This axial contrast-enhanced CT nicely illustrates a defect in the membranous portion of the IVS (arrow). This defect permits a communication between the ventricular cavities, and causes blood shunting from the high pressure left systemic circulation to the low-pressure pulmonary circulation. The magnitude of the shunt is influenced by the size of the defect, the pressure in right ventricle (RV) and left ventricle (LV) and the pulmonary and systemic vascular resistance. RA right atrium, LA left atrium