45 Impact of sex on the assessment of the microvascular resistance reserve INTRODUCTION The vasodilator capacity of the coronary circulation plays a crucial role in the development of myocardial ischemia and its clinical sequelae.1 It is increasingly recognized that an abnormal vasodilator capacity portends important clinical and prognostic value in patients with chronic coronary syndromes (CCS), even in the absence of epicardial coronary stenosis.2 Consequently, the diagnostic work-up of these patients is more frequently geared towards the assessment of the vasodilator capacity as part of comprehensive evaluation of chest pain syndromes. Recently, the microvascular resistance reserve (MRR) was introduced as a novel index to assess the vasodilator capacity of the coronary circulation.3 The MRR corrects the traditional index of coronary flow reserve (CFR) for the presence of concomitant epicardial disease and the hemodynamic effects of the administration of potent vasodilators. By this capacity, MRR allows assessment of microvascular vasodilator function in the presence of epicardial coronary stenosis, and addresses intrinsic limitations of CFR while providing an index that can be derived by either the bolus or continuous thermodilution and Dopplerflow technique. Following these theoretical advantages, MRR was documented to add diagnostic and prognostic value over CFR for the assessment of microvascular vasodilator dysfunction.4 However, the hemodynamic characteristics of women merit specific consideration. In women, the fractional flow reserve (FFR) is generally higher, and the vasodilatory capacity (CFR) is generally lower.5 As such, correcting the CFR for FFR might affect the applicability of MRR specifically in women. Unfortunately, limited data on the sex-specific considerations in the assessment of the MRR exists. The aim of this study was to assess the diagnostic and prognostic applicability of the MRR in women and assess the potential differences across different sexes. METHODS Study population The ILIAS (Inclusive Invasive Physiological Assessment in Angina Syndromes) registry is a retrospective global, multi-center initiative pooling vessel-level coronary pressure and flow data, as well as vessel level clinical outcome data. All studies included were approved by local medical ethics committees. The registry is composed of 20 expert medical institutes from the Netherlands, Korea, Japan, Spain, Denmark, Italy, and the United States of America. All data were gathered in local study protocols between 1998 and 2018. Patients who underwent clinically indicated invasive coronary angiography and comprehensive invasive physiological assessment of at least one native coronary 3
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