FFR, or fractional flow reserve, is a test used during a heart catheterization to measure how much a narrowed coronary artery is actually restricting blood flow to the heart muscle. It produces a simple number between 0 and 1, where a healthy artery with no blockage scores 1.0 and lower values indicate more significant obstruction. The key cutoff is 0.80: a reading at or below that level means the narrowing is significant enough to consider treatment with a stent or other intervention.
How FFR Works
The core idea behind FFR is comparing blood pressure at two points in the same artery. A cardiologist threads a thin wire equipped with a tiny pressure sensor through the artery until it sits just past the narrowed segment. The wire measures blood pressure downstream of the blockage, while a separate reading captures the pressure upstream, near the opening of the artery. Dividing the downstream pressure by the upstream pressure gives the FFR value.
A completely open artery would deliver nearly all of its pressure downstream, producing an FFR close to 1.0. A significant blockage creates a pressure drop across the narrowed segment, pulling the FFR value lower. An FFR of 0.70, for example, means the narrowing is cutting blood delivery by roughly 30% compared to a healthy artery.
These measurements must be taken while blood flow is at its maximum, a state called hyperemia. To achieve this, the cardiologist administers a drug (most commonly adenosine) that dilates the tiny blood vessels downstream and forces the heart’s circulation to work at full capacity. This is important because at rest, even a moderately blocked artery can maintain normal-looking pressure. It’s only under peak demand that the blockage reveals its true impact. The hyperemic state is maintained for three to four minutes while readings are recorded. One practical note: caffeine can interfere with adenosine’s effect, so patients are typically asked to avoid coffee and caffeinated drinks before the procedure.
What the Numbers Mean
The widely adopted threshold for treatment decisions is an FFR of 0.80 or below. This cutoff was established through the FAME clinical trials and has been incorporated into major cardiology practice guidelines. At or below 0.80, the narrowing is considered functionally significant, meaning it’s restricting enough blood flow to potentially cause symptoms or damage during exertion.
There is some nuance in the range between 0.75 and 0.80, sometimes called the “grey zone.” Earlier research originally pegged the cutoff at 0.75 based on comparisons with stress tests, but values in that grey zone showed inconsistent results on noninvasive testing. The FAME trials simplified the decision by consolidating the cutoff at 0.80, which better predicted patient outcomes over time. Above 0.80, the narrowing is generally not restricting flow enough to warrant a stent, and medication alone tends to produce good results.
Why FFR Matters for Treatment Decisions
Before FFR became routine, cardiologists relied heavily on angiography, which shows how narrow an artery looks on an X-ray image. The problem is that visual appearance doesn’t always match functional impact. A blockage that looks 60% narrowed on an angiogram might restrict very little actual blood flow, while another that looks similar might be starving the heart muscle. FFR solves this by measuring the real physiological effect rather than estimating from an image.
The clinical evidence supporting FFR-guided decisions is strong. In the landmark FAME 2 trial, published in the New England Journal of Medicine, patients with stable coronary artery disease whose blockages tested positive on FFR were randomly assigned to receive either a stent plus medication or medication alone. The trial was stopped early because the difference was so clear: only 4.3% of patients in the stent group experienced a major cardiac event (death, heart attack, or emergency procedure), compared to 12.7% in the medication-only group. The gap was driven almost entirely by a dramatic reduction in emergency revascularization procedures, dropping from 11.1% to just 1.6%.
Equally telling was what happened to patients whose blockages tested negative on FFR. Those patients did well on medication alone, with only 3.0% experiencing a major event. In other words, FFR helped identify not just who needs a stent, but who can safely avoid one.
The Procedure From a Patient’s Perspective
FFR is performed during a cardiac catheterization, so if you’re having one, the FFR measurement can be done at the same time without a separate procedure. You’ll already have a catheter in place, typically inserted through the wrist or groin. The cardiologist advances the pressure-sensing wire through that catheter and into the coronary artery being evaluated.
When adenosine is given intravenously to induce maximum blood flow, you may feel brief chest tightness, flushing, or shortness of breath. These sensations are temporary and resolve within a minute or two after the drug is stopped. The measurement itself takes only a few minutes. The FFR value appears on a monitor in real time, giving the cardiologist an immediate answer about whether to proceed with a stent or leave the artery alone.
Noninvasive FFR From CT Scans
A newer approach called CT-derived FFR (sometimes written as FFR-CT) estimates fractional flow reserve from a standard CT heart scan, without the need for a catheter at all. Specialized software uses the CT images to build a 3D model of the coronary arteries and then simulates blood flow through them using computational fluid dynamics.
The accuracy is impressive. In a multicenter study, CT-derived FFR achieved 96% sensitivity and 92% specificity compared to the gold-standard invasive measurement, meaning it correctly identified the vast majority of significant blockages while rarely flagging normal arteries as abnormal. This technology can help avoid unnecessary catheterizations for patients whose blockages turn out to be functionally insignificant.
CT-derived FFR does have limitations. It’s not suitable for patients who have had coronary bypass surgery, and the quality of the CT scan itself must be high enough to produce reliable results. Factors like a very high heart rate, significant calcium buildup in the arteries, or severe obesity can reduce scan quality and make the analysis less reliable.
Alternatives: iFR and Other Resting Indexes
One downside of traditional FFR is the need for adenosine, which causes brief but uncomfortable side effects and adds time to the procedure. A newer measurement called instantaneous wave-free ratio (iFR) addresses this by calculating the pressure gradient across a blockage at rest, during a specific window of the heartbeat when the downstream blood vessels are naturally at their lowest resistance.
A large trial published in the New England Journal of Medicine found that iFR-guided treatment decisions produced outcomes similar to FFR-guided decisions. Because iFR doesn’t require adenosine, the measurement is faster, more comfortable, and avoids the rare patient who can’t tolerate the drug. Both FFR and iFR are now used in clinical practice, and the choice between them often comes down to the cardiologist’s preference and the specific clinical situation.