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iFR as a Catalyst for Adoption of Physiologic Assessment of CAD
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As an interventional cardiologist, I am committed to staying current and adopting evidence-based technologies that best help treat my patients at the University of New Mexico. It was during my cardiology fellowship training that the results of the FAME trial were published, showing a clear benefit in favor of fractional flow reserve-guided PCI over angiography only-guided PCI.
Despite the evidence, adoption of FFR has been slow, albeit improving. In 2014, 25% of my PCI practice was FFR-guided, and I continue to think more about the hemodynamic relevance of a lesion and not just its angiographic appearance.
FFR: Physiologic Assessment for Better Outcomes
Coronary angiography comes with several limitations, one being the 2D rendering of the 3D arterial tree. Second, it is subjective and for lesions that are in the intermediate zone (50% to 70%), fairly inaccurate in determining the hemodynamic relevance of the lesion.
The FAME I, FAME II and DEFER trials strongly support physiologic assessment of intermediate lesions to guide PCI, which has been shown to lead to better outcomes compared with angiography-based assessment. Physiologic assessment should be the gold standard; however, many physicians are reluctant to change the way they evaluate patients, despite mounting nearly incontrovertible evidence that a superior method exists. Some reasons for this surround what is required to perform FFR, including the cost of adenosine, its side-effect profile and additional time it may add to the overall procedure. Perhaps more physicians might consider physiologic testing for CAD if a simpler way were available.
iFR: An Alternative to FFR Diagnosis
The most recent physiologic assessment tool is instant wave-free ratio (iFR Modality, Volcano Corp). Cleared by the FDA in March 2014, iFR is based on the same principle of pressure assessment across the lesion to evaluate its hemodynamic effect on myocardial perfusion but without the use of adenosine. iFR identifies a pressure gradient during the “wave-free period” in diastole where there is a linear relationship between pressure and flow. It is during this period that competing forces (waves) that affect coronary flow are inactive and microcirculatory resistance is negligible. The performance of iFR requires the use of the same high-fidelity pressure wires used for FFR to measure the ratio of distal coronary pressure (Pd) to the aortic pressure (Pa). An iFR value of less than 0.89 is correlated with reduction in flow similar to an FFR value of less than 0.80.
The decision to adopt iFR in my lab to guide PCI treatments was influenced by a number of major studies, presentations and networking with colleagues. One of the main advantages of iFR is its simplicity.
Initially, we brought in iFR on a trial agreement. We had already used the Volcano IVUS and FFR systems, so the integration was fairly simple once we overcame a few IT hurdles. The iFR software was integrated into each of our cath labs, training for the entire staff was easy and it has become an extremely simple procedure.
Although FFR remains the gold standard for hemodynamic assessment of intermediate coronary lesions, it mandates the appropriate use of IV or intracoronary adenosine. With iFR, for a majority of lesions, we can eliminate the use of adenosine, resulting in significant cost and time savings.
Hybrid Approach Alleviates Physician Concern
For those who are FFR users in clinical practice, there is some trepidation regarding the reliability and correlation of iFR with FFR. When evaluating the data, the most optimal and reliable use of iFR comes as a hybrid approach.
In this approach, an iFR greater than 0.93 and less than 0.86 correlates strongly with non–flow-limiting and flow-limiting lesions, respectively. Stenoses with an iFR reading between 0.86 and 0.93 are considered in the “gray zone” and require adenosine administration to measure FFR to guide further therapy. The hybrid approach raises the sensitivity and specificity to more than 90%, which should assuage concerns in terms of the reliability of the data.
Application for Multivessel CAD Assessment
Another advantage of iFR is that it can lower the time and cost to assess the hemodynamic effect of angiographic multivessel CAD. For example, formerly, we might have identified a patient with angiographic three-vessel disease and recommended surgical revascularization. There is good evidence to suggest that FFR assessment of multivessel disease will reclassify a majority of lesions, often times changing the diagnosis from multivessel disease to less disease and changing the management plan. But, performing FFR on lesions in multiple vessels comes with increased hurdles of cost and time, given the requirement to assess each lesion with a 2-minute adenosine infusion. The “simpler” iFR approach may persuade more physicians to obtain additional data regarding the hemodynamic relevance of the each individual lesion and not just rely on angiographic appearance alone.
Post-PCI iFR assessment
Physiologic assessment may also be used to assess improvement in hemodynamics after PCI.
With FFR, post-PCI assessment requires re-dosing of the adenosine and may dissuade a re-assessment. With iFR, there is the ability to more simply recheck post-PCI iFR. Hopefully, hemodynamics have improved and one can be confident in the end result. If the post-PCI reading is in the gray zone, adenosine should be given for a conclusive measurement. It may be clinically relevant to not miss a significant edge dissection or plaque shift that is not angiographically severe. Having these data may prompt the decision to perform further intravascular imaging to be confident that the patient has received the best stent result possible.
iFR Pullback to Guide Treatment Decisions
Another application of iFR can be made when assessing serial lesions or diffuse disease. Cases with diffuse disease without a clear focal area of stenosis can be challenging, making the target for treatment difficult to identify. iFR pullback measurement identifies the area with the largest pressure drop across the length of the artery and can help target the part of the vessel that would benefit from revascularization.
Reimbursement
As of August 2014, the last impediment to reimbursement of iFR was removed. The American College of Cardiology and the Society for Cardiovascular Angiography and Interventions are now recommending iFR measurements be coded using the FFR CPT codes with a -52 modifier. With reimbursement, physicians will be more inclined to adopt the procedure and adoption should accelerate.
Future of Physiologic Assessments
Overall, the most exciting aspect of iFR is that it may lower the threshold to assess the hemodynamic relevance of coronary lesions and not rely solely on findings of coronary angiography. By simplifying the acquisition of the information, iFR might be easier to adopt for physicians who are not already using FFR. For clinicians already doing FFR, like we were, iFR offers the ability to avoid adenosine in a subset of patients, evaluate multivessel CAD and foster rapid post-PCI assessment.
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- Bina Ahmed, MD, is an interventional cardiologist and assistant professor of medicine at the University of New Mexico School of Medicine. She can be reached at bahmed@salud.unm.edu.
Disclosure: Ahmed reports no relevant financial disclosures.