Diagnostic and Prognostic Value of Resistive Reserve Ratio in Coronary Microvascular Dysfunction

Introduction to RRR

The European Society of Cardiology recommends using continuous thermodilution-derived measurements, including RRR, as a reliable diagnostic parameter for coronary microvascular dysfunction, with higher reproducibility than bolus thermodilution methods 1

Patients with coronary microvascular dysfunction have a 9.4% rate of death or myocardial infarction by 4 years, even without obstructive coronary disease 2
Impaired coronary flow reserve, which correlates with low RRR, is associated with elevated hazard for major coronary events at 10-year follow-up 3
Nonobstructive coronary artery disease with microvascular dysfunction increases risk of all-cause death and myocardial infarction 3

Index of microcirculatory resistance (IMR) is abnormal if ≥25 units, indicating structural microvascular disease, and should be interpreted alongside RRR 1, 4
Coronary flow reserve (CFR) is abnormal if <2.0, representing combined epicardial and microvascular function, and correlates with RRR 5, 6

The American Heart Association recommends beta-blockers, such as bisoprolol, as first-line antianginal therapy for patients with microvascular dysfunction (RRR 1.5), targeting a resting heart rate of 55-60 bpm 4
Ivabradine may be considered as a superior alternative to bisoprolol, demonstrating better effects on coronary collateral flow and coronary flow reserve 4
ACE inhibitors or ARBs, high-intensity statin therapy, and aspirin are recommended as baseline therapy for patients with microvascular dysfunction (RRR 1.5) 3, 4

A coronary flow reserve (CFR) of 1.6 is considered abnormal, as normal CFR should be ≥2.0-2.7, indicating impaired coronary flow reserve affecting both epicardial and microvascular circulation, according to the American College of Cardiology 7, 8, 9, 10
CFR is influenced by hemodynamics: tachycardia reduces CFR by 10% for each 15-beat increase in heart rate, so rate control is essential, as stated by the American Heart Association 7, 8, 9, 10

The American Heart Association and European Society of Cardiology recommend beta-blockers as first-line treatment for coronary microvascular dysfunction (CMD) with a vasospastic component, with a target resting heart rate of 55-60 bpm 7, 8, 9, 10
ACE inhibitors or ARBs are recommended to improve endothelium-dependent vasodilation in patients with CMD, according to the European Society of Cardiology 7, 8, 9, 10