FFR and iFR: How We Decide Whether a Coronary Narrowing Actually Needs a Stent

A 62-year-old woman gets a coronary angiogram for chest pain. The angiogram shows a 60 percent narrowing in her left anterior descending artery. Some operators would stent it. Others wouldn't. The honest answer is that 60 percent on a picture doesn't tell us whether the narrowing is causing real flow limitation. So her interventional cardiologist passes a thin pressure wire across the lesion, gives a vasodilator, and measures the pressure drop. The number that comes back is 0.85, comfortably above the cutoff. The lesion isn't flow-limiting. No stent. The patient goes home on optimized medical therapy.

I'm Dr. Damian Rasch, a cardiologist in Encinitas. Fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR) are physiologic measurements that tell us whether a coronary narrowing is actually causing reduced blood flow to the heart muscle, as opposed to just looking impressive on an angiogram. The reason this matters is that a stent placed in a non-flow-limiting lesion doesn't help the patient, exposes them to procedure complications, and commits them to dual antiplatelet therapy and stent surveillance. Stenting only the lesions that are physiologically significant, while leaving the others alone, produces better outcomes than stenting based on visual appearance. This article walks through what FFR and iFR are, when they're used, what the numbers mean, and why they have changed how interventional cardiology is practiced.

The Problem with Visual Estimation

Coronary angiography produces beautiful pictures, but it's a 2-dimensional projection of a 3-dimensional vessel, and the visual estimate of stenosis severity is famously unreliable. Studies have shown that the same lesion can be read as 50 percent by one operator and 70 percent by another. Eccentric lesions look different from different angles. Reference vessel size affects the perception of how tight a narrowing is. A 60 percent narrowing in a small vessel may not limit flow at all, while a 60 percent narrowing in a large vessel may.

There's a deeper problem too. Visual stenosis severity correlates poorly with actual flow limitation. A lesion that looks 70 percent on angiogram may have a normal FFR. A lesion that looks 50 percent may have an abnormal FFR. The angiogram is a roadmap. The pressure measurement tells us whether traffic is flowing.

What FFR Actually Measures

Fractional flow reserve is the ratio of the maximum blood flow that can be achieved through a narrowed artery, compared to what flow would be if the artery were normal. It's measured by passing a thin pressure wire across the lesion, giving an IV vasodilator (usually adenosine) to maximally dilate the microvasculature, and dividing the distal pressure by the proximal pressure during peak hyperemia.

The number is unitless, ranging from 0 to 1. A normal artery has an FFR of 1.0. An FFR of 0.80 or below indicates that the lesion is causing significant flow limitation, and stenting it is likely to improve symptoms or outcomes. An FFR above 0.80 indicates the lesion isn't flow-limiting, and the patient is better served by leaving the lesion alone and managing the underlying disease medically.

The 0.80 cutoff comes from large randomized trials. Patients with FFR-guided stenting (stenting only the lesions below 0.80) had fewer adverse events than patients stented based on angiogram alone, even when angiogram showed disease that "looked tight enough to stent."

What iFR Measures

Instantaneous wave-free ratio is a similar physiologic measurement that doesn't require adenosine or other vasodilators. It uses the natural period of the heart cycle (the wave-free period in late diastole) when microvascular resistance is naturally low and stable. The pressure ratio during that window is calculated, with a cutoff of 0.89 indicating significant flow limitation.

iFR was developed because adenosine causes uncomfortable side effects (chest pressure, flushing, shortness of breath) that some patients find distressing. iFR avoids those side effects while providing similar diagnostic accuracy. Large trials including DEFINE-FLAIR and iFR-SWEDEHEART showed that iFR-guided revascularization is non-inferior to FFR-guided revascularization for clinical outcomes.

In modern interventional cardiology, both FFR and iFR are accepted, with the choice often depending on operator preference, patient comfort considerations, and lab workflow. Some operators use iFR as the first-line assessment and FFR for borderline cases.

When FFR or iFR Is Used

Intermediate Lesions on Angiogram

The classic indication is a lesion that looks 50 to 70 percent on angiogram, where the operator isn't sure whether it's flow-limiting. FFR or iFR resolves the question. Below the cutoff: stent. Above: leave alone. This is the most common use of physiologic assessment in the cath lab.

Multivessel Disease

Patients with disease in multiple coronary arteries pose a particular challenge. Stenting every visually narrowed segment exposes the patient to multiple stents, each with their own risks. FFR or iFR identifies which lesions are actually flow-limiting and need treatment. Trials including FAME and FAME 2 showed that FFR-guided revascularization in multivessel disease produced fewer stents per patient and better outcomes than angiography-guided stenting.

After Stable Negative Stress Test

Patients with significant chest pain and a negative or equivocal stress test sometimes get an angiogram that shows moderate disease. Whether to stent that disease is unclear without physiologic assessment. FFR or iFR clarifies whether the moderate disease explains the symptoms.

Tandem and Diffuse Disease

When multiple lesions are present in a single artery, FFR or iFR can identify which is the dominant flow-limiting lesion. Stenting just the dominant one sometimes restores adequate flow without committing the patient to stents in serial sites.

Left Main Disease

Significant left main disease usually warrants treatment, often surgical. But borderline left main lesions sometimes get FFR or iFR assessment to clarify whether they're flow-limiting. The cutoff for the left main is sometimes set slightly differently (FFR 0.80 to 0.85 range), reflecting the higher stakes of getting the decision wrong.

After Stenting

FFR can be used after stenting to assess whether the stent achieved adequate hemodynamic results. A post-stent FFR above 0.90 generally indicates a successful procedure. Lower values suggest residual disease that may need further intervention.

Limitations and Pitfalls

FFR and iFR aren't perfect. Several situations limit their accuracy or interpretation.

Microvascular Disease

If the microvasculature can't dilate normally, the maximum flow that can be achieved is limited regardless of epicardial stenosis severity. Patients with diabetes, longstanding hypertension, or other causes of microvascular dysfunction may have abnormal microvascular function that affects FFR interpretation. The coronary flow reserve (CFR) and microvascular resistance reserve (MRR), measured separately, can clarify whether microvascular disease is contributing.

Recent Heart Attack

In the days following an acute myocardial infarction, the microvasculature in the infarcted territory is dysfunctional, and FFR measurements may not accurately reflect epicardial disease severity. Many operators delay FFR-guided assessment of non-culprit lesions until the patient has stabilized weeks after the index event.

Severe Hypertrophy or Aortic Stenosis

Patients with severe LV hypertrophy or severe aortic stenosis have altered microvascular and macrovascular hemodynamics that can affect FFR interpretation. The values still provide useful information but require interpretation in clinical context.

Diffuse Disease

When a vessel has diffuse atherosclerosis throughout its length rather than focal lesions, FFR can show a gradual pressure drop that doesn't localize to a specific stentable lesion. In these cases, the physiologic measurement informs the conversation about medical therapy versus surgical revascularization rather than guiding focal stenting.

FFR-CT: The Non-Invasive Cousin

FFR-CT (also called HeartFlow FFR-CT or computational FFR) takes a coronary CT angiogram dataset and uses computational fluid dynamics to estimate the FFR of any narrowed segment, without requiring an invasive catheterization. The same 0.80 cutoff applies. FFR-CT has been validated in multiple trials and is now widely available with insurance coverage.

For patients with intermediate disease on CCTA, FFR-CT often resolves the question of whether further invasive evaluation is needed. A patient with moderate disease on CCTA and FFR-CT above 0.80 can usually be managed medically. A patient with FFR-CT below 0.80 may need invasive evaluation. FFR-CT has reduced unnecessary catheterizations in this population substantially.

What the Numbers Mean for Treatment

FFR or iFR results drive immediate decisions during the procedure.

FFR above 0.80 (or iFR above 0.89): the lesion isn't flow-limiting. The patient is better served by medical therapy alone. Stenting wouldn't improve outcomes and would expose the patient to procedure complications and antiplatelet therapy requirements.

FFR 0.75 to 0.80 (or iFR 0.86 to 0.89): borderline. Decision is individualized based on symptom burden, clinical context, and patient preference. Some operators stent in this range; others don't.

FFR below 0.75 (or iFR below 0.86): clearly flow-limiting. Stenting is likely to improve symptoms and outcomes.

FFR below 0.50: very severe. Strong indication for revascularization, with consideration of bypass surgery if multiple vessels are involved or surgical anatomy is favorable.

The Evidence That Changed Practice

Several large randomized trials drove the adoption of physiologic assessment in interventional cardiology.

FAME (2009) randomized patients with multivessel disease to FFR-guided versus angiography-guided revascularization. The FFR-guided group had fewer stents per patient and lower rates of major adverse cardiovascular events at one year and at follow-up.

FAME 2 (2012) randomized patients with stable coronary disease and at least one lesion with FFR below 0.80 to FFR-guided PCI plus medical therapy versus medical therapy alone. The PCI group had reduced urgent revascularization, suggesting that flow-limiting disease benefits from treatment beyond medical therapy alone.

DEFINE-FLAIR and iFR-SWEDEHEART (2017) compared iFR-guided to FFR-guided revascularization. iFR was non-inferior to FFR for clinical outcomes, establishing iFR as a clinically equivalent option with better patient comfort.

FAVOR III China (2021) and other trials have evaluated quantitative flow ratio (QFR), an angiography-derived computational measurement, with promising results. The technology continues to evolve.

Common Patient Questions

Will FFR or iFR delay my procedure?

Adding FFR or iFR to a coronary angiogram extends the procedure by 5 to 15 minutes per lesion. The added time is small relative to the diagnostic value, and most catheterization labs build it in routinely when intermediate lesions are present.

Will FFR cause uncomfortable symptoms?

FFR uses adenosine, which causes chest pressure, flushing, and shortness of breath that lasts about 30 seconds. Most patients tolerate it without trouble, though it's an unusual sensation. iFR avoids adenosine and the associated symptoms entirely, which is one of the reasons many operators prefer it.

If my FFR is normal, why am I still having chest pain?

FFR measures the impact of epicardial coronary disease on flow. If your FFR is above 0.80, the epicardial disease isn't the cause of your symptoms. Other possibilities include microvascular disease (which standard FFR doesn't directly assess), coronary vasospasm, non-cardiac causes (esophageal, musculoskeletal, pulmonary), or anxiety/somatic causes. Further workup may include microvascular function testing, provocation testing for vasospasm, or evaluation for non-cardiac causes.

If my FFR is abnormal, do I always need a stent?

Not always. An abnormal FFR is one input among several. The decision also considers symptom burden, the specific anatomy (some lesions are anatomically unsuitable for stenting), the option of bypass surgery, and the patient's overall situation. Most patients with abnormal FFR and significant symptoms benefit from revascularization, but the choice between PCI and CABG and the decision about how aggressively to treat are individualized.

Why didn't my last cardiologist do FFR?

FFR isn't always done, and the reasons vary. Sometimes the lesion is clearly significant on angiogram (90 percent narrowing) and FFR isn't needed. Sometimes the lesion is clearly not significant (less than 50 percent) and FFR isn't needed. Sometimes the operator prefers visual assessment, although this is becoming less common as evidence has accumulated. If you're being told a moderate lesion needs stenting based on angiogram alone, asking about FFR or iFR is reasonable.

What is FFR-CT and how is it different?

FFR-CT is a non-invasive estimate of FFR derived from a coronary CT angiogram dataset. It uses computational fluid dynamics to model the pressure drop across narrowed segments. The cutoff (0.80) and interpretation are similar to invasive FFR. FFR-CT can spare some patients from invasive catheterization when their CCTA shows moderate disease that turns out to not be flow-limiting.

Is FFR safe?

FFR is generally safe. The wire used for FFR is small and flexible. Complication rates from the FFR portion of the procedure (beyond the baseline risks of angiography) are very low. The transient symptoms from adenosine resolve within a minute. Allergic reactions to adenosine are rare. The risk-benefit calculation strongly favors FFR or iFR when intermediate lesions are present.

When to Escalate Care

Call 911 immediately for severe chest pain, shortness of breath, or symptoms that suggest acute coronary syndrome. FFR or iFR is for stable disease evaluation; acute presentations need different management.

Contact your cardiologist the same day for new chest pain, palpitations with chest pressure, or any concern about coronary symptoms in the days or weeks following an FFR-guided decision.

Schedule a clinic visit within one to two weeks for stable, predictable chest pain that you've been managing on your own. The conversation about whether FFR or iFR is appropriate happens in the context of a planned cardiac evaluation.

A Final Note From Me

Physiologic assessment of coronary lesions has changed how I think about angiography. The 60 percent lesion that looks impressive on a picture may have a normal FFR and not need any intervention. The 50 percent lesion that looks insignificant may have an abnormal FFR and benefit from stenting. The visual estimate just isn't reliable enough to guide decisions about who gets stented and who doesn't, and the evidence is now strong that FFR or iFR-guided decisions produce better outcomes than visual estimation alone.

If you've been told you have moderate coronary disease and you're trying to understand whether you need a stent, ask your cardiologist whether FFR, iFR, or FFR-CT could clarify the question. The answer is sometimes that the lesion is clearly significant or clearly not significant and the assessment isn't needed. But for the borderline cases that make up so much of clinical practice, physiologic assessment can spare you an unnecessary stent or, conversely, identify a lesion that needs treatment that visual estimation might have missed.

If you've had FFR or iFR-guided decisions made about your care, the most important thing is to understand what the numbers showed and what the next step is. A normal FFR with persistent symptoms doesn't mean nothing is wrong; it means the next step is investigating non-epicardial causes. An abnormal FFR with stenting means the lesion needed treatment, and your post-stent care plan (dual antiplatelet therapy duration, secondary prevention, surveillance imaging) becomes the focus. The numbers are most useful when they're put in clinical context with a cardiologist who knows your overall situation.

References

1. Tonino, Pim A. L., Bernard De Bruyne, Nico H. J. Pijls, et al. "Fractional Flow Reserve versus Angiography for Guiding Percutaneous Coronary Intervention." New England Journal of Medicine 360, no. 3 (2009): 213-224.

2. De Bruyne, Bernard, Nico H. J. Pijls, Bindu Kalesan, et al. "Fractional Flow Reserve-Guided PCI versus Medical Therapy in Stable Coronary Disease." New England Journal of Medicine 367, no. 11 (2012): 991-1001.

3. Davies, Justin E., Sayan Sen, Hakim-Moulay Dehbi, et al. "Use of the Instantaneous Wave-Free Ratio or Fractional Flow Reserve in PCI." New England Journal of Medicine 376, no. 19 (2017): 1824-1834.

4. Götberg, Matthias, Evald H. Christiansen, Ingibjörg J. Gudmundsdottir, et al. "Instantaneous Wave-Free Ratio versus Fractional Flow Reserve to Guide PCI." New England Journal of Medicine 376, no. 19 (2017): 1813-1823.

5. Norgaard, Bjarne L., Jonathon Leipsic, Sara Gaur, et al. "Diagnostic Performance of Noninvasive Fractional Flow Reserve Derived from Coronary Computed Tomography Angiography in Suspected Coronary Artery Disease." Journal of the American College of Cardiology 63, no. 12 (2014): 1145-1155.

6. Pijls, Nico H. J., Pim A. L. Tonino, William F. Fearon, et al. "Functional Measurement of Coronary Stenosis." Journal of the American College of Cardiology 59, no. 12 (2012): 1045-1057.

7. Lawton, Jennifer S., Jacqueline E. Tamis-Holland, Sripal Bangalore, et al. "2021 ACC/AHA/SCAI Guideline for Coronary Artery Revascularization." Journal of the American College of Cardiology 79, no. 2 (2022): e21-e129.

8. Xu, Bo, Jian-an Wang, Lefeng Wang, et al. "Diagnostic Performance of Angiography-Based Quantitative Flow Ratio Measurements for Online Assessment of Coronary Stenosis." Journal of the American College of Cardiology 70, no. 25 (2017): 3077-3087.

9. Fearon, William F., Bernard De Bruyne, Pim A. L. Tonino, et al. "Clinical Outcomes and Cost-Effectiveness of Fractional Flow Reserve-Guided Percutaneous Coronary Intervention in Patients with Stable Coronary Artery Disease: Three-Year Follow-Up of the FAME 2 Trial." European Heart Journal 38, no. 24 (2017): 1923-1930.

10. Curzen, Nick, Martin Rana, Ziyad Nicholas, et al. "Does Routine Pressure Wire Assessment Influence Management Strategy at Coronary Angiography for Diagnosis of Chest Pain? The RIPCORD Study." Circulation: Cardiovascular Interventions 7, no. 2 (2014): 248-255.

Published on damianrasch.com. The above information was composed by Dr. Damian Rasch, drawing on individual insight and bolstered by digital research and writing assistance. The information is for educational purposes only and does not constitute medical advice.