8.1 FFR & iFR Studies

8.1 FFR & iFR Studies

Quick Answer: FFR measures pressure across a coronary lesion during pharmacologic hyperemia; ≤ ~0.80 generally indicates ischemia-producing stenosis warranting revascularization. iFR is a resting index measured in the wave-free period with a cutoff of ~0.89, avoiding adenosine. RCIS candidates must know both thresholds, how adenosine is used, wire technique basics, and the pitfalls that invalidate results.

Physiologic lesion assessment has moved beyond angiography alone. Two pressure-derived indices dominate contemporary cath lab practice: fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR). Both quantify whether a stenosis meaningfully restricts blood flow, but they differ in timing, pharmacology, and interpretation. As an RCIS, you prepare equipment, monitor the patient during hyperemia, recognize artifact, and support accurate data acquisition.

What FFR Measures

FFR is the ratio of mean distal coronary pressure to mean aortic pressure during maximal hyperemia, when microvascular resistance is minimized and flow is proportional to the pressure gradient across a stenosis.

Formula (conceptual): FFR = Pd / Pa (during hyperemia)

Landmark trials (DEFER, FAME) established that deferring PCI for FFR > 0.80 is safe, while revascularizing FFR ≤ 0.80 improves outcomes. The ~0.80 cutoff is the exam anchor number.

Hyperemia and Adenosine

Standard FFR requires pharmacologic hyperemia so microvascular resistance falls and the pressure gradient reflects the stenosis alone. Without hyperemia, resting flow may not expose the full functional severity of moderate lesions.

Adenosine is the most common agent in the United States:

• Intracoronary (IC): Rapid bolus through the guiding catheter (e.g., 40–100 mcg depending on vessel and protocol); quick onset, short duration; useful when systemic effects must be minimized
• Intravenous (IV): Continuous infusion (commonly 140 mcg/kg/min through a dedicated line); sustained hyperemia for pull-back assessments and multiple vessels

Alternatives include ATP (similar mechanism, IV infusion) and regadenoson (selective A2A agonist, IV bolus). Each causes transient side effects—flushing, dyspnea, chest discomfort, AV block, bronchospasm in reactive airways—that the RCIS must anticipate and communicate to the physician.

RCIS responsibilities during hyperemia:

1. Confirm dedicated IV access and infusion pump settings when IV adenosine is used
2. Monitor HR, rhythm, BP, and SpO₂; adenosine can provoke bradycardia, AV block, or hypotension
3. Ensure the pressure wire signal remains stable before and during hyperemia
4. Document hemodynamic tracings and any arrhythmia or patient symptoms

iFR: Resting Assessment Without Adenosine

iFR (instantaneous wave-free ratio) is measured during the wave-free period of diastole, when microvascular resistance is naturally lower and distal pressure more reliably reflects flow through the stenosis. Because hyperemia is not required, iFR reduces cost, time, and patient discomfort.

An iFR ≤ ~0.89 correlates with ischemia in validation studies (VERIFY, DEFINE-FLAIR). Values above ~0.89 generally support deferral. Hybrid strategies (iFR-first, FFR if borderline) appear in many labs; know both numbers for the exam.

Pressure-Wire Technique Essentials

Accurate FFR/iFR depends on signal quality. The RCIS supports setup and troubleshooting:

1. Zeroing: Zero the transducer at atmospheric pressure with the wire in a neutral reference (often the catheter hub or saline flush port per manufacturer protocol)
2. Equalization: Match wire and guide pressures in the aorta before crossing the lesion—waveforms should overlay
3. Crossing: Advance the pressure wire distal to the lesion; confirm d waveforms are not damped and not in a side branch
4. Recording: Acquire stable tracings during hyperemia (FFR) or in the resting wave-free window (iFR)
5. Pull-back: Slowly withdraw the wire during sustained hyperemia to identify pressure drops across tandem lesions

Waveform inspection is as important as the numeric result. Equalization mismatch, damped aortic pressure, or ventricularized distal tracings invalidate the index.

Pitfalls and Sources of Error

FFR vs. iFR selection pitfalls: Applying the 0.80 FFR threshold to iFR values (or vice versa) is a common exam trap. iFR is not adenosine-free FFR—they are different indices with different cutoffs.

Clinical Integration in the Cath Lab

Physiologic assessment guides PCI versus medical therapy. When FFR ≤ ~0.80 or iFR ≤ ~0.89, stenting typically improves symptoms and outcomes; when values are above threshold, deferral is often appropriate even if angiography looks moderate.

The RCIS role extends beyond button presses: maintain infusion lines, monitor for bronchospasm or high-grade AV block during adenosine, archive hemodynamic records, and verbalize when waveforms look damped. Catching artifact before the physician commits to revascularization is a high-value safety contribution.

Exam Focus Checklist

• FFR cutoff ~0.80 during hyperemia
• iFR cutoff ~0.89 at rest, no adenosine
• Adenosine routes (IC bolus vs IV infusion) and side effects
• Equalization, zeroing, pull-back technique
• Pitfalls: drift, damped pressures, serial lesions, microvascular disease

Master these concepts and you can anticipate physician needs, protect data integrity, and answer RCIS items on functional coronary assessment with confidence.