#fractional-flow-reserve #coronary-physiology #coronary-artery-disease #pci-guidance #acute-coronary-syndrome

Fractional Flow Reserve (FFR)

Definition

Fractional flow reserve (FFR) is a pressure wire-based physiologic index measuring the ratio of maximum achievable coronary flow distal to a stenosis to the theoretical maximum without the stenosis, expressed as distal coronary pressure (Pd) divided by aortic pressure (Pa) during maximum hyperemia (induced by adenosine). A value of ≤0.80 is the established ischemic threshold warranting revascularization; values >0.80 favour medical management or deferral. FFR is validated as superior to angiography alone for identifying hemodynamically significant coronary stenoses in stable and chronic coronary artery disease, but its role in the ACS setting is more complex.

Key Concepts

Physiology

Requires pharmacologic hyperemia (IV or intracoronary adenosine) to minimise microvascular resistance and isolate the pressure gradient across the epicardial stenosis
FFR ≤0.80 = hemodynamically significant (ischemia-producing); FFR >0.80 = defer revascularization safely
Unlike quantitative coronary angiography, FFR accounts for lesion length, shape, location, and the downstream microvascular territory
Critical limitation: FFR measures current flow limitation only — it cannot detect plaque vulnerability (thin fibrous cap, large lipid core) or assess risk of future MI from plaque rupture

Validation in Stable and Chronic Coronary Artery Disease

DEFER trial: FFR-guided deferral safe in stable CAD — 5-year MACE comparable to normal coronary arteries when FFR >0.75
FAME trial (NEJM 2009): FFR-guided vs angiography-guided PCI in multivessel stable CAD — fewer stents, lower MACE at 1 year
FAME-2 trial (NEJM 2014): FFR-guided PCI vs medical therapy in stable CAD — lower urgent revascularization; no all-cause mortality benefit
Current guideline position: Class I recommendation (ESC) for intermediate stenoses 50–90% in stable/chronic CAD (sources/CCS-AHA-2023, rating: high)

FFR in ACS — Culprit vs Nonculprit Assessment

FFR of the culprit vessel acutely is unreliable: microvascular dysfunction elevates resistance → overestimates FFR → false-negative (may miss ischemia-producing stenosis)
FFR of nonculprit vessels is considered reliable acutely and comparable to repeat measurement at 35 days (WAVE study; Ntalianis 2010)
This distinction underpins all ACS FFR trials: culprit PCI is guided angiographically; only nonculprit assessment uses FFR

FFR-Guided Complete Revascularization in ACS — FULL REVASC Trial

Design: 1,542 patients with STEMI or very-high-risk NSTEMI and multivessel CAD; 1:1 FFR-guided complete revascularization vs culprit-only PCI; 32 centres, 7 countries; median 4.8-year follow-up (sources/ffrpci-mi-fullrevasc-nejm-2024, rating: very high)
Primary outcome (death/MI/unplanned revascularization): 19.0% vs 20.4%; HR 0.93 (95% CI 0.74–1.17); P=0.53 — not superior
Death or MI: HR 1.12 (NS); Unplanned revascularization: HR 0.76 (NS)
Any revascularization: 10.2% vs 16.5% (HR 0.59) — fewer total procedures with FFR strategy
Stent thrombosis: 2.5% vs 0.9% (HR 2.80) — significantly higher with FFR-guided complete revascularization
Key FFR finding: 40% of complete-revascularization patients had ALL nonculprit lesions deferred (FFR >0.80); only 47% of tested vessels had FFR ≤0.80
Trial terminated early (74% power) after COMPLETE trial publication (sources/ffrpci-mi-fullrevasc-nejm-2024, rating: very high)

FFR vs Angiography Guidance — A Critical Distinction in ACS

COMPLETE trial (NEJM 2019; n=4,041): Angiography-guided complete revascularization → 26% lower risk of CV death/MI; benefit driven by stenoses ≥80% by visual estimation or ≥60% by QCA; FFR used in <1% of patients
FULL REVASC trial (NEJM 2024; n=1,542): FFR-guided complete revascularization → neutral (HR 0.93, P=0.53) (sources/ffrpci-mi-fullrevasc-nejm-2024, rating: very high)
Mechanistic hypothesis for divergence: Angiography-guided approach treats high-grade stenoses (≥60–80%) that are not only flow-limiting but also harbor vulnerable plaques — FFR defers some of these because FFR >0.80, yet these plaques carry future MI risk that FFR cannot detect
COMPLETE OCT substudy confirmed vulnerable plaque features in nonculprit lesions, independent of stenosis severity
AIR-STEMI trial (NEJM 2026; n=1,823) — the direct comparison, and it reverses the inference: physiology-guided complete revascularization beat angiography-guided completion in STEMI (primary composite 8.9% vs 13.7%; HR 0.62; 95% CI 0.47–0.83; P<0.001), and was also safer (contrast-associated AKI or major bleeding 4.6% vs 7.1%; HR 0.63). Mortality did not differ. Caveat on transferability: AIR-STEMI tested functional coronary angiography (angiography-derived FFR, no pressure wire or hyperaemic agent, core-lab analysed), applied within 48 h of culprit PCI and used for both lesion selection and PCI planning via pullback — not wire-based FFR used as a binary gate (sources/AIR-STEMI-NEJM-2026, rating: very high)
Synthesis across the four trials — timing and implementation, not modality: FLOWER-MI applied wire FFR predominantly in staged procedures (96%) as a binary gate and was neutral; FULL REVASC applied wire FFR to guide completion vs culprit-only and was neutral; FRAME-AMI integrated physiology into the index procedure (60%) and improved outcomes, driven by procedure-related MI; AIR-STEMI used angiography-derived physiology at the index hospitalization for selection and planning and won decisively. The pattern suggests benefit requires integration into a predefined physiological strategy, not an isolated binary deferral decision. Whether the vulnerable-plaque argument (FFR >0.80 deferring plaques that still cause MI) survives AIR-STEMI — whose benefit was in MI — remains open
The ongoing COMPLETE-2 trial will provide a further direct physiology-vs-angiography comparison in ACS

FFR vs iFR

Both measure hemodynamic significance via intracoronary pressure wire
iFR uses resting diastolic pressure ratio (no adenosine); FFR uses hyperemic ratio (adenosine required)
Validated equivalent for MACE in stable CAD (DEFINE-FLAIR, iFR-SWEDEHEART, NEJM 2017)
iFR may have a higher false-positive rate (~11%) in STEMI due to elevated resting flow; FFR requires adenosine which may cause transient AV block, dyspnoea, chest discomfort
iFR-guided immediate complete revascularization in STEMI was also neutral vs deferred MRI-guided approach at 3 years (iMODERN, NEJM 2026; HR 0.95, P=0.81)
See concepts/Instantaneous-Wave-Free-Ratio

FFR vs Angiography-Based vFFR — FAST III Trial

Design: 2,235 patients with intermediate lesions (30–80% stenosis); chronic or acute coronary syndrome; 37 European centers; 1:1 vFFR-guided vs FFR-guided revascularization; 1-year follow-up (sources/vffr-fastiii-nejm-2026, rating: very high)
vFFR (vessel FFR; Pie Medical Imaging) derives an FFR equivalent from 3D quantitative coronary angiography using two angiographic projections — no intracoronary wire and no adenosine required; same ≤0.80 threshold as wire FFR
Primary endpoint (death/MI/revascularization at 1 year): 7.5% (vFFR) vs 7.5% (FFR); risk difference −0.02 pp (95% CI −2.25 to 2.21); P=0.004 for noninferiority — vFFR noninferior to FFR (sources/vffr-fastiii-nejm-2026, rating: very high)
vFFR identified more functionally significant lesions (40.9% vs 31.3%) and led to more revascularizations (45.0% vs 36.0%), yet clinical outcomes were equivalent; procedure time was ~5 min shorter with vFFR and intraprocedural complications were lower (3.7% vs 6.0%)
The higher lesion detection rate with vFFR reflects its reliance on assumed geometric flow rather than actual measured hyperemic flow — angiography-based tools do not account for microvascular resistance, demand, or patient-specific hyperemia responses
See concepts/Angiography-Based-Coronary-Physiology for full detail including FAVOR III Europe vs FAST III comparison

FFRangio vs Pressure-Wire Physiology — ALL-RISE Trial

Design: 1,930 patients with intermediate stenoses (50–90%), 59 international sites, 1:1 FFRangio (angiography-derived FFR, no wire or hyperaemic agent) versus pressure-wire assessment (FFR or a non-hyperaemic pressure ratio); 1-year follow-up; 3.5 percentage-point noninferiority margin (sources/ffrangio-allrise-nejm-2026, rating: very high)
Primary endpoint (death/MI/unplanned clinically indicated revascularization at 1 year): 6.9% vs 7.1%; HR 0.98 (95% CI 0.70–1.39); difference −0.2 percentage points; upper one-sided 97.5% CI 2.1 points; P<0.001 for noninferiority (sources/ffrangio-allrise-nejm-2026, rating: very high)
FFRangio classified 43.0% vs 37.0% of lesions as significant and led to more PCI (44.3% vs 35.4% of lesions; OR 1.45) with equivalent outcomes — the same direction as vFFR in FAST III (sources/ffrangio-allrise-nejm-2026, rating: very high)
Assessment was faster (6 vs 8 min), with shorter procedural time (39 vs 42 min), less fluoroscopy (9 vs 12 min) and less contrast (100 vs 105 mL) (sources/ffrangio-allrise-nejm-2026, rating: very high)
Unlike QFR in FAVOR III Europe, FFRangio met noninferiority; the trial cannot establish why one angiography-derived index succeeds where another fails (sources/ffrangio-allrise-nejm-2026, rating: very high)

Guideline Positions — FFR in ACS

ESC 2023 ACS Guidelines: Angiographic severity is the primary basis for nonculprit PCI in STEMI; functional invasive evaluation "may be considered" — not mandated; functional evaluation recommended for nonculprit lesions in NSTEMI
AHA/ACC 2025 ACS Guidelines: FFR "may be useful" to guide nonculprit PCI for intermediate stenoses in STEMI (Class IIa); complete revascularization remains Class I recommendation (without mandating FFR)
TAVI candidates with CAD: FFR ≤0.80 or ≥90% angiographic stenosis — PCI reduces MACE (NOTION-3, NEJM 2024) (sources/PCI-TAVI-NOTION3-NEJM-2024, rating: very high)

Contradictions / Open Questions

FFR-guided vs angiography-guided complete revascularization in ACS: COMPLETE shows angiography-guided strategy reduces CV death/MI (HR ~0.74); FULL REVASC shows FFR-guided strategy is neutral (HR 0.93, P=0.53). Whether FFR-guided deferral systematically under-treats patients with vulnerable high-grade lesions (FFR >0.80 but rupture-prone) is the central unanswered question. COMPLETE-2 (ongoing) will directly compare the two approaches (sources/ffrpci-mi-fullrevasc-nejm-2024, rating: very high)
Stent thrombosis excess (HR 2.80) with FFR-guided complete revascularization: Each additional nonculprit stent in an ACS patient carries thrombotic risk that may not be offset by ischemia prevention when FFR borderline. The 40% deferral rate in FULL REVASC still left 60% with additional stents, driving excess stent thrombosis without commensurate outcome benefit (sources/ffrpci-mi-fullrevasc-nejm-2024, rating: very high)
FIRE trial contradiction (NEJM 2023): Physiology-guided complete revascularization in elderly STEMI/NSTEMI (≥75yr) showed 36% lower CV death/MI at 1 year — directly contradicts FULL REVASC's neutral result at 4.8 years. Key differences: older/sicker FIRE population, multiple physiology tools (not FFR alone), shorter follow-up, and different composite endpoint dynamics
Optimal FFR threshold in ACS: The ≤0.80 cutoff was established in stable CAD; whether a lower threshold (e.g., ≤0.75) would improve specificity in the ACS setting and avoid the vulnerable-plaque problem remains unexplored
vFFR detects more lesions than FFR (FAST III): vFFR classified 40.9% vs 31.3% of lesions as functionally significant, driving a 9 pp higher revascularization rate (45% vs 36%) with equivalent outcomes. Whether this indicates vFFR better identifies prognostically relevant lesions, or systematically over-detects, adding stents without benefit, is unresolved — prespecified cost-effectiveness analysis pending (sources/vffr-fastiii-nejm-2026, rating: very high)
FFRangio repeats the pattern (ALL-RISE): a higher significant-lesion rate (43.0% vs 37.0%) and more PCI (44.3% vs 35.4% of lesions) with identical 1-year outcomes. Two independent angiography-derived indices now show the same "more lesions, same outcomes" behaviour, which may be a class effect of geometry-based flow modelling rather than a defect of one product — but whether the extra stents help or harm beyond 1 year is untested (sources/ffrangio-allrise-nejm-2026, rating: very high)
Which angiography-derived index to use is unresolved: QFR failed noninferiority (FAVOR III Europe), while vFFR (FAST III) and FFRangio (ALL-RISE) succeeded. No trial compares them head to head, and the explanations offered — computational method, operator dependence, image quality — are hypotheses rather than demonstrated mechanisms (sources/ffrangio-allrise-nejm-2026, rating: very high)

Connections

Related to concepts/Angiography-Based-Coronary-Physiology — wire-free alternatives; vFFR (FAST III) and FFRangio (ALL-RISE) noninferior to FFR, QFR (FAVOR III Europe) not
Related to concepts/Complete-Revascularization — FFR-guided nonculprit lesion selection in MI
Related to concepts/Instantaneous-Wave-Free-Ratio — resting pressure-wire alternative to FFR
Related to concepts/Multivessel-PCI-STEMI-Timing — FFR vs angiography guidance in STEMI nonculprit decisions
Related to concepts/PCI-Before-TAVI — FFR threshold used in NOTION-3 trial
Related to concepts/Myocardial-Viability — physiologic vs imaging approaches to assessing myocardial significance