Functinal assessment in Angina and Non-Obstructive Coronary Arteries (ANOCA) patients: from Microvascular Resistance Reserve (MRR) to different subtypes of Coronary Microvascular Dysfunction (CMD)

Abstract Background Coronary microvascular dysfunction (CMD) is a heterogeneous condition defined by a reduced coronary flow reserve (CFR) and low/high values of index of microvascular resistance (IMR). A new index, the microvascular resistance reserve (MRR) has been developed, but its clinical significance is unclear. Purpose We aimed to investigate the relationships between functional indices in ANOCA (Angina and No Obstructive Coronary Arteries) patients and to assess the hemodynamic and clinical characteristics of different CMD subtypes. Materials and methods We prospectively enrolled consecutive ANOCA patients who underwent a comprehensive functional assessment of microvascular domain by thermodilution technique. CFR, IMR and MRR were estimated and correlated each other. Patients were divided in two groups according to the presence of CMD (defined by a CFR<2.5). Subsequently High Hyperaemic Resistance (HHR) and Low Hyperaemic Resistance (LHR) CMD subtypes were defined according to low or high IMR values respectively (cut-off 25). Microvascular flow and resistance were estimated both at rest and during hyperaemia with Tmnrest /IMRrest and Tmnhyp/IMR respectively. All functional indices were compared between groups. Results one hundred and eight patients were available for the analysis. 66 patients in the Normal Group (CFR≥2.5), 20 patients in HHR CMD (CFR<2.5 and IMR≥25), 22 patients in LHR CMD (CFR<2.5 and IMR<25). MRR showed a strong correlation (r 0.966, p <0.01) with CFR, while a mild and negatively correlation was found between CFR and IMR (r -0.242, p=0.01) and MMR and IMR (r -0.261, p<0.01). MRR showed a good discriminatory power (AUC 0.97, 95% CI 0.94 – 0.99) and accuracy (85%) in detecting CMD when compared to CFR. LHR CMD patients showed reduced microvascular resistance (IMRrest 34.3±15.1 vs 90.1±54.5 Normal Group vs 75.8±28.9 HHR CMD, p<0.01) and increased flow at rest (Tmnrest 0.37±0.17 vs 0.96±0.62 Normal Group vs 0.81±0.43 HR CMD, p<0.01), while HHR CMD patients had impaired flow during hyperemia (Tmnhyp 0.45±0.24 vs 0.26±0.18 Normal Group vs 0.21±0.07 LR CMD p<0.01). MRR was reduced in CMD patients (Normal 4.70±1.78 vs CMD 2.20±0.59, p<0.01), with no differences between CMD subtypes (HHR 2.17±0.47 vs LHR CMD 2.24±0.70, p=0.66). Conclusions In INOCA patients, MRR and CFR are strongly correlated and could be considered as functionally interchangeable tools. LHR CMD subtype is characterized by a pathological low resistance and high flow at rest while HHR CMD presented an impaired flow during hyperaemia.