Novel Angiography-based Virtual FFR Offers Accurate, Less Invasive Option

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Computer reconstruction using angiographic data alone enables assessment of myocardial fractional flow reserve (FFR) with no need for invasive measurement or induction of hyperemia. In a small study published in the February issue of JACC: Cardiovascular Interventions, the ‘virtual’ FFR (vFFR) method showed a 97% accuracy rate in identifying functionally significant coronary lesions.

Researchers led by Paul D. Morris, MD, of the University of Sheffield (Sheffield, United Kingdom), evaluated the method in 19 patients with stable CAD at a single institution who underwent angiography in anticipation of elective PCI.

FFR was measured invasively with a pressure wire (mFFR), after which 13 ischemia-inducing lesions (FFR < 0.80) were stented. Angiography and mFFR were then repeated, followed by reconstruction of three-dimensional arterial anatomy pre- and post-stenting and application of generic boundary conditions for computational fluid dynamics analysis. Finally, vFFR and mFFR values were compared.

High Diagnostic Accuracy

Overall, 35 matched anatomical and physiological data sets were obtained: 10 right coronary arteries (5 pre- and post-stenting), and 12 left coronary arteries (8 pre- and post-stenting). The average Syntax score of the vessels studied was 4.9.

Overall diagnostic accuracy of vFFR was 97%. Applying a more stringent threshold of significance for FFR (< 0.75) produced a sensitivity of 71% (2 false negatives), a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 93% for an overall diagnostic accuracy of 94%.

In terms of quantitative accuracy, the mean difference between mFFR and vFFR was fairly small, as was the average absolute error (table 1). The correlation coefficient of the vFFR and mFFR values was 0.84.

Table 1. Quantitative Accuracy of vFFR vs. mFFR

To investigate whether accuracy was consistent across all values of FFR, the researchers compared the agreeability of vFFR and mFFR excluding data from post-stent cases. There was a small increase in average absolute error, but this was not significant (P = 0.412).

Virtual Advantages for Operators

According to the authors, the level of accuracy achieved by the virtual FFR is “excellent” considering the small size of the study.

Furthermore, they say, the innovative angiographic model carries several advantages:

• It only requires knowledge of vessel geometry
• There is no need for induction of hyperemic flow
• No additional procedure time is required
• It avoids the hazard of passing an intracoronary wire
• No additional equipment, training, or cost are required

“A computational tool such as this would improve operator and patient access to physiologically

guided decision making with potential impact on clinical outcomes and cost,” they add. “The current workflow will be simplified for use by a nonspecialist cardiologist or radiographer for use at the time of diagnostic angiography to plan revascularization strategy.”

A further advantage of vFFR, they note, is that the effects of multiple lesions or collateral vessels can be included in the simulation.

The main disadvantage of the virtual method, however, is that it requires a rotational coronary angiogram, which is not universally available, can be cumbersome to perform, and is invasive, Dr. Morris and colleagues acknowledge. It also could underestimate the severity of stenosis due to “alterations in the resistance of the downstream microcirculation.” The virtual model also makes assumptions about downstream resistance and applies a “one-size fits all” approach.

“Consequently, in cases with abnormally high microcirculatory resistance or when maximal hyperemia has not been achieved, the vFFR is more likely to deviate from the mFFR,” the investigators note.

Although somewhat similar to calculation of FFR by coronary CT angiography (eg, Koo B-K et al. J Am Coll Cardiol. 2011;58:1989-1997), the vFFR approach relies on vessel geometry alone derived from angiography and “does not involve an estimation of myocardial mass or computed tomography,” which the authors say may result vFFR achieving more accurate results than the CTA calculation method.

Source:

Morris PD, Ryan D, Morton AC, et al. Virtual fractional flow reserve from coronary angiography: Modeling the significance of coronary lesions. Results from the VIRTU-1 (VIRTUal fractional flow reserve from coronary angiography) study. J Am Coll Cardiol Intv. 2013;6:149-157.

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