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_wmeasurement. Methods: A method to calculate IMR by estimating FFR_corfrom myocardial FFR (FFR_myo), which does not require P_wmeasurement, 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_corand FFR_myo(FFR_cor= 1.34 × FFR_myo- 0.32, r²= 0.87, p < 0.001) by regression analysis. With this equation to estimate FFR_corin the validation cohort, there was no significant difference between IMR calculated from estimated FFR_corand 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 < 0.001) and agreement by Bland-Altman analysis between calculated and measured IMR. 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.