Clinical meaning
Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart defect, accounting for approximately 10% of all congenital heart disease. It results from a single embryologic defect: anterior and superior deviation of the infundibular (outlet) septum, which produces four characteristic anatomic abnormalities: (1) Right ventricular outflow tract obstruction (RVOTO/pulmonary stenosis) — the degree of RVOTO determines the severity of cyanosis and is the primary determinant of clinical presentation; (2) Ventricular septal defect (VSD) — a large, non-restrictive perimembranous VSD allowing equalization of right and left ventricular pressures; (3) Overriding aorta — the aortic root is positioned directly over the VSD, receiving blood from both ventricles; (4) Right ventricular hypertrophy — a compensatory response to the increased right ventricular pressure from RVOTO. The hemodynamic consequence is right-to-left shunting of deoxygenated blood through the VSD into the systemic circulation, producing cyanosis. The degree of cyanosis depends on the severity of RVOTO: mild obstruction may produce acyanotic ('pink') TOF with predominantly left-to-right shunting, while severe obstruction produces significant right-to-left shunting and severe cyanosis. Hypercyanotic ('tet') spells are life-threatening episodes of acute cyanosis occurring when RVOT infundibular muscle spasm or decreased systemic vascular resistance (SVR) increases right-to-left shunting. During a tet spell, the infant becomes profoundly cyanotic, tachypneic, and may lose consciousness. Immediate management includes knee-to-chest positioning (squatting in older children), which increases SVR by compressing the femoral arteries, thereby reducing right-to-left shunting and redirecting blood through the pulmonary circulation.