## Atrial Septal Defect (ASD) **Key Point:** ASD is a defect in the interatrial septum allowing left-to-right shunting at the atrial level, resulting in increased pulmonary blood flow without cyanosis. **High-Yield:** ASDs account for ~10% of all congenital heart disease and are the most common acyanotic lesion diagnosed in adulthood. ### Classification of ASDs | Type | Location | Frequency | Association | |------|----------|-----------|-------------| | Ostium secundum | Mid-atrial septum (fossa ovalis) | 70–75% | Isolated | | Ostium primum | Lower atrial septum | 15–20% | Down syndrome, cleft palate | | Sinus venosus | Near SVC/IVC entry | 10% | Partial anomalous pulmonary venous return | | Unroofed coronary sinus | Rare | <1% | Severe hemodynamics | **Clinical Pearl:** Most ASDs remain asymptomatic in childhood and are discovered incidentally. Symptoms (dyspnea, fatigue) typically emerge in the 3rd–4th decade due to progressive pulmonary vascular disease and atrial arrhythmias. **Mnemonic — ASD Features: "SECUNDUM"** - **S**eptal defect (interatrial) - **E**arly diastolic murmur (tricuspid flow) - **C**ontinuous left-to-right shunt - **U**nder-recognized in childhood - **N**o cyanosis (acyanotic) - **D**ilated right atrium and ventricle - **U**nusually presents in adulthood - **M**idline chest X-ray (cardiomegaly, pulmonary plethora) ### Pathophysiology Left-to-right shunt → increased pulmonary blood flow (Qp:Qs ratio) → pulmonary overcirculation → eventual right heart dilation and pulmonary hypertension (if untreated). **Tip:** On examination, look for a **fixed, widely split S₂** (hallmark finding) due to right ventricular volume overload delaying pulmonary valve closure, and a **systolic ejection murmur** at the left upper sternal border from increased flow across the pulmonary valve. 
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