A 6-month-old female infant from rural Maharashtra presents with progressive cyanosis, squatting posture, and recurrent respiratory tract infections. On examination, she is deeply cyanosed (SpO2 68% on room air), with clubbing of fingers. A loud systolic ejection murmur is heard at the left upper sternal border. Chest X-ray shows a boot-shaped heart with decreased pulmonary vascularity. Echocardiography confirms a large ventricular septal defect (VSD) with right ventricular outflow tract (RVOT) obstruction. Which of the following is the most likely diagnosis?

Explanation

## Diagnostic Analysis: Classic Tetralogy of Fallot ### The Four Components of TOF **Key Point:** Tetralogy of Fallot is defined by four anatomical features: 1. **Ventricular septal defect (VSD)** — large, non-restrictive 2. **Right ventricular outflow tract (RVOT) obstruction** — infundibular and/or valvular pulmonary stenosis 3. **Right ventricular hypertrophy (RVH)** — secondary to RVOT obstruction 4. **Overriding aorta** — aorta positioned over the VSD, receives blood from both ventricles ### Clinical Features in This Case | Feature | Observation | Significance | |---------|-------------|-------------| | **Age of presentation** | 6 months | Classic TOF; cyanosis develops as RV pressure rises | | **Cyanosis severity** | Deep (SpO2 68%) | Moderate-to-severe right-to-left shunt | | **Squatting posture** | Present | Increases systemic vascular resistance, decreases right-to-left shunt, improves oxygenation | | **Clubbing** | Present | Chronic hypoxemia | | **Murmur** | Loud systolic at left upper sternal border | Pulmonary stenosis (RVOT obstruction) | | **CXR: Boot-shaped heart** | Characteristic | RVH + small pulmonary artery | | **CXR: Decreased pulmonary vascularity** | Present | Right-to-left shunt (deoxygenated blood bypasses lungs) | | **Echo findings** | Large VSD + RVOT obstruction | Confirms TOF | **High-Yield:** The **boot-shaped heart** (coeur en sabot) on CXR is virtually pathognomonic for TOF. ### Pathophysiology: Why Cyanosis Develops ```mermaid flowchart TD A[Large VSD + RVOT obstruction]:::outcome --> B[RV pressure rises]:::outcome B --> C{RV pressure > LV pressure?}:::decision C -->|Yes| D[Right-to-left shunt through VSD]:::action D --> E[Deoxygenated blood enters systemic circulation]:::outcome E --> F[Cyanosis]:::urgent F --> G[Squatting increases SVR]:::action G --> H[Decreases R-to-L shunt]:::action H --> I[Improves SpO2 temporarily]:::outcome ``` ### Natural History of TOF 1. **Infancy (0–3 months):** Cyanosis may be absent or mild; RVOT obstruction is not yet severe 2. **Early infancy (3–12 months):** Progressive cyanosis as RV hypertrophy worsens RVOT obstruction (as in this case) 3. **Childhood:** Squatting develops as a compensatory mechanism 4. **Untreated:** Progressive cyanosis, clubbing, polycythemia, stroke risk, sudden death from arrhythmia **Clinical Pearl:** Squatting is a **spontaneous maneuver** that children with TOF learn to perform; it increases systemic vascular resistance (SVR), which decreases the right-to-left shunt and temporarily improves oxygenation. ### Management Approach **Mnemonic: PROS** (Prostaglandin, Rest, Oxygen, Squatting) - **Prostaglandin E1:** Keeps ductus arteriosus patent (in neonates) - **Rest:** Reduces metabolic demand - **Oxygen:** Decreases pulmonary vascular resistance - **Squatting:** Increases SVR, reduces R-to-L shunt - **Definitive:** Surgical repair (Blalock-Taussig shunt or primary repair) **Key Point:** The definitive treatment is surgical correction, typically performed in infancy to prevent progressive cyanosis and complications. [cite:Park 26e Ch Congenital Heart Disease; Pediatric Cardiology Essentials]