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SAN FRANCISCO -- A new pressure-based index to measure the severity of coronary lesion stenosis without adenosine compared favorably to fractional flow reserve (FFR) with adenosine, researchers found.

Calculating the instantaneous wave-free ratio (iFR), a measure of resting intracoronary resistance, researchers found it correlated very closely with FFR (r=0.90, P<0.001), Justin Davies, MBBS, MRCP, PhD, from Imperial College London, and colleagues found.

The iFR measurement also showed excellent diagnostic efficiency (area under curve 93%, at FFR<0.80), with equally good specificity (91%), sensitivity (85%), and negative and positive predictive values (85% of PPV and 91% for NPV), Davies reported here at the Transcatheter Cardiovascular Therapeutics meeting.

A panel of experts praised the new technology.

"I congratulate you and your team for bringing the concept of FFR forward into a new way to examine physiology," commented Morton J. Kern, MD, from the University of California Irvine. "I'm very impressed that a demonstration of a resistance at rest has matched to adenosine hyperemia," he said, adding that it changes the paradigm of lesion assessment through a much simpler mode.

Although the panelists were enthusiastic about the new concept, they raised questions that weren't answered in the small (131 patients, 157 lesions) pilot study.

The concerns included how the constancy of the resistance would hold up under a number of different hemodynamic conditions, such as changes in blood pressure and heart rate. Also, another issue that was raised was how to obtain a strong enough iFR signal in left main or proximal LAD lesions where there is sometimes very little vessel gradient.

Relaying data not presented here, Davis said they had found no statistical relationship related to changes in blood pressure or heart rate, and that the reproducibility of iFR was significantly above FFR.

"One of the reasons for the excellent reproducibility is because we're not relying on inducing a physiologic response to make a measure; we don't have big fluctuations in blood pressure, which you get with adenosine," he said.

William F. Fearon, MD, from Stanford University Medical Center in Palo Alto, Calif., who reported the two-year results of the FAME study at TCT in 2009, wondered about the barriers to FFR use, which are many, but may not include the use of adenosine.

"Having said that, anything that increases physiologic-guided PCI is going in the right direction," Fearon added.

To test the novel technology, Davies and colleagues conducted the ADVISE (ADenosine Vasodilation Independent Stenosis Evaluation) trial, a nonrandomized, international, multi-center study.

The plot showing close correlation between iFR and FFR (almost a 1:1 ratio, Davies said) comprised samples from both the left and right coronary arteries, indicating that iFR works between different arteries.

Investigators also adjusted for inherent variability in FFR, which increased the sensitivity (93%), specificity (97%), and PPV and NPV (97% and 93%, respectively). The diagnostic accuracy also increased from 88% to 95%.

Davies concluded that they've identified a wave-free period in the cardiac cycle when resistance is naturally stabilized and minimal, avoiding the need for administration of adenosine.

The instantaneous wave-free ratio measured during this wave-free period gives a measure of stenosis severity similar to FFR, he said.

The clinical implications of iFR, according to Davies, are that it can remove barriers to adoption of physiological assessment, improve workflow in the cath lab, and improve patient experience.

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