## Pathophysiology of Tetralogy of Fallot **Key Point:** TOF is characterized by a RIGHT-to-LEFT shunt, not left-to-right. The direction of shunting depends on the relative resistance in the pulmonary and systemic circulations. ### The Four Anatomic Components 1. **Ventricular septal defect (VSD)** — large, non-restrictive 2. **Right ventricular outflow tract obstruction** — pulmonary stenosis (primary lesion) 3. **Right ventricular hypertrophy** — secondary to increased RV workload 4. **Overriding aorta** — receives blood from both ventricles ### Direction of Shunting **High-Yield:** The degree of cyanosis in TOF depends on the severity of RVOT obstruction, not the size of the VSD. When pulmonary stenosis is severe, RV pressure exceeds systemic pressure, forcing deoxygenated blood through the VSD directly into the aorta — this is a **RIGHT-to-LEFT shunt**, causing cyanosis. | Feature | TOF (Right-to-Left) | ASD/VSD (Left-to-Right) | | --- | --- | --- | | **Primary obstruction** | RVOT stenosis | None | | **Shunt direction** | Right → Left | Left → Right | | **Cyanosis** | Present (deoxygenated blood systemic) | Absent | | **Pulmonary blood flow** | Decreased | Increased | ### Clinical & Radiological Findings **Clinical Pearl:** Squatting posture increases systemic vascular resistance, which decreases the right-to-left shunt and improves oxygenation — a classic compensatory mechanism in TOF. **Mnemonic: RVOT** = Right Ventricular Outflow Tract obstruction is the PRIMARY lesion that drives the pathophysiology. ### Chest X-ray: "Boot-Shaped" Heart - Caused by RV hypertrophy (upturned apex) + decreased pulmonary vascularity - Pulmonary artery segment is concave ("coeur en sabot") - This is a characteristic radiological sign **Warning:** Option D states "left-to-right shunting" — this is INCORRECT. TOF produces a right-to-left shunt because the RVOT obstruction raises RV pressure above systemic pressure, forcing blood across the VSD into the aorta. [cite:Park 26e Ch 3]
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