IVUS Cannot Replace FFR But Can Help Rule Out Ischemia-Producing Stenoses

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Intravascular ultrasound (IVUS) imaging can be used to exclude severe lesions apt to have functional significance, but only fractional flow reserve (FFR) measurement can provide adequate physiological assessment in intermediate cases, according to a paper published online January 25, 2011, ahead of print in Circulation: Cardiovascular Interventions.

Investigators led by Seung-Jung Park, MD, PhD, of Asan Medical Center (Seoul, South Korea), assessed 236 lesions in 201 consecutive patients using IVUS, angiography, and FFR measurement prior to intervention. The researchers sought to pinpoint the optimal IVUS criteria for predicting FFR values.

Anatomy vs. Physiology

Overall, 49 lesions (21%) exhibited FFR < 0.80 at maximum hyperemia induced by intravenous adenosine infusion. Independent predictors of FFR were minimal lumen area (MLA), plaque burden, lesion length with a lumen area of < 3.0 mm2, and left anterior descending artery lesion location.

MLA measurements below a threshold of 2.4 mm2 showed the greatest potential to predict FFR < 0.80. That cutoff had 90% sensitivity and 60% specificity, amounting to an overall diagnostic accuracy of 68%. In contrast, MLA below 4.0 mm2 had a sensitivity of 100% but a specificity of only 13% to predict FFR < 0.80. 

Among lesions with MLA ≥ 2.4 mm2, 96% also had FFR ≥ 0.80, indicating absence of functional significance. Conversely, only 37% of lesions with MLA < 2.4 mm2 had abnormal values of FFR < 0.80, indicating functional significance. 

“IVUS-measured MLA is only one of many factors affecting coronary flow hemodynamics,” Dr. Park and colleagues conclude. Although MLA ≥ 2.4 mm2 may be a “useful criterion” for ruling out abnormal FFR, lower MLA measurements do not always “equate with functional significance. Thus, physiological assessment such as direct FFR measurement or stress tests may be necessary for identifying ischemia-inducible stenosis that require PCI to reduce unnecessary procedures, especially in lesions with MLA < 2.4 mm2 or small-vessel disease.” 

An Answer to Ongoing Controversy

In a telephone interview with TCTMD, John McBarron Hodgson, MD of the Geisinger Health System (Wilkes-Barre, PA), emphatically agreed with that conclusion. “This is about shedding a little more light on this clinical myth that we can use IVUS to assess the functional significance of coronary stenoses. And we just can’t. That’s just incorrect,” he concluded. “It’s about the physiology. We should stent lesions based on their demonstrated reduction in blood flow during conditions of exercise.” 

Importantly, “there is no single number for MLA,” he said, stressing that only 5 papers thus far have validated IVUS-measured MLA as a predictor for abnormal FFR. Clinicians commonly misinterpret that small body of literature, “totally incorrectly” employing the cut point of 4.0 mm2—saying that a significant lesion is anything below that number and using it as an excuse to stent, Dr. Hodgson noted. 

“While high MLA can be useful to exclude FFR < 0.80, poor specificity limits its value for assessment of lesions,” he explained. “The take-home message here is that IVUS is a very poor way to try to assess the physiologic significance of intermediate lesions.” 

Akiko Maehara, MD, of Columbia University Medical Center (New York, NY), agreed in an e-mail communication with TCTMD that the current paper is attempting to revalidate IVUS estimates of stenosis severity that were previously developed in the late 1990s and have since caused some debate.

“FFR is very good for assessment of lesion severity, but not good for optimization of stent implantation,” Dr. Maehara said. IVUS can do both of these things as well as evaluate lesion morphology, and in addition, IVUS is better suited than FFR for evaluating multiple lesions per artery and left main disease. “Ideally, if we can have very good IVUS criteria that correlate with abnormal FFR, we can use IVUS for all measures of lesion severity: physiological significance, morphology, and [whether PCI should be performed].”

Dr. Maehara pointed out that one drawback to this study is that many of the vessels were small, and therefore did not carry much myocardial burden distal to the lesion. “So we need to be very careful when using the cutoff in this paper,” she advised.

For researchers, this paper provides as much practical information as did the earlier retrospective studies, but more data and better methodology are required. For clinicians, the main message is that FFR is still necessary for physiological assessment, Dr. Maehara concurred. 

Choosing the Right Test for the Job 

Both FFR and IVUS are “fantastic, and they’re complementary,” Dr. Hodgson commented. “But they’re for different purposes.” 

In instances where clinicians already know they want to treat a particular lesion, IVUS can provide valuable information. Using IVUS to guide intervention allows operators “to do a good job and end up with a well-placed stent that covers the lesion, is maximally expanded, etc,” he said, adding that FFR, on the other hand, is the only way to understand whether a lesion restricts blood flow or causes symptoms. 

But “trolling for lesions,” with IVUS and choosing to stent based on IVUS alone is wrong, Dr. Hogson emphasized. “That is total hogwash.” 

Clinicians often make “the incorrect assumption that if there’s atherosclerosis present, it must be limiting blood flow and therefore I must do surgery or stenting or something,” he commented. “Lots of people have atherosclerosis. Few of them need an interventional procedure. All of them need a statin to lower their cholesterol and [should] be on aspirin.” 

Note: Gary S. Mintz, MD, a coauthor of the paper, serves as Medical Director and Editor-in-Chief of TCTMD. 

 


Source:
Kang S-J, Lee J-Y, Ahn J-M, et al. Validation of intravascular ultrasound-derived parameters with fractional flow reserve for assessment of coronary stenosis severity.Circ Cardiovasc Interv. 2011;4:65-71.


Disclosures:

  • Dr. Park reports no relevant conflicts of interest.
  • Dr. Hodgson reports serving on the speaker’s bureau for Volcano and receiving educational grants from St. Jude Medical and Volcano.
  • Dr. Maehara reports receiving a research grant from Boston Scientific and speaker’s fees from Volcano.


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