Calculation of the Index of Microcirculatory Resistance Without Coronary Wedge Pressure Measurement in the Presence of Epicardial Stenosis

ObjectivesThis 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 (Pw).BackgroundThe 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 (FFRcor), which requires balloon dilation within the coronary artery for Pw measurement.MethodsA method to calculate IMR by estimating FFRcor from myocardial FFR (FFRmyo), which does not require Pw 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.ResultsFrom the derivation cohort, a strong linear relationship was found between FFRcor and FFRmyo (FFRcor = 1.34 × FFRmyo − 0.32, r2 = 0.87, p < 0.001) by regression analysis. With this equation to estimate FFRcor in the validation cohort, there was no significant difference between IMR calculated from estimated FFRcor and measured FFRcor (21.2 ± 12.9 U vs. 20.4 ± 13.6 U, p = 0.161). There was good correlation (r = 0.93, p < 0.001) and agreement by Bland-Altman analysis between calculated and measured IMR.ConclusionsThe FFRcor, and, by extension, microcirculatory resistance can be derived without the need for Pw. This method enables assessment of coronary microcirculatory status before or without balloon inflation, in the presence of epicardial stenosis.