Fractional Flow Reserve and Optical Coherence Tomography

Abstract

Angiography has long been considered the gold standard in guiding coronary interventional therapies. However, well-documented limitations of angiography necessitate the use of more advanced imaging and functional hemodynamic evaluation to arrive at optimal decisions regarding revascularization.1 Contemporary guideline-driven practice dictates revascularization only for ischemia-causing lesions and medical therapy for other lesions.2,3 For this reason, accurate assessment of the functional significance of a lesion (ischemic vs. nonischemic) using noninvasive or invasive techniques is very important.

Support
The publication of this information was supported by St. Jude Medical.
Accepted date
01 August 2014
Citation
RadcliffeCardiology.com, August 2014

Introduction
Angiography has long been considered the gold standard in guiding coronary interventional therapies. However, well-documented limitations of angiography necessitate the use of more advanced imaging and functional hemodynamic evaluation to arrive at optimal decisions regarding revascularization.1 Contemporary guideline-driven practice dictates revascularization only for ischemia-causing lesions and medical therapy for other lesions.2,3 For this reason, accurate assessment of the functional significance of a lesion (ischemic vs. nonischemic) using noninvasive or invasive techniques is very important.

Inherent and logistical limitations of noninvasive stress testing decrease the applicability of this diagnostic technique both in- and outside the catheterization laboratory in regard to making decisions on revascularization (e.g., multivessel disease and balanced ischemia, artifact, tolerance to test). Angiographic evaluation of intermediate lesions for ischemic significance is also challenging and often requires a noninvasive functional evaluation or lab assessments. Fractional flow reserve (FFR), in contrast, enables functional evaluation in the cath lab and delivers gold standard ischemia detection.

Fractional Flow Reserve
FFR assesses the physiologic significance of a coronary stenosis by calculating the ratio of distal coronary pressure (Pd) to proximal coronary pressure (Pa) during hyperemia.4 Coronary flow (Q) can be determined during maximal hyperemia with the coronary pressure measurement (Pa/Pd). The venous pressure (Pv) and the resistance of the myocardium (R) remain constant. By virtue of the physiology behind FFR measurement, attributes such as the extent of a perfusion territory, myocardial blood flow and the presence or absence of inducible ischemia are taken into account in evaluating stenosis severity.

How to Induce Maximal Hyperemia
Multiple agents can be used to induce maximal hyperemia. The author prefers the use of intravenous (IV) adenosine, 140 μg/kg/min (Table 1). Papaverine is no longer favored, and nitroprusside has shown promise in several small trials.5
Adenosine to Induce Hyperemia
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IV adenosine administered via a central (femoral) vein has been compared with an antecubital (peripheral) vein. The mean FFR difference between these IV infusion access routes at 140 μg/kg/min, although statistically significant, was only 0.0126.9 The author believes that experienced operators have a preference for IV adenosine due to the more reliable hyperemia state and fewer artifacts seen due to drug administration. Additionally, use of adenosine enables the operator to perform pullbacks for diffuse disease and provides the ability to easily disengage a catheter during FFR measurements of ostial disease.

How Reproducible is FFR? What is the Sensitivity and Specificity of FFR?
Multiple evaluations have shown the reproducibility of FFR measurements despite changes in loading conditions. In fact, serial evaluations have demonstrated the reproducibility of FFR measurements with the use of chronotropic and inotropic agents and afterload reducing agents.10,11 In a landmark study, Pijls et al. demonstrated that the sensitivity of FFR is 88 % while the specificity is 100 %.12

FFR and Microvascular Disease
It is known that multiple factors determine the functional severity of a stenosis in addition to the stenosis itself, including the extent and distribution of the perfusion territory, the state of the myocardium and the presence or absence of collaterals. The presence of microvascular disease can definitely be a confounder in the diagnosis and treatment of ischemic lesions and may contribute to a finding of reversible ischemia. FFR, however, can still indicate the contribution of an epicardial stenosis to the degree of inducible ischemia in patients with microvascular disease. FFR will show the exact maximum flow that can be achieved by removing the epicardial stenosis. Although coronary flow reserve (CFR) has been traditionally used to diagnose the presence of microvascular disease, measuring FFR can provide valuable information in these patients. Details regarding the validation of this concept have been presented elsewhere and are worthy of discussion.13 The evaluation of 150 lesions, with both FFR and CFR, found agreement between outcomes of FFR and CFR in 109 lesions (73%), and discordant outcomes in 41 lesions (27%). In 26 of these 41 lesions, FFR was < 0.75 and CFR ≥ 2.0. In 15 of these 41 lesions, FFR was ≥ 0.75 and CFR < 2.0.13 Concordance between CFR and FFR is seen in patients where FFR and CFR are normal and where FFR and CFR are abnormal. Cases where CFR is normal and FFR is abnormal are of concern. In this small subgroup of individuals, higher CFR levels fail to show the significance of a stenosis, which is evident with an abnormal FFR. Another group, and perhaps the most important, is that of patients with microvascular disease who have an abnormal CFR and a normal FFR. FFR fails to show the microvascular disease by virtue of the patient’s normal value. As a result, the lesion is not treated as the FFR is normal. The clinician arrives at the correct decision, despite the confounding information. In essence, FFR still determines accurately to what extent the maximum myocardial perfusion can be increased by coronary intervention.

Coronary Physiology vs. Coronary Anatomy
(Intravascular Imaging/Angiography)

The interventional community has long used intravascular imaging to answer the physiologic question of lesion significance (ischemic vs. nonischemic). While validation studies may have shown some correlation between intravascular imaging and the ischemic significance of a lesion, this diagnostic modality has its obvious limitations. There is a known discordance between FFR and intravascular ultrasound (IVUS) when it comes to lesion significance.14, 15 As such, physiologic questions require a physiologic answer, while anatomical questions require an anatomical answer (Table 2).
Specificity and Sensitivity of IVUS and FFR
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This should not be surprising as multiple physiologic measures of a lesion’s significance cannot be evaluated with an anatomical evaluation. Evaluation of a single cross-sectional area (CSA) does not take into account the lesion entrance effect, friction-derived loss of pressure, flow separation losses, blood viscosity and lesion turbulence. Moreover, the inherent value of a vessel’s myocardial distribution (small or large) and the condition of the myocardium (microvascular disease or not) must be evaluated physiologically not anatomically. Therefore, the limitations of an anatomical evaluation should not be surprising. A similar limitation may exist when comparing myocardial perfusion imaging with FFR.17

Recent studies have also shown the direct relationship between IVUS and FFR for a given vessel size. The FIRST trial (Fractional Flow Reserve and IVUS Relationship Study) showed 64 % sensitivity and a 65 % specificity finding that a minimum lumen area (MLA) > 3.07 corresponded to a FFR < 0.80.18 The accuracy of IVUS improved when a reference vessel analysis was performed (Table 3).18
Results of the FIRST Trial
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The inherent limitations of angiography also provide the basis for some subjectivity in the evaluation of ischemic lesions. The limitations on revascularization decision making can be seen in a comparison of FFR and angiographically guided evaluation on lesion significance during routine assessment (Table 4).19
FFR Compared with Standard Angiography
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The limitations of angiography and IVUS in identifying intermediate lesions able to produce ischemia have led contemporary interventional cardiology to call for more advanced and reliable means of evaluating lesion significance. FFR has been able to change that paradigm.

FFR in Diffuse Disease
Diffuse disease represents a challenge in the evaluation of significant lesions. Often, tandem lesions may not look significant on angiography but may have a physiologic impact in combination. IVUS may not identify a significant minimal CSA or MLA among the tandem lesions. It is also challenging to find reference segments when diffuse disease is present. FFR provides the ability to evaluate the significance of those lesions. It also allows for a pullback (with the use of IV adenosine), which can be helpful in identifying delta changes that unmask the significance of one lesion versus another. FFR provides for great spatial resolution. Lesions with the largest significant gradient are treated first. Subsequent FFR evaluations will be able to reveal if the remaining lesions are still ischemia producing or not.

FFR vs. Intravascular Imaging in Left Main Disease
The angiographic evaluation of left main (LM) disease is another situation that can present challenges and require additional intravascular imaging in order to make the correct decision regarding revascularization. The angle of the ostium in particular often presents challenges for the accurate angiographic evaluation of disease. Multiple validation studies have been performed to show the MLA at which severe disease is present (Table 5).

It is important to remember that when evaluating ostial LM disease one has to disengage the catheter from the ostium during pressure measurements. In this case, IV adenosine may facilitate the recording process. FFR evaluation of the LM is critically dependent not only on the lesion characteristics but also on the territory supplied by the vessel. In the hypothetical case that there is an area of infarct in the territory of a given vessel, the quantity of viable myocardium supported by the vessel will be reduced and the FFR could be higher. This is not to be considered an underestimation of the severity of the ischemia by FFR but simply that the lumen is adequate for the amount of viable myocardium present. Alternatively, the FFR may be lower when a given vessel also supplies the collateral flow to another vessel territory. In this case, the supplied territory for a given vessel has increased rendering of an otherwise insignificant lesion to become FFR positive. When comparing FFR and intravascular imaging in isolated LM disease, it seems that the MLA at which LM disease shows an FFR < 0.80 is < 4.8 mm2.14

Correlation between IVUS and FFR in Predicting LM and Non-LM Disease
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Evaluation of LM disease in the presence of significant proximal disease of the left anterior descending and circumflex arteries is still controversial. A clinically relevant effect on the FFR assessment of LM disease with the pressure guidewire in a nonstenosed downstream vessel occurs only when the stenosis in the other vessel is proximal and very severe, as validated in an animal study.22 This means that FFR in the presence of isolated LM disease is very reliable. As a general rule, it seems to be accepted that an FFR in the LM < 0.8 or an MLA < 5.9-6.5 mm2 is the threshold for revascularization.

FFR in Ostial Disease and Jailed Side Branches
Ostial disease often represents a challenge in the interventional treatment of coronary disease. Lesion location and characteristics may make it difficult to stent ostial lesions while trying to preserve the parent vessel that is disease-free. Most side branch lesions and ostial disease do not have functional significance, and quantitative coronary angiography is unreliable in assessing the functional severity of these lesions.23, 24 However, various studies have shown the ability of FFR to clearly evaluate these lesions (Table 6).23, 24
FFR Measurements of Jailed Side Branch and Ostial Lesions
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FFR in Acute Coronary Syndromes
Questions have been raised as to the value and safety of FFR in the setting of acute coronary syndromes (ACS). It is standard practice to avoid measuring FFR in an infarct-related artery and territory until after 5 to 7 days. However, FFR in a non-infarct-related artery or other territories can still be evaluated in the acute setting as long as it is clinically safe and required. The benefit of using FFR to guide percutaneous coronary intervention (PCI) in multivessel disease does not differ in patients with unstable angina (UA) or non-ST-segment elevation myocardial infarction (NSTEMI) compared with patients with stable angina.25

What Do the DEFER, FAME 1 and FAME 2 Studies Show About FFR?
Assessing individual coronary lesions in an effort to find if they are responsible for ischemia has important prognostic implications. The DEFER study is a prospective, randomized, multicenter study that evaluated lesion significance with FFR (< 0.75) and followed patients for future events. The 5-year follow-up of these patients demonstrated that event-free survival was similar for patients with or without interventions at FFR > 0.75.26 Medical treatment of patients with a hemodynamically nonsignificant stenosis (FFR > 0.80) in the proximal left anterior descending artery (LAD) is associated with an excellent long-term clinical outcome with survival at 5 years similar to an age-related and sex-matched control population.27

The routine measurement of FFR during PCI with drug-eluting stents in patients with multivessel disease was compared with a strategy of angiography alone in the FAME 1 study. The study demonstrated an approximate 30 % reduction of the composite endpoint of death, myocardial infarction (MI), revascularization and bypass at 12 months. It also showed that the FFR-guided strategy resulted in cost saving, did not prolong the procedure, reduced the total numbers of stents used, decreased the amount of contrast used and resulted in a similar, if not better, functional status. The results of FAME 1 support the paradigm of a “functionally complete revascularization” by stenting only the significant lesions and medically treating nonsignificant lesions.28-30

The FAME 2 study compared FFR-guided PCI plus optimal medical therapy (OMT) versus OMT alone. The primary endpoint was a composite of death, MI and urgent revascularization. FAME 2 was halted prematurely as events favored PCI plus OMT. The results were largely driven by urgent revascularization (1.6 % vs. 11.1 %). In patients with stable CAD, PCI plus OMT decreased the need for urgent revascularization when compared with OMT alone. In essence, FFR may identify stable CAD patients who may fare worse with OMT alone.31

Anatomic Evaluation with Optical Coherence Tomography
Optical coherence tomography (OCT) is an advanced imaging technique that enables ultra-high resolution evaluation of crosssectional biological systems. The near-infrared light source used in OCT allows for the evaluation of coronary artery disease (CAD) and vessel characteristics enabling PCI guidance with 10 times the resolution of IVUS.32 For this reason, previously difficult to image neointimal hyperplasia, fibrous caps or more subtle atherosclerosis can be seen with OCT.32 This allows for better visualization of the lumen, intima, media and adventitia where coronary pathology requiring evaluation is most prevalent (Figure 1).

OCT Imaging Technique
The automated OCT system allows for a 75 mm pullback (survey mode) and 54 mm pullback (high resolution mode) after contrast administration at the level of the coronary vessel being evaluated via an automatic injector or manual injection. Mastering the technique for optimal OCT image acquisition is relatively easy. To minimize artifact, a 6 F catheter is recommended. A coaxial position with the catheter nicely engaged in the coronary artery is ideal. Predilatation of lesions may be necessary. Artifacts seen with OCT can include residual blood in the vessel lumen or swirl; blood inside the probe; nonuniform distortion; fold over, saturation or sew-up artifact; and Z-offset drift. All of these artifacts are relatively uncommon with the exception of residual blood in the lumen and blood inside the probe. The first one is easily remedied with better contrast injection, and the second one by simply flushing the OCT catheter.
OCT Image of Coronary Lumen Showing Intima, Media and Adventitia
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Angiography and OCT
The limitations of angiography in the evaluation of CAD were previously discussed. When comparing angiography alone with angiography plus OCT to guide decision-making during PCI, the CLI-OPCI study suggests that the use of OCT can improve clinical outcomes of patients.33 Though an observational study with obvious limitations, it is nonetheless hypothesis generating and is the first study suggesting a direct relationship between the use of OCT and a clinical outcome. OCT found adverse features requiring further interventions in 34.7 % of cases. The unadjusted analysis showed that the OCT group had a significantly lower 1-year risk of cardiac death, MI or repeat revascularization (9.6 % vs. 14.8 %, p=0.044). OCT did not increase short-term or mid-term adverse events demonstrating proof of its safety. The statistically significant reductions in cardiac death or nonfatal MI suggest that OCT guidance may minimize procedural issues, particularly periprocedural and in-hospital MI.33

IVUS vs. OCT
Both IVUS and OCT can be used to visualize minimum lumen diameter (MLD), MLA, lesion length, calcium, fibrosis and lipids. However, when it comes to evaluation of strut apposition, stent coverage, cap fibroatheroma evaluations, thrombus and macrophages, OCT tends to be better by virtue of the higher resolution. Post-intervention findings such as dissection, tissue prolapse and apposition tend to be seen better with OCT.34,35Table 7 compares attributes and characteristics of IVUS and OCT.

FFR and OCT
There is a benefit to integrating FFR and OCT technologies for the evaluation of disease and PCI optimization. These tools offer physicians the ability to identify, diagnose and treat CAD while improving outcomes. FFR is considered the gold standard in the detection of myocardial ischemia and enables high-fidelity hemodynamic and physiologic evaluation. FFR has been shown to improve outcomes in the treatment of multivessel disease as evident by the results of the FAME 1 study. FFR also enables evaluation of multiple other lesion subsets that could facilitate interventional procedures. See Table 8 for appropriate use criteria.43
Comparison of OCT and IVUS
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OCT represents a leap in intravascular imaging and provides superior resolution when compared with IVUS. Advanced imaging enabled by OCT facilitates procedural decisions and may result in improved outcomes. Specifically, OCT allows for better plaque characterization and superior evaluation of post-interventional results and facilitates research endeavors requiring high resolution.
Use of FFR for Lesion Severity in the Diagnostic Appropriate Use Criteria
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Conclusion
Contemporary guideline-driven practice dictates revascularization only for ischemia-causing lesions and medical therapy for other lesions. FFR provides accurate assessment of the functional significance of a lesion in a variety of clinical scenarios (e.g.: single lesions, diffuse disease, left main, ostial disease, side branches). OCT further enables assessment and treatment of disease via high resolution lesion evaluation (pre- and post-intervention).

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