The interplay between coronary microvascular resistance, arterial reservoir function and coronary flow reserve: unravelling the significance of high microvascular resistance
Authors: Tas A , Alan Y , Kara Tas I , Turkmenoglu E , Yilmaz S , Umman S , Parker K H , Sezer M 
Published: October 2024

Abstract

Background
Beyond the observation that patients with low CFR indicating presence of coronary microvascular dysfunction (CMD) have an unfavourable prognosis compared to patients with preserved CFR, recent large multicentre trials [1-2] have shown that increased minimal hyperaemic microvascular resistance (HMR or IMR) ,neither on top of the low CFR nor alone, does not play a role as a prognostic factor for ischemia in nonobstructive coronary artery disease (INOCA) patients.

Purpose
This study investigates the pathophysiological interactive processes in CMD and no-CMD groups with high and low HMR taking into account potential adaptive mechanisms that alter arterial reservoir function [3] associated with microvascular resistance and systemic vasomotor response in INOCA patients.

Methods
Analysis was conducted on 312 unique unobstructed vessels (FFR&gt;0.80) from 258 patients (73% male) enrolled in the DEFINE-FLOW [4] study. CFR and HMR were quantified using intracoronary Doppler and pressure measurements. Vessels were categorized into four groups based on high and low CFR and HMR using the best available cut-off values (2.5 for both). Arterial reservoir capacity (ARC) and related parameters [3] were quantified using reservoir-excess pressure (Preservoir and Pexcess) analysis of averaged aortic and distal coronary pressure waveforms in CMD (CFR&lt;2.5) and no-CMD (CFR≥2.5) subgroups. (Figure 1-2)

Results
Compared to vessels without CMD, functional and structural CMD have increased Pexcess (p:0.007) and a rapid decline in diastolic pressure (DRCcoronary p&lt;0.001, DRCaortic, p&lt;0.05), indicating the ongoing systemic hemodynamic superfluous state despite locally suppressed resting blood flow in the structural CMD. Higher baseline microvascular resistance (BMR) was associated with increased reservoir pressure (r:0.302, p&lt;0.001). The vessels with preserved CFR despite high HMR are characterised by uniquely increased ARC (p:0.014), effectively diminished Pexcess (p:0.010), which parallels the local (highest BMR p&lt;0.001) and global (lowest DRCaortic, p&lt;0.05) vasomotor response, that normalizes the systemic and local hyperperfusion, and ultimately regains an adequate flow reserve (CFR) (p&lt;0.001). The high HMR in this group was mainly due to increased resting tonus unlike structural CMD, evident by the highest RRR (BMR/HMR).(p&lt;0.001) (Figure 1-2)

Conclusion(s)
The high HMR despite preserved CFR vessels form a unique-adaptive group, characterised by substantially increased arterial reservoir capacity and effectively diminished Pexcess, by means of an adaptive vasomotor response which manifests locally in the coronary bed as pronouncedly augmented BMR & RRR, and globally as reduced DRC in the aortic pressure waveform. This adaptive myogenic response normalising resting flow and ongoing systemic and local hyperperfusion ultimately regains an adequate flow reserve (CFR).Figure 1.Methods and Main FindingsFigure 2.Visual Abstract