Calculation of the index of microcirculatory resistance without coronary wedge pressure measurement in the presence of epicardial stenosis

Objectives This study sought to investigate a novel method to calculate the index of microcirculatory resistance (IMR) in the presence of significant epicardial stenosis without the need for balloon dilation to measure the coronary wedge pressure (P w ). Background The IMR provides a quantitative measure of coronary microvasculature status. However, in the presence of significant epicardial stenosis, IMR calculation requires incorporation of the coronary fractional flow reserve (FFR cor ), which requires balloon dilation within the coronary artery for P w measurement. Methods A method to calculate IMR by estimating FFR cor from myocardial FFR (FFR myo ), which does not require P w measurement, was developed from a derivation cohort of 50 patients from a single institution. This method to calculate IMR was then validated in a cohort of 72 patients from 2 other different institutions. Physiology measurements were obtained with a pressure-temperature sensor wire before coronary intervention in both cohorts. Results From the derivation cohort, a strong linear relationship was found between FFR cor and FFR myo (FFR cor = 1.34 × FFR myo − 0.32, r 2 = 0.87, p cor in the validation cohort, there was no significant difference between IMR calculated from estimated FFR cor and measured FFR cor (21.2 ± 12.9 U vs. 20.4 ± 13.6 U, p = 0.161). There was good correlation (r = 0.93, p Conclusions The FFR cor , and, by extension, microcirculatory resistance can be derived without the need for P w . This method enables assessment of coronary microcirculatory status before or without balloon inflation, in the presence of epicardial stenosis.