Tetralogy of Fallot

Tetralogy of Fallot is a conotruncal congenital heart defect produced by anterocephalad deviation of the outlet septum during cardiac development. That single developmental error creates four linked findings: - a ventricular septal defect, usually large and nonrestrictive; - right ventricular outflow tract obstruction, which may be infundibular, valvar, supravalvar, or involve the pulmonary arteries; - an aorta positioned over the ventricular septal defect and receiving blood from both ventricles; and - right ventricular hypertrophy from pressure overload. The ventricular septal defect is not usually the feature that determines the child’s oxygenation. The degree of obstruction between the right ventricle and pulmonary artery is more influential. When obstruction is mild, pulmonary blood flow may be adequate and the infant may have little or no cyanosis at rest. This is sometimes called “pink” tetralogy. When obstruction is severe, blood is preferentially ejected through the overriding aorta rather than toward the lungs, producing right-to-left shunting and systemic desaturation. The governing relationship is the balance between resistance to pulmonary outflow plus pulmonary vascular resistance and systemic vascular resistance. A fall in systemic vascular resistance...