Fractional flow reserve (FFR) and index of microcirculatory resistance (IMR) relationship in patients with chronic or stabilized acute coronary syndromes

Aims: To investigate the influence of index of microcirculatory resistance (IMR) on fractional flow reserve (FFR) and adenosine-induced hyperemia (ΔPd/Pa-FFR) in patients with chronic (CCS) or stabilized acute coronary syndromes (ACS), utilizing various IMR threshold values. Methods: Data were extracted from two ongoing Italian registries involving patients with CCS or stabilized ACS who underwent a #FullPhysiology approach [Pd/Pa, FFR, IMR, coronary flow reserve (CFR)] by bolus thermodilution technique in the left anterior descending artery. Correlations between IMR and both FFR and ΔPd/Pa-FFR were analyzed both globally and within three IMR-defined groups: Group 1 (IMR <25), Group 2 (25 ≤ IMR <40), and Group 3 (IMR ≥40). A multiple linear regression was employed to adjust for confounding factors. Results: Of 275 patients, 163 were in Group 1, 60 in Group 2, and 52 in Group 3. Globally, a weak but significant correlation was observed between IMR and both FFR (r = 0.170, p < 0.01) and ΔPd/Pa-FFR (r = −0.159, p < 0.01). After stratification only patients in Group 3 exhibited a significant and more pronounced, though still weak, correlation between FFR and IMR (r = 0.387, p < 0.01) coupled with a reduction of ΔPd/Pa-FFR as IMR values increased (r = −0.411, p < 0.01). After adjustment, a significant increase of 0.01 in FFR values was observed for every 14-unit rise in IMR (p < 0.01) in Group 3. Conclusions: FFR appears particularly influenced by elevated IMR values (≥40) highlighting the importance of the #FullPhysiology approach in understanding epicardial and microvascular contribution of myocardial ischemia.