Coronary flow characteristics in cases with discordance between coronary flow reserve and the index of microvascular resistance in the assessment of coronary microvascular dysfunction

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Abstract Background/Introduction Coronary flow reserve (CFR) and the index of microvascular resistance (IMR) are commonly used for the diagnosis of coronary microvascular dysfunction, but the two indices can reveal discordance. Purpose The aim of this study was to investigate the mechanisms of the discordance between CFR and IMR. Methods Haemodynamic data from patients who underwent invasive coronary functional assessment using a guidewire equipped with pressure and temperature sensors was analysed. We performed vessel-based analysis, and classified vessels with reference to the criteria of IMR ≥25 and CFR <2.0 as pathological into four groups: normal IMR/normal CFR (Group 1: G1), abnormal IMR/normal CFR (Group 2: G2), normal IMR/abnormal CFR (Group 3: G3), and abnormal IMR/abnormal CFR (Group 4: G4). Results Of 249 vessels form 176 patients, 82 (33%) were classified into G1, 28 (11%) into G2, 94 (38%) into G3, and 45 (18%) into G4. Median and interquartile ranges of IMR of the four groups were 14.3 [10.7–18.9], 34.2 [29.8–45.1], 16.9 [12.7–19.7], and 30.7 [28.4–43.5] (P <0.001), respectively, and those of CFR were 3.21 [2.48–4.29], 2.73 [2.34–3.03], 1.27 [1.06–1.55], and 1.28 [1.07–1.67] (P <0.001), respectively (Figure). In G2, hyperemic flow velocity was as low as G4 (1.97 [1.47–2.29] vs. 1.95 [1.41–2.50] sec-1, P=1.00), and the baseline flow velocity was lower than G4 (0.71 [0.49–0.85] vs. 1.44 [0.93–1.96] sec-1, P <0.001), leading to apparently normal CFR despite the diminished hyperemic flow. In G3, the hyperemic (3.60 [2.77–4.73] vs. 1.95 [1.41–2.50] sec-1, P <0.001) and the baseline flow velocity (2.85 [2.19–3.92] vs. 1.44 [0.93–1.96] sec-1, P <0.001) were higher than G4, leading to apparently decreased CFR despite the maintained hyperemic flow. Conclusion(s) The discordance between IMR and CFR can be explained by decreased coronary flow in abnormal IMR/normal CFR, and by increased baseline coronary flow in normal IMR/abnormal CFR.