Overview
Anatomy Determines the Shunt
Tetralogy of Fallot is a conotruncal congenital heart lesion produced by anterocephalad deviation of the outlet, or conal, septum.
Tetralogy of Fallot is a conotruncal congenital heart lesion produced by anterocephalad deviation of the outlet, or conal, septum. That single embryologic malalignment creates four linked findings: - a large malalignment ventricular septal defect (VSD) - right ventricular outflow tract obstruction (RVOTO), which may be infundibular, valvar, or both - an aorta that overrides the VSD and receives blood from both ventricles - right ventricular hypertrophy caused by chronically elevated right ventricular pressure The VSD is usually large and nonrestrictive, so it does not independently limit flow. The important question is where the blood can exit the right ventricle. When RVOTO is severe, blood is directed across the VSD into the overriding aorta rather than through the pulmonary arteries. This right-to-left shunt lowers arterial oxygen saturation and produces cyanosis. Mild obstruction may permit enough pulmonary blood flow that cyanosis is subtle or absent at first. The direction and size of the shunt change with haemodynamics. Increased resistance in the right ventricular outflow tract promotes right-to-left flow. A fall in systemic vascular resistance (SVR), such as with vasodilation or sudden loss...