Contrast index of microcirculatory resistance (cIMR): a novel index of coronary microvascular resistance

Background The Index of Microvascular Resistance (IMR) has emerged as a pivotal index for characterizing the specific endotype of patients with ischemic heart disease and also for predicting their prognosis. However, this index can be calculated only using pressure wires equipped with a thermodilution sensor and using a vasodilator drug, such as adenosine, to induce maximal hyperemia. Aims Our objective is to evaluate the feasibility and the diagnostic accuracy of a novel index, "contrastIMR" for assessing microvascular resistance. Methods We prospectively enrolled consecutive patients with a suspected chronic coronary syndrome (CCS) undergoing who a invasive coronary functional testing according to the #FullPhysiology protocol. Fractional Flow Reserve (FFR) and Thermo-IMR served as the standard of reference. cIMR was calculate combining invasive and angiographic findings using the following formula: [contrast distal pressure (cPd) * transit time (Tt)]. cPd was obtained as the lowest Pd value after contrast medium injection; Tt was estimated from the same angiogram as the ratio between the number of frames needed for contrast dye to travel from the guiding catheter to a distal reference and the acquisition rate. cIMR and Thermo-IMR were estimated and correlated each other. Results Analysis included 94 patients accounting for 100 lesions. The majority (87%) exhibited no significant epicardial stenoses (FFR>0.80) during the index procedure, with the left anterior descending (LAD) artery being the most frequently assessed (98%). Median IMR and cIMR values were 22.0 (14.0, 31.0) and 37.0 (31.7, 45.4) units, respectively. cIMR demonstrated a strong correlation (r=0.86; p<0.001) and excellent discriminatory power [AUC 0.92, CI (0.87-0.97)] compared to thermo-IMR. The optimal cut-off value of cIMR, assessed by the Youden Index, was 38, showing an accuracy of 85.0%, a positive predictive value (PPV) of 81.2% (95%CI, 74.3- 89.4%) and a negative predictive value (NPV) of 87.5% (95% CI, 81.0-94.0%) in detecting high resistance compared to thermo-IMR (≥25 U). Conclusion this study demonstrates that cIMR is not only feasible but also very accurate for assessing microvascular resistance and provides a valid surrogate for IMR, that can be calculated with any available pressure wire without the need of a vasodilator drug.correlation between cIMR and IMR Roc curve