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The interesting study by Stuijfzand et al1 in this issue of Circulation: Cardiovascular Imaging compares positron emission tomography (PET)-measured coronary flow reserve (CFR), maximum absolute stress perfusion (MBF) in cc/min/g, and relative CFR (RFR) for predicting pressure-derived fractional flow reserve (FFR) in patients undergoing coronary angiography for suspected CAD.2,3 RFR was determined as the ratio of regional MBF of stenotic arteries to MBF of nonstenotic arteries. The authors report no significant difference between flow-based RFR and pressure-derived FFR (Δ=0.04, paired P value 0.12) but imperfect agreement (standard deviation of difference =0.16, area under the curve 0.82, accuracy 82%). All flow measurements showed notable discordances with FFR, specifically illustrated by the RFR–FFR correlation and Bland–Altman analysis.See Article by Stuijfzand et al Combining MBF with RFR provided the best performance compared with FFR, likely because of its reflecting both relative and absolute perfusion abnormalities that integrate segmental stenosis, diffuse disease, and preserved microvascular vasodilation capacity for most mild-to-moderate stenosis. The authors conclude that Noninvasive estimation of FFR with quantitative perfusion PET by calculating RFR is feasible, yet only a nonsignificant trend toward slight improvement of diagnostic accuracy over hyperemic MBF assessment, with discussion of methodological variability and discordances among CFR, RFR, MBF, and FFR.Although their conclusions are appropriately specific and focused, their raw data suggests a more complex, nuanced message that makes this article interesting and important beyond a simplistic correlation of CFR, MBF, and RFR with invasive FFR. The authors’ conclusions reflect a binary view that either MBF or RFR or CFR as a single test is right or wrong, better or worse for defining complex stenosis severity compared with FFR for doing or not doing a procedure, rather than asking what the data tells us physiologically.The fluid dynamics of coronary stenosis, their associated …

Physiologic Stenosis Severity, Binary Thinking, Revascularization, and “Hidden Reality”