10.3 Flow Reserve (FFR, iFR, RFR)

Flow Reserve (FFR, iFR, RFR)

Angiography is a lumen picture. Flow reserve asks whether that picture is actually ischemic. Procedures additional diagnostic imaging expects you to know FFR, iFR, and RFR as physiologic indices, how the wire is equalized, why damping wrecks the number, and when an intermediate stenosis should not be stented on eyeball alone. Cutoffs below are clinical teaching thresholds, not unpublished ARRT law.

Quick Answer: FFR = Pd/Pa during maximal hyperemia (IV or IC adenosine). Teaching ischemic threshold ≤ 0.80. Pd 72 / Pa 90 = 0.80. iFR is a resting instantaneous wave-free ratio, teaching threshold 0.89, no adenosine. RFR is a resting full-cycle ratio with similar resting intent. Calibrate, equalize, check drift. Damped guide pressure invalidates the run. IV adenosine is off-limits in high-grade AV block, severe asthma/COPD, and hypotension. Physiology decides PCI versus deferral when the angiogram is about 50–70%.

FFR: Pd over Pa at hyperemia

Fractional flow reserve (FFR) is the ratio of distal coronary pressure (Pd) to aortic (guide) pressure (Pa) when the microvasculature is fully dilated — maximal hyperemia. At hyperemia, pressure loss across a stenosis is proportional to the flow limitation that stenosis can impose.

Hyperemia is produced with adenosine:

• Intravenous (IV) infusion (typical teaching 140 mcg/kg/min) gives systemic, sustained hyperemia — flushing, chest warmth, brief AV delay, and a blood-pressure dip are expected if the drug is working.
• Intracoronary (IC) boluses are shorter and more local. Teaching doses are often lower in the RCA than the left system because the AV node lives on the RCA in right-dominant anatomy — exact milligrams follow lab protocol, not an ARRT table.

Teaching ischemic threshold: FFR ≤ 0.80. Values at or below 0.80 are treated as ischemia-producing in the FAME-era teaching most labs still use. Some older deferral literature used 0.75 as a more specific line. For this exam, remember ≤ 0.80 as teaching, labeled as teaching, not ARRT-published law.

Worked example. Distal pressure Pd = 72 mmHg, aortic pressure Pa = 90 mmHg, recorded at maximal hyperemia.

FFR = Pd / Pa = 72 / 90 = 0.80

That ratio sits exactly on the teaching ischemic cutoff. The number is the ratio; it is not 1.25 (which would be Pa/Pd, backwards), and it is not an iFR just because 0.80 appears.

Pullback FFR distinguishes a focal step-up (one treatable stenosis) from diffuse distal-to-proximal pressure loss (a long diseased vessel that may not love a single stent).

iFR and RFR: resting indices

Not every intermediate lesion needs adenosine. Resting indices use the fact that diastolic wave-free flow still carries information about stenosis significance.

iFR (instantaneous wave-free ratio) is measured in a diastolic wave-free period at rest. No hyperemic drug. Teaching ischemic threshold: 0.89. Below that line, labs treat the lesion as significant; above it, they often defer. A gray zone in some hybrid algorithms then triggers FFR — know that hybrid exists; do not memorize a lab-specific gray band as ARRT law.

RFR (resting full-cycle ratio) scans the entire cardiac cycle for the lowest Pd/Pa at rest. Intent is the same family as other resting ratios: no adenosine, a resting physiologic yes/no. Teaching cutoffs cluster near the 0.89 resting-index neighborhood. If a stem says RFR, think resting, full-cycle, similar decision to iFR, not a third flavor of hyperemic FFR.

Do not mix the drugs and the cutoffs. FFR needs hyperemia and uses ~0.80. iFR/RFR are resting and use ~0.89. An iFR of 0.80 is well below its own teaching line; an FFR of 0.89 is not ischemic by FFR teaching.

Wire calibration, equalization, and drift

A pressure wire is only as honest as its zero and its match to the guide.

1. Anticoagulate as for any coronary wire, per operator.
2. Calibrate / zero the transducer system per IFU so both Pa and Pd start from a true zero at the cath-lab transducer height.
3. Advance the wire until the pressure sensor sits at the guide-catheter tip, still in aorta, not yet across the lesion.
4. Equalize: Pd should match Pa (Pd/Pa = 1.00, or the vendor’s equalization command). If they do not match, you do not cross yet — check height, connections, and whether the guide is damped.
5. Cross the lesion, rest the sensor distal, record resting Pd/Pa and the resting index if you are using iFR or RFR.
6. For FFR, start adenosine, wait for stable hyperemia, record Pd/Pa.
7. Pull back toward the guide. A sudden step-up localizes a focal lesion; a long slow climb is diffuse disease.
8. When the sensor returns to the equalization point, Pd should again match Pa. If it does not, that is drift — the whole run is suspect. Repeat equalization and the measurement; do not stent a drifted 0.79 as if it were gospel.

Pressure damping invalidates physiology. If the guide wedges or damps at the ostium, displayed Pa is falsely low. FFR is Pd/Pa; a too-low Pa raises the ratio and can make a significant lesion look “fine.” Unseat the guide until a crisp aortic waveform returns, re-equalize, and only then record. Ostial and left-main lesions are classic damping traps. A ventricularized Pa is not a Pa.

Adenosine contraindications and when physiology decides PCI

IV adenosine teaching contraindications:

• High-grade AV block without a backup pacemaker — adenosine slows AV conduction on purpose.
• Severe asthma or COPD — IV adenosine can provoke bronchospasm. IC adenosine is sometimes discussed as an alternative in labs; it is not a free pass to ignore airway disease, and the stem that says severe asthma is pointing at do not infuse IV adenosine.
• Hypotension — hyperemia drops SVR; a crashing pressure is not a hyperemia protocol.

Relative teaching issues: caffeine and other methylxanthines can blunt adenosine hyperemia and yield a falsely high FFR. If the patient is full of coffee and the FFR is 0.84 on a lesion that looks 80%, the number may be unreliable, not reassuring.

When the angiogram is about 50–70%, visual percent stenosis is a poor ischemia test. That is the zone physiology is built for: FFR/iFR/RFR decide PCI versus medical deferral. A 90% culprit in STEMI with TIMI 0 flow is not waiting for adenosine. A hazy 60% proximal LAD in stable symptoms is exactly the stem.

Index / hyperemia? / typical ischemic cutoff (teaching)

Worked FFR row: Pd 72, Pa 90 → 0.80, on the FFR teaching line.

Worked case

Stable angina. Mid-LAD looks 60% in one view and 50% in the orthogonal view — classic intermediate. The pressure wire is equalized at the guide tip (Pd/Pa = 1.00), then parked distal. Resting iFR is 0.92 (above the 0.89 teaching line). The operator still wants FFR. IV adenosine at a teaching 140 mcg/kg/min is started; the patient flushes; Pa is a crisp aortic tracing (not damped). Pd 72, Pa 90, FFR = 0.80. Pullback shows a single step at the angiographic stenosis; back at the guide, Pd again matches Pa (no drift). Using FFR ≤ 0.80 teaching, this lesion is in the ischemic bin and PCI is discussed. If the same patient had severe asthma, you would not run IV adenosine — use a resting index or IC strategy the physician owns, or do not force hyperemia. If the guide had damped when you engaged, you would unseat and re-equalize rather than trust 0.80.

Exam traps

• Computing FFR as Pa/Pd (72/90 inverted to 1.25).
• Applying the 0.80 cutoff to iFR, or 0.89 to FFR.
• Calling iFR a hyperemic index or giving it adenosine by default.
• Recording FFR through a damped guide and calling the high ratio reassuring.
• Skipping equalization or ignoring drift on return to the tip.
• Infusing IV adenosine in high-grade AV block, severe asthma/COPD, or hypotension.
• Stenting or deferring a 50–70% stenosis on one angiographic view without physiology when the stem is offering you a wire.