? IS IT PFO OR SMALL ASD differentiating a small atrial septal defect (ASD) from a patent foramen ovale (PFO) on transthoracic echocardiography (TTE) with left-to-right shunting can be tricky, because both may appear as a small color Doppler jet across the interatrial septum. The key is to look at anatomic features, shunt characteristics, and associated findings: 1--Anatomic/Structural Clues PFO Flap-like tunnel between septum primum and septum secundum. Often not a true tissue defect, but a sli