Intracardiac shunts that result in increased pulmonary aerial flow can enlarge the pulmonary vascular system giving the lung fields a plethoric appearance. The most common lesions causing increased vascularity without cyanosis are, Atrial septal defect (ASD) Ventricular septal defect (VSD) Patent ductus aeriosus (PDA) The main cardiac lesions with cyanosis and increased pulmonary vascularity are, Transposition of the great vessels Truncus aeriosus Total anomalous pulmonary venous connection (TAPVC) Ref: Ravenel J.G. (2011). Chapter 3. Imaging of the Hea and Great Vessels. In M.Y. Chen, T.L. Pope, D.J. Ott (Eds), Basic Radiology, 2e.