Coronary microvascular dysfunction (CMD), once relegated to the periphery of cardiovascular medicine, is now recognized as a major determinant of outcomes following ST-segment elevation myocardial infarction (STEMI). While epicardial reperfusion remains the procedural target, resistance within the coronary microcirculation—an invisible yet critical determinant of myocardial perfusion—has emerged as a key driver of infarct size, adverse remodeling, heart failure, and recurrent ischemic symptoms. Invasive physiologic indices, most notably coronary flow reserve (CFR) and the index of microcirculatory resistance (IMR), have established strong prognostic validity in this setting. ,, Yet, their routine use remains limited by cost, workflow disruption, the need for pressure wires and hyperemia, and operator familiarity. Angiography-derived alternatives therefore represent an appealing evolution: the possibility of physiologic insight without additional intracoronary instrumentation.
In a recent issue of The American Journal of Cardiology , Benvenuto and colleagues present a single-center, retrospective study evaluating an angiography-derived IMR (AngioIMR) in 180 patients undergoing successful primary percutaneous coronary intervention (PCI) for anterior STEMI. Using standard angiographic data, AngioIMR was calculated offline as the product of mean arterial pressure (MAP), quantitative flow ratio (QFR), and TIMI frame count (TFC). Over a median 5-year follow-up, an AngioIMR threshold ≥43—similar to the invasive IMR cut-off of 40—identified patients at substantially higher risk of death, target-vessel myocardial infarction, or hospitalization for heart failure, despite restoration of TIMI 3 flow and nonischemic post-PCI QFR. Although overall event rates were low (∼9%), AngioIMR demonstrated high sensitivity and an excellent negative predictive value, suggesting potential utility as a rule-out tool for adverse outcomes following apparently successful reperfusion.
Wire-based IMR, measured using thermodilution during maximal hyperemia, has emerged as the reference standard for invasive assessment of microvascular resistance. Its strengths are well established: relative independence from epicardial stenosis, reproducibility, and consistent associations with infarct size, microvascular obstruction, and long-term clinical outcomes after STEMI. However, IMR requires pressure-wire instrumentation, adenosine administration, and additional procedural time—factors that have limited its adoption outside specialized centers.
AngioIMR seeks to approximate this physiology using computational modeling. Pressure is estimated via QFR, while flow is inferred from contrast transit time (TFC), allowing resistance to be calculated using an Ohm’s law framework. ,, The appeal is obvious: no pressure wire, no hyperemia, and no deviation from standard PCI workflow. Yet, this convenience comes at the cost of additional assumptions. Unlike wire-based IMR, AngioIMR relies on surrogate estimates of both pressure and flow, each influenced by image quality, acquisition angles, heart rate, contrast injection technique, and systemic hemodynamics. These distinctions underscore that AngioIMR is not simply “IMR without a wire,” but a related—though fundamentally modeled—index.
Importantly, AngioIMR cannot fully replicate the physiologic specificity of invasive measurements such as CFR, IMR, or microvascular resistance reserve (MRR, 8). CFR integrates both epicardial and microvascular compartments, while MRR captures vasodilatory capacity across physiologic states. By contrast, AngioIMR provides a single post-PCI snapshot under resting or near-resting conditions, without direct assessment of hyperemic reserve. As such, AngioIMR should be viewed as complementary to, rather than a replacement for, comprehensive wire-based physiologic interrogation.
The principal strength of this study lies in its focused design. By restricting analysis to anterior STEMI patients with final TIMI 3 flow and QFR ≥0.80, the authors isolate microvascular injury from overt epicardial disease, reinforcing the concept that angiographic success does not ensure myocardial recovery. The derived AngioIMR cut-off of 43—slightly higher than the canonical invasive IMR threshold—raises the possibility of vessel-specific or context-dependent thresholds, an area warranting further investigation.
Several limitations, however, merit emphasis. The study is retrospective, single-center, and modest in size, with substantial attrition: fewer than half of screened patients were included, largely due to angiographic images unsuitable for QFR or TFC analysis. This degree of exclusion raises concerns regarding generalizability and underscores the need for prospective, standardized acquisition protocols if angiography-derived physiology is to scale beyond highly selected cohorts.
Additionally, AngioIMR rests on physiologic and mathematical assumptions that may be challenged in the acute coronary setting. Systemic blood pressure, intracoronary nitrates, sedatives, catecholamines, and contrast kinetics can all influence calculated resistance. Although patients in cardiogenic shock or requiring mechanical circulatory support were excluded, applicability across the full spectrum of STEMI care—including contemporary use of ventricular unloading or adjunctive therapies—remains uncertain. Exclusion of patients with final TIMI flow <3, while methodologically defensible, also limits insight into whether angiography-derived resistance adds value beyond visual flow assessment in the highest-risk population.
The observation that patients with elevated AngioIMR had higher admission troponin levels yet similar post-PCI and discharge values is particularly instructive. This dissociation reinforces the concept that CMD is not merely a surrogate for infarct size, but a distinct pathophysiologic entity influencing long-term outcomes, especially heart failure and recurrent angina.
At present, AngioIMR should be viewed as a promising risk stratification tool rather than an actionable therapeutic target. No intervention has yet been shown to reliably reverse established microvascular injury following STEMI. Nevertheless, a practical, wire-free index could meaningfully expand awareness of residual microvascular disease, identify high-risk patients for closer surveillance or clinical trial enrollment, and serve as a gateway to more selective use of invasive physiology.
Future prospective, multicenter studies comparing AngioIMR directly with wire-based IMR, CFR, MRR, and cardiac magnetic resonance markers of microvascular obstruction will be essential. As angiographic software evolves toward real-time, automated analysis, angiography-derived physiology may narrow—but not eliminate—the gap between visual assessment and true microvascular phenotyping.
For now, AngioIMR represents an important step toward democratizing coronary physiology by making the assessment of CFR along with microvascular resistances as available as coronary angiography. Its additional value lies not in replacing established invasive measures, but in allowing physicians to make a full physiologic evaluation to optimize patient care. Ultimately, expanding our coronary physiology tools and availability reminds us—quickly and pragmatically—that opening the artery is only the first step in restoring myocardial perfusion.
CRediT authorship contribution statement
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