## Pathophysiology of Tetralogy of Fallot **Key Point:** TOF is characterized by four anatomical defects: VSD, RVOT obstruction, RV hypertrophy, and overriding aorta. The degree of cyanosis depends primarily on RVOT obstruction severity, not LV changes. ### Cardiac Chamber Changes in TOF | Chamber | Change | Reason | |---------|--------|--------| | Right Ventricle | Hypertrophy (concentric) | Increased afterload from RVOT obstruction | | Left Ventricle | Normal or underdeveloped | Receives decreased pulmonary venous return | | Right Atrium | Dilation | Secondary to RV hypertrophy | **High-Yield:** RV hypertrophy, not LV hypertrophy, is the hallmark of TOF. The left ventricle is typically spared because pulmonary blood flow is reduced. ### Physiology of Cyanosis 1. RVOT obstruction creates high RV pressure 2. VSD allows right-to-left shunting when RV pressure exceeds LV pressure 3. Deoxygenated blood bypasses the lungs → systemic cyanosis 4. Severity depends on: degree of RVOT obstruction, VSD size, and systemic vs. pulmonary vascular resistance ratio **Clinical Pearl:** Squatting increases systemic vascular resistance (SVR), which decreases the pressure gradient across the VSD and reduces right-to-left shunting—this is why children instinctively squat during cyanotic spells. **Mnemonic for TOF:** **RVOT** — Right Ventricular Outflow Tract obstruction is the primary driver; the other three defects (VSD, RV hypertrophy, overriding aorta) are secondary consequences.
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