Shedding light on the gray zone

Over two decades have passed since Pijls et al. proposed that invasively-determined myocardial fractional flow reserve (FFR) could serve as an index of the functional severity of coronary artery stenosis (1). FFR is derived from the ratio of mean pressure at the distal epicardial coronary conduit of an atheromatous coronary lesion to that of the aortic root in pharmacologically induced hyperemia, and represents the very fraction of maximal myocardial blood flow (MBF) that can be maintained despite coronary artery stenosis. The theoretical normal value of FFR is 1.00 regardless of the patient, the specific vessel studied or concurrent hemodynamic changes. However, when the microcirculation remains intact, the FFR value falls with the progression of a flow-limiting coronary lesion and as an increasing amount of myocardium becomes supplied by a flow-limiting coronary artery.