Visual Estimates of Coronary Slow Flow Are Not Associated with Invasive Wire-Based Diagnoses of Coronary Microvascular Dysfunction

Background: Coronary slow flow (CSF) by invasive coronary angiography is frequently understood to be an indicator of coronary microvascular dysfunction (CMD) in patients with ischemia with non-obstructive coronary arteries (INOCA). However, the relationship between visual estimates of CSF and quantitative wire-based invasive diagnosis of CMD is uncertain.Methods: We prospectively enrolled adults age ≥18 years with stable ischemic heart disease who were referred for invasive coronary angiography. Individuals with ≥50% epicardial coronary artery stenosis were excluded. Invasive coronary angiography was reviewed for CSF, defined as ≥3 cardiac cycles to opacify distal vessels with contrast. Coronary function testing was performed in the left anterior descending (LAD) coronary artery using bolus coronary thermodilution techniques to measure coronary flow reserve (CFR) and the index of microcirculatory resistance (IMR). Invasively determined CMD was defined as abnormal CFR (<2.5), abnormal IMR (≥25), or both.Results: Among 104 participants, the median age was 61.5 and 79% were female. The median CFR was 3.6 (IQR 2.5-4.7) and the median IMR was 21 (IQR 13.3-28.0). Overall, 24.0% of participants had abnormal CFR, 34.6% had abnormal IMR, and 48.1% had a final diagnosis of invasively determined CMD. CSF was present in 23 participants (22.1%). The proportions of patients with CMD (56.5% vs 45.7%, p=0.36), abnormal CFR (17.4% vs 25.9%, p=0.40) and abnormal IMR (43.5% vs. 32.1%, p=0.31) were not different in patients with versus without CSF.Conclusions: Among patients with INOCA, CSF was not associated with abnormal CFR, IMR, or either abnormal CFR or IMR. CSF is not a reliable angiographic surrogate of abnormal CFR or IMR as determined by invasive, wire-based physiology testing.