To the editor: The paper by De Bruyne and Sarma 1 reads well as a tutorial on how to use fractional flow reserve (FFR) in a variety of clinical circumstances. However, there are several inaccurate statements in this tutorial and important aspects of coronary physiology are completely neglected. A major misconception is the underlying assumption of the tutorial as expressed by their fig 1.1

It is erroneous and misleading to regard the coronary system as being linear—that is, presenting the coronary pressure–flow relation as straight and passing through the origin of the pressure–flow plot. The course of such a relationship implies that coronary …