Soft systolic ejection murmur**: At left upper sternal border (pulmonary area), reflecting increased pulmonary blood flow. - **Diastolic flow murmur**: May be present at the tricuspid area if significant shunt. **Clinical Pearl:** Ostium primum ASD (endocardial cushion defect) is more commonly associated with mitral regurgitation and a pansystolic murmur, whereas ostium secundum typically presents with a pure systolic ejection murmur and fixed split S2. [cite:Park 26e Ch 11]