The diagnostic and therapeutic strategies in patients with coronary artery disease (CAD) have focused on identifying and alleviating both extent and severity of myocardial ischemia as it is the most important prognosticator. Thus, fractional flow reserve (FFR) has been a standard method for identifying ischemia-related epicardial coronary stenosis, accruing an abundance of clinical evidence on the benefit of FFR-guided treatment decisions. However, a high FFR value (>0.80) does not necessarily imply freedom from future events. Indeed, clinical events still occur in patients who are deferred based on high FFR.7 The microvasculature is one of the main components of coronary circulatory system, and the presence of microvascular disease can be the cause of clinical events in patients without epicardial coronary stenosis. In a cardiac catheterization laboratory, its presence can be assessed using a single pressure/temperature-sensor coronary wire or a Doppler wire. Previous studies have demonstrated the added prognostic implications of coronary flow reserve (CFR) and index of microcirculatory resistance (IMR) in patients with high FFR, and the recent European guidelines supported the importance of invasive physiologic assessment using CFR and IMR in patients with stable CAD. Furthermore, recent Expert Consensus Documents and the European Society of Cardiology guideline of Chronic Coronary Syndrome underlined an importance of evaluating coronary microvascular disease (CMD) in patients with ischemic heart disease (IHD) and proposed an universal definition of CMD based on 1) functionally non-obstructive CAD defined by a FFR>0.80 and 2) impaired coronary microvascular function determined by abnormal CFR and/or microvascular resistance.~Another important issue in contemporary practice is how to improve patient's prognosis after percutaneous coronary intervention (PCI). Previous trials demonstrated that intravascular imaging-guided PCI optimization has significantly better clinical outcomes than angiography-only guided PCI. However, previous trial were limited with small sample size, dealt with very selected lesion subsets such as chronic total occlusion or long lesion, or could not explain the exact mechanism that can explain the potential benefit of intravascular imaging-guided PCI optimization for better clinical outcome. Although the fundamental purpose of PCI is to resolve inducible myocardial ischemia originated from the epicardial coronary stenosis, several studies have demonstrated that a substantial proportion of patients who underwent angiographically successful PCI had suboptimal post-PCI FFR18-20 or non-hyperemic pressure ratios that was independently associated with worse clinical outcomes. Previous studies demonstrated that intravascular imaging devices could identify correctable cause of suboptimal post-PCI FFR. In this regard, it can be expected that intravascular imaging-guided PCI optimization would have better post-PCI physiologic results such as higher post-PCI FFR and CFR, compared with angiography-only guided PCI.~However, the abovementioned 2 important issues have not been fully clarified. For the first issue regarding the prognostic impact of CMD, only limited data has been available on the prognostic implications of CMD defined by the universal definition among patients with IHD, especially in patients with insignificant epicardial coronary disease defined by FFR>0.80. For the second issue regarding the potential physiologic benefit of intravascular imaging-guided PCI optimization, only 1 prospective study evaluated optical coherence tomography (OCT)-guided PCI for post-PCI FFR in patients with non-ST segment elevation myocardial infarction. None of prospective study evaluated potential physiologic benefit of intravascular imaging-guided PCI optimization using intravascular ultrasound (IVUS) or OCT in unselected patient population.~Therefore, the primary objectives of the current multicenter prospective registry are 1) to evaluate prognostic implications of CMD in patients with IHD undergoing revascularization decision using FFR or other non-hyperemic pressure ratios 2) to evaluate physiologic benefit of intravascular imaging-guided PCI optimization over angiography-only guided PCI.
MulticenterFlow is a prospective, multi-center, registry study.~The aim of the study is twofold:~To evaluate prognostic implications of coronary microvascular disease (CMD) in patients with ischemic heart disease (IHD) undergoing revascularization decision using fractional flow reserve (FFR) or other non-hyperemic pressure ratios in deferred population~To evaluate the efficacy of intravascular imaging-guided optimization to enhance post-revascularization coronary circulatory function, compared with angiography-only guided revascularization in revascularized population.