Fetal circulation is illustrated, showing right-to-left shunting (dashed arrows) through patent ductus arteriosus (PDA), pulmonary artery (PA) to aorta (Ao), and patent foramen ovale (PFO), right atrium (RA) to left atrium (LA). RV = right ventricle. LV = left ventricle. (Modified from Graves, E., et al: Persistent pulmonary hypertension in the neonate. Chest 93:638, 1988, with permission.)