The validation and clinical applicability of angiography-derived assessment of coronary microcirculatory resistance: a [15O]H2O PET study

Background The introduction of wire-free microcirculatory resistance index from functional angiography (angio-IMR) promises swift detection of coronary microvascular dysfunction, however it has not been properly validated. We sought to validate angio-IMR against invasive IMR and PET derived microvascular resistance (MVR). Moreover, we studied if angio-IMR could aid in the detection of ischemia with non-obstructive coronary arteries (INOCA). Methods In this investigator-initiated study symptomatic patients underwent [ 15 O]H 2 O positron emission tomography (PET) and invasive angiography with 3-vessel fractional flow reserve (FFR). Invasive IMR was measured in 40 patients. Angio-IMR and QFR were computed retrospectively. MVR was defined as the ratio of mean distal coronary pressure to PET derived coronary flow. PET and QFR/angio-IMR analyses were performed by blinded core labs. The right coronary artery was excluded. Results A total of 211 patients (mean age 61±9, 148 (70%) male) with 312 vessels with successful angio-IMR analyses were included. Angio-IMR correlated moderately with invasive IMR (r=0.48, p<0.01), whereas no correlation was found between angio-IMR and MVR (r=-0.07, p=0.25). Angio-IMR did not differ for vessels without obstructive coronary artery disease (CAD) (FFR-) but with reduced stress perfusion (PET+) compared to vessels without obstructive CAD (FFR-) with normal stress perfusion (PET-) (median 28.19 IQR 20.42 – 38.99 vs 31.67 IQR 23.47 – 40.63, p=0.40). Conclusion Angio-IMR correlated moderately with invasively measured IMR, whereas angio-IMR did not correlate with PET derived MVR. Moreover, angio-IMR was similar in patients without obstructive CAD, irrespective of ischaemia status, hampering the identification of INOCA.