Which cyanotic congenital heart defect is characterized by complete mixing of systemic and pulmonary venous blood with a single arterial trunk arising from the heart?

Explanation

## Truncus Arteriosus: Definition and Pathophysiology ### Embryologic Basis **Key Point:** Truncus arteriosus results from failure of the **truncus arteriosus** (embryologic common arterial trunk) to divide into the aorta, pulmonary artery, and coronary arteries during weeks 4–6 of gestation. Normally, the truncus arteriosus is divided by the **truncoconal septum** into three separate vessels. In truncus arteriosus, this septation fails completely or partially. ### Anatomical Features | Feature | Truncus Arteriosus | | --- | --- | | **Number of arterial trunks** | Single trunk arising from the heart | | **Pulmonary artery origin** | Arises from the common trunk (not from RV) | | **Aorta origin** | Arises from the common trunk | | **Coronary arteries** | Arise from the common trunk | | **Associated defect** | VSD (always present) | | **Valve** | Single truncal valve (often dysplastic, regurgitant) | ### Classification (Van Praagh) 1. **Type A1** — Pulmonary arteries arise from the common trunk (most common, ~50%) 2. **Type A2** — Right and left pulmonary arteries arise separately from the trunk 3. **Type A3** — Pulmonary arteries arise from the descending aorta (rare) 4. **Type A4** — No pulmonary arteries (pulmonary blood supply from aortic branches) ### Mechanism of Cyanosis ```mermaid flowchart TD A[Single Arterial Trunk]:::outcome --> B[Complete mixing of systemic and pulmonary venous blood]:::action B --> C[Deoxygenated blood enters systemic circulation]:::action C --> D[Mild to moderate cyanosis]:::urgent E[VSD present] --> B F[Single truncal valve] --> G[Valve regurgitation common]:::outcome G --> H[Volume overload to lungs]:::action H --> I[Pulmonary edema, heart failure]:::urgent ``` **High-Yield:** Truncus arteriosus causes **complete mixing** of oxygenated and deoxygenated blood, resulting in mild-to-moderate cyanosis. The cyanosis is usually less severe than in TOF because there is some mixing of oxygenated pulmonary venous blood with systemic venous blood. **Clinical Pearl:** Truncus arteriosus is often associated with **22q11 deletion (DiGeorge syndrome)**, which may present with hypocalcemia, thymic hypoplasia, and facial dysmorphism. Always screen for these features. **Mnemonic:** **TRUNCUS** — Truncal valve (dysplastic), Right-to-left shunt (complete mixing), Undivided trunk, Neonatal cyanosis, Common arterial trunk, Univentricular origin, Single outlet.