Functional assessment in angina and non-obstructive coronary arteries: from microvascular resistance reserve to subtypes of coronary microvascular dysfunction

Aims Coronary microvascular dysfunction (CMD) is a heterogeneous condition defined by reduced coronary flow reserve (CFR). The new index 'microvascular resistance reserve' (MRR) has been developed, but its role is unclear. We investigate the relationships between functional indices in ANOCA (angina and non-obstructive coronary arteries) patients and evaluate the hemodynamic features of different CMD subtypes. Methods We enrolled consecutive ANOCA patients assessed by using the bolus thermodilution technique. CFR, index of microcirculatory resistance (IMR) and MRR were estimated and correlated with each other. Patients were divided into two groups based on CMD presence (CFR < 2.5). Subsequently, high-hyperaemic-resistance (HHR) and low-hyperaemic-resistance (LHR) CMD subtypes were defined according to IMR values (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 In total, 108 patients were enrolled: 66 patients in the normal group (CFR ≥ 2.5), 20 in the HHR-CMD group (CFR < 2.5 and IMR ≥ 25) and 22 in the LHR-CMD group (CFR < 2.5 and IMR < 25). MRR strongly correlated (r = 0.968, P < 0.01) with CFR, showing a good discriminatory power (area under the curve = 0.97) and accuracy (85%) for detecting CMD. LHR-CMD patients showed reduced microvascular resistance (IMRrest 34.3 ± 15.1, P < 0.01) and increased resting flow (Tmnrest 0.37 ± 0.17, P < 0.01), while HHR-CMD patients had impaired hyperaemic flow (Tmnhyp 0.45 ± 0.24 P < 0.01). MRR was reduced in CMD patients (P < 0.01), with no differences between CMD subtypes (P = 0.66). Conclusions In ANOCA patients, MRR and CFR are strongly correlated and could be considered as functionally interchangeable tools. IMR is crucial for differentiating CMD endotypes.