## Distinguishing Tetralogy of Fallot (TOF) from Transposition of the Great Arteries (TGA) ### Key Anatomical Differences **Key Point:** Pulmonary stenosis with RV outflow tract obstruction is the hallmark feature that separates TOF from TGA. This obstruction is ABSENT in TGA. | Feature | TOF | TGA | | --- | --- | --- | | **RVOT obstruction** | Present (pulmonary stenosis) | Absent | | **VSD** | Present | Present (ASD/PFO usually) | | **Aorta origin** | Overrides VSD | Arises from RV | | **Pulmonary artery origin** | Stenosed, from RV | Arises from LV | | **Cyanosis onset** | Gradual (weeks to months) | Acute (hours to days) | | **Squatting posture** | Common | Rare | ### Clinical Presentation Differences **High-Yield:** TOF cyanosis develops *gradually* because the child can compensate initially via the PDA and ASD/PFO. Squatting increases systemic vascular resistance, reducing right-to-left shunting through the VSD — a compensatory mechanism unique to TOF. In TGA, cyanosis is *acute and severe* within hours because there is complete separation of pulmonary and systemic circulations. The infant is entirely dependent on mixing at the PDA/ASD level. ### Chest X-ray Findings - **TOF:** Boot-shaped heart (coeur en sabot) — due to RV hypertrophy and small pulmonary artery - **TGA:** Egg-on-string (narrow mediastinum, egg-shaped silhouette) — due to anterior aorta and posterior pulmonary artery **Clinical Pearl:** The presence of pulmonary stenosis with RVOT obstruction is the *sine qua non* of TOF and is completely absent in TGA. This is the single most discriminating feature. ### Why Squatting Occurs in TOF Squatting → ↑ SVR → ↓ right-to-left shunt → ↓ cyanosis. This reflex is pathognomonic for TOF and does not occur in TGA (where shunting is obligatory and fixed). [cite:Park 26e Ch 11] 
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.