Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart defect, accounting for approximately 10% of all CHD. It results from anterior malalignment of the infundibular (conal) septum during fetal development, producing four characteristic defects: (1) Right ventricular outflow tract obstruction (RVOTO)/pulmonary stenosis — the primary determinant of clinical severity; ranges from mild infundibular narrowing to complete pulmonary atresia; (2) Ventricular septal defect (VSD) — large, nonrestrictive, subaortic; allows pressure equalization between ventricles; (3) Overriding aorta — the aortic root is positioned over the VSD, receiving blood from both ventricles; (4) Right ventricular hypertrophy (RVH) — develops as a compensatory response to increased right ventricular pressure from RVOTO. The hemodynamic consequence is a right-to-left shunt across the VSD, with the degree of shunting determined by the severity of RVOTO relative to systemic vascular resistance (SVR). Increased RVOTO or decreased SVR worsens the right-to-left shunt and deepens cyanosis. Hypercyanotic episodes (tet spells) occur when dynamic infundibular spasm acutely increases RVOTO, dramatically increasing right-to-left shunting. These spells are most common between 2-4 months of age and are the most dangerous acute...