## Most Common Cyanotic Congenital Heart Defect **Key Point:** Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart defect, accounting for approximately 7–10% of all congenital heart disease and 10% of all cyanotic heart disease. ### Pathophysiology of TOF TOF consists of four anatomical components: 1. Ventricular septal defect (VSD) 2. Right ventricular outflow tract obstruction (pulmonary stenosis) 3. Right ventricular hypertrophy 4. Overriding aorta The degree of right ventricular outflow obstruction determines the magnitude of right-to-left shunting and severity of cyanosis. ### Clinical Presentation **High-Yield:** Classic presentation includes: - Cyanosis (degree varies with severity of RVOT obstruction) - "Tet spells" — acute hypercyanotic episodes triggered by crying, feeding, or defecation - Squatting posture (increases systemic vascular resistance, reduces right-to-left shunt) - Poor feeding and failure to thrive ### Diagnostic Features | Feature | TOF | TGA | TAPVR | TA | | --- | --- | --- | --- | --- | | **Boot-shaped heart** | Yes (characteristic) | Egg-on-string | Snowman sign | Yes | | **Pulmonary vascular markings** | Decreased | Increased | Increased | Decreased | | **Age at presentation** | Days to weeks | First hours of life | Days to weeks | First days of life | | **Shunt level** | Ventricular | Atrial/arterial | Atrial | Atrial | ### Epidemiology **Clinical Pearl:** TOF is the most common cyanotic heart defect in children beyond infancy and is associated with trisomy 21 and DiGeorge syndrome (22q11 deletion). ### Management - **Acute tet spell:** Knee-chest position, oxygen, morphine, propranolol - **Definitive:** Surgical repair (primary repair in infancy; historically Blalock-Taussig shunt as palliation) **Mnemonic: TOF = The Only Frequent cyanotic lesion** — emphasizing its dominance among cyanotic defects.
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