Congenital heart defects (CHDs) are classified by hemodynamic effect: acyanotic (left-to-right shunts increasing pulmonary blood flow) and cyanotic (right-to-left shunts or mixing lesions causing systemic desaturation). Acyanotic defects include VSD (most common CHD), ASD, and PDA; blood shunts from high-pressure left heart to low-pressure right heart, causing volume overload and eventual pulmonary hypertension. If untreated, chronic left-to-right shunting causes irreversible pulmonary vascular remodeling (Eisenmenger syndrome), where pulmonary resistance exceeds systemic resistance and the shunt REVERSES to right-to-left, causing cyanosis. Cyanotic defects include Tetralogy of Fallot (most common cyanotic CHD: VSD + overriding aorta + RV outflow tract obstruction + RV hypertrophy) and transposition of the great arteries (TGA: aorta from RV, pulmonary artery from LV -- incompatible with life without mixing).