Invasive diagnosis of coronary microvascular dysfunction: a comparison between microvascular resistance reserve (MRR) and coronary flow reserve (CFR)

The assessment of INOCA/ANOCA patients requires the measurement of Coronary Flow Reserve (CFR) and Index of Microcirculatory Resistance (IMR), while Acetylcholine (Ach) test allows for the evaluation of coronary vasomotor function. In this way, the functional coronary angiography (FCA) allows to stratify INOCA/ANOCA patients in different endotypes including Coronary Microvascular Dysfunction (CMD), Vasospastic Angina (VSA), a combined class (CMD and VSA) or Non-Cardiac Chest Pain (NCCP). Microvascular Resistance Reserve (MRR) is recently emerging as a new index to assess coronary microvascular dysfunction. Aim of the study was to evaluate the diagnostic accuracy of MRR as compared with CFR/IMR to correctly identify INOCA patients.

Patients presenting with typical angina and evidence of myocardial ischemia undergoing invasive functional coronary angiography (FCA) were enrolled in our Institution. During the FCA arterial pressure (Pa), distal pressure (Pd) and mean Transit Time both at rest (mTTREST) and during maximal hyperemia (mTTHYP) were measured. The latter was achieved by intravenous administration of adenosine (140 μg/kg/min). IMR and MRR were finally calculated. Ach test was performed with incremental i.c. boluses of Acetylcholine ranging from 20mg up to 200mg.

A total of 77 patients were enrolled. Mean age was 61±10 years and female patients were mostly represented (57%). INOCA endotypes stratification according to the conventional functional assessment (CFR/IMR) led to CMD identification in 33 (42%) patients, VSA in 19 (25%) patients and NCCP in 25 (33%) patients. Among CMD patients, compensated CMD (C-CMD), structural CMD (S_CMD) and functional CMD (F_CMD) were identified respectively in 19 (58%), 8 (24%) and 6 (18%) patients. INOCA endotypes stratification according to the solely MRR evaluation led to CMD identification in 23 (30%) patients, VSA in 26 (34%) patients and NCCP in 28 (36%) patients. No further differentiation was possible among CMD patients without considering the IMR value. INOCA endotypes stratification according to MRR/IMR combination led to a significantly higher CMD identification (38 patients, 49%) and a lower NCCP proportion (20 patients, 26%) as compared with the conventional assessment (p < 0.05; Figure 1). In addition, among CMD patients, compensated CMD (C-CMD), structural CMD (S_CMD) and functional CMD (F_CMD) were identified respectively in 13 (34%), 10 (26%) and 15 (40%) patients and significantly changed as compared with the conventional approach (p < 0.05; Figure 2).

The combined use of IMR and MRR for invasive functional coronary angiography (FCA) could improve INOCA endotype diagnosis, clinical management and prognostic stratification potentially leading to a more effective targeted therapeutic strategy.

CMD and NCCP reclassification