Fractional flow reserve (FFR) is a pressure measurement taken inside a coronary artery to determine whether a blockage is actually restricting blood flow to the heart muscle. A cardiologist threads a thin wire with a pressure sensor past the narrowed area, then compares the pressure downstream of the blockage to the pressure upstream. The result is a ratio between 0 and 1, where 1.0 means completely normal flow and lower numbers indicate greater restriction. An FFR of 0.80 or below is the widely accepted threshold for a blockage significant enough to benefit from treatment.
Why FFR Exists
Coronary angiography, the standard imaging used during a heart catheterization, shows the shape of a blockage but not how much it actually limits blood delivery. A narrowing that looks severe on camera may still allow adequate flow, and a moderate-looking blockage may be more restrictive than it appears. FFR fills that gap by measuring what the blockage physically does to blood pressure and, by extension, blood supply.
The distinction matters because treating a blockage that isn’t functionally significant (with a stent, for example) exposes a patient to procedural risk without meaningful benefit. The landmark FAME trial demonstrated this clearly: when doctors used FFR to decide which blockages to stent rather than relying on angiography alone, the rate of major adverse cardiac events dropped by roughly 30% over two years. Patients received fewer unnecessary stents and had better outcomes.
How the Measurement Works
FFR is performed during a cardiac catheterization. A guide catheter is already positioned at the opening of the coronary artery, and the cardiologist advances a specialized pressure-sensing wire through it and past the narrowed segment. Two pressures are then compared simultaneously: the pressure at the tip of the guide catheter (representing aortic pressure before the blockage) and the pressure recorded by the wire beyond the blockage.
The critical step is inducing “hyperemia,” a state of maximum blood flow through the artery. At rest, the heart’s small blood vessels can compensate for a partial blockage by dilating, which masks the true pressure drop. To reveal the full effect of the narrowing, a drug is given to force those vessels wide open. Adenosine is the most commonly used agent, delivered either directly into the coronary artery or through an IV. Once maximum blood flow is achieved, the pressure ratio is recorded. The formula is simply the distal pressure divided by the aortic pressure.
The entire measurement typically adds only a few minutes to an already-planned catheterization procedure.
What the Numbers Mean
A normal coronary artery without any disease produces an FFR of 1.0, meaning no pressure is lost as blood passes through. As a blockage worsens, the number drops. Current U.S. guidelines from the ACC and AHA recommend using an FFR of 0.80 or below as the cutoff for proceeding with revascularization (stenting or bypass) in patients with stable coronary artery disease. This is a Class 1 recommendation with the highest level of supporting evidence.
There is a “gray zone” between 0.75 and 0.80 that creates some clinical uncertainty. The value of 0.75 was the original threshold validated against stress testing for detecting ischemia, while 0.80 emerged from later trials as the more practical decision point. Blockages producing FFR values in this narrow band may or may not cause meaningful ischemia, and the best strategy for these patients is still debated. Values above 0.80 generally indicate the blockage is not flow-limiting, and medical therapy alone (cholesterol-lowering drugs, blood thinners, lifestyle changes) is typically sufficient.
When FFR Is Recommended
FFR is most useful for intermediate blockages, the ones that look like they could go either way on angiography. For a clearly severe or clearly mild narrowing, the visual information is usually enough. The 2021 ACC/AHA guidelines specifically recommend FFR to guide stenting decisions in patients with stable ischemic heart disease. For patients experiencing a heart attack involving one artery but who also have blockages in other arteries, FFR of those non-culprit arteries is considered a reasonable approach to decide which additional blockages need treatment.
Risks and Preparation
Because FFR is performed during cardiac catheterization, it carries the same baseline risks as that procedure: a small chance of coronary artery dissection, vessel occlusion, or bleeding at the catheter insertion site. These complications are uncommon but not negligible.
The adenosine used to induce hyperemia can cause transient side effects including chest tightness, shortness of breath, flushing, and a brief drop in heart rate. These typically resolve within seconds once the drug is stopped. FFR is generally not performed in patients with significant heart rhythm abnormalities (second- or third-degree heart block), those on theophylline-based medications (which block adenosine’s effect), or patients with severe reactive airway disease.
If you’re scheduled for an FFR measurement, you’ll likely be asked to avoid caffeine-containing foods and drinks for at least 24 hours beforehand. Caffeine directly interferes with adenosine’s ability to dilate blood vessels, which can produce a falsely higher FFR reading and potentially cause a significant blockage to be missed.
FFR vs. Resting Pressure Ratios
A newer alternative called instantaneous wave-free ratio (iFR) measures the pressure gradient across a blockage during a specific window in the cardiac cycle when the heart is relaxed, without requiring adenosine or any other drug. This makes the test faster and eliminates drug-related side effects.
The two methods agree on the clinical decision (treat or don’t treat) about 79% of the time. The roughly 21% discordance rate exists because the tests measure slightly different things: FFR captures pressure loss during artificially maximized flow across the full heartbeat, while iFR captures resting pressure loss during mid-diastole only. Large clinical trials have shown that outcomes are similar whether doctors use FFR or iFR to guide stenting decisions, so both are considered acceptable in current practice.
CT-Based FFR Without a Catheter
A noninvasive version of FFR can now be estimated from a standard coronary CT scan using computational fluid dynamics, essentially a computer simulation of blood flow through the arteries visible on the scan. This approach, called FFR-CT, avoids catheterization entirely.
A large meta-analysis found that FFR-CT has an overall diagnostic accuracy of about 82%, with sensitivity around 81% and specificity around 83% when compared to invasive FFR as the reference standard. When the analysis is performed on-site rather than sent to an off-site processing center, accuracy improves slightly to about 84%. The negative predictive value of on-site FFR-CT reaches 88%, meaning that when it says a blockage is not significant, it’s right nearly nine times out of ten.
FFR-CT is increasingly used as a gatekeeper: patients whose CT-derived FFR suggests no significant flow limitation can often avoid invasive catheterization altogether, while those with concerning values proceed to the catheterization lab where invasive FFR can confirm the finding and treatment can be performed in the same session.