The following are key points to remember from this JACC review topic of the week on assessing microvascular dysfunction in angina with unobstructed coronary arteries:

1. Coronary microvascular dysfunction is a highly prevalent condition of both structural and functional coronary disorders in patients with angina and nonobstructive coronary artery disease (ANOCA).
2. Current diagnostic modalities to assess microvascular function are related to prognosis, but these modalities have several technical shortcomings and lack the opportunity to determine true coronary blood flow and microvascular resistance.
3. Intracoronary continuous thermodilution assessment of absolute coronary flow (Q) and microvascular resistance (R) was recently shown to be safe and feasible in ANOCA.
4. Abnormal flow and resistance values can be useful for diagnosis of microvascular dysfunction in patients with ANOCA. Recently the continuous thermodilution–derived coronary flow reserve (CFR) and microvascular resistance reserve (MRR) have become available.
5. CFR is the classical ratio of maximum blood flow and resting blood flow (and as such is confounded by epicardial disease), whereas MRR is the ratio of true resting MVR (which is not confounded by epicardial disease) and hyperemic MVR, and might be the most specific index of microvascular disease.
6. An important potential advantage is that CFR and MRR theoretically rely on baseline and hyperemic measurements in the same epicardial territory and therefore are independent of myocardial mass.
7. Continuous thermodilution data also have the potential to guide therapy in patients with coronary microvascular dysfunction.
8. Implementation of continuous thermodilution in more catheterization laboratories might improve our understanding of the true prevalence of coronary microvascular dysfunction within ANOCA and provide a better understanding of the underlying physiology of the different endotypes.
9. The next steps for continuous thermodilution will be the association of the individual parameters absolute coronary flow (Q), absolute coronary resistance (R), CFR, and MRR with clinical outcomes, and the demonstration of improvements in anginal complaints and clinical outcomes by individual patient-tailored (pharmacologic) care of microvascular dysfunction.
10. An ongoing randomized, controlled trial will assess the effect of calcium channel antagonists on vasomotor dysfunction endotypes at repeated invasive coronary function test compared with placebo (EDIT-CMD [Efficacy of Diltiazem to Improve Coronary Microvascular Dysfunction: A Randomized Clinical Trial]; NCT04777045).

Clinical Topics: Heart Failure and Cardiomyopathies, Prevention, Stable Ischemic Heart Disease, Vascular Medicine, Atherosclerotic Disease (CAD/PAD), Chronic Angina

Keywords: Angina Pectoris, Calcium Channel Blockers, Catheterization, Coronary Artery Disease, Hemodynamics, Hyperemia, Microvascular Angina, Myocardial Ischemia, Secondary Prevention, Thermodilution, Vascular Resistance

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