Angio-based Quantitative Flow Ratio Virtual PCI Versus Conventional Angio-guided PCI in the Achievement of an Optimal Post-PCI QFR

Post-percutaneous coronary intervention (PCI) physiology The results of functional assessment performed after percutaneous coronary intervention (PCI) taught us that a "successful" angiography-guided PCI is often suboptimal in terms of physiology and that physiology result after PCI is closely linked to outcome. Yet, post-PCI physiology is rarely utilized in clinical practice, being employed in less than 10% of lesions investigated with physiology pre-PCI. Reasons for the low use of functional assessment post-PCI and for subsequent intervention are manifold. First, physiology is actually used after PCI only in cases where it was utilized pre-PCI. Second, randomized clinical trials (RCT) addressing the use of physiology to assess PCI results have not been performed, therefore clear instructions and cutoffs for its use are lacking. Third, when fractional flow reserve (FFR) is measured, the need to administer adenosine several times during the same procedure results in increased procedure time, cost and adverse side effects. Fourth, in case of a post-PCI suboptimal functional result, it may be difficult to ascertain the underlying cause.

New indices and tools have been developed in an effort to overcome barriers to the widespread adoption of functional assessment. Non-hyperaemic pressure indices (NHPI) including, resting distal to aortic coronary pressure ratio (Pd/Pa), and other resting indices have enabled functional evaluation without pharmacological arteriolar vasodilation, while angiography-based functional assessment have eliminated the need for a dedicated pressure wire.

Importantly, these newer tools may allow operators to understand the mechanism underlying an abnormal physiology value after angiographically successful intervention. In fact, the real novelty related to their development is the shift from a binary interpretation of physiology (positive/negative) to a quantitative site-specific one. For these reasons, they are extremely appealing post-PCI and several studies have been recently conducted to validate them in this setting.

Quantitative flow ratio (QFR) In particular, quantitative flow ratio (QFR) is an angiographically-derived estimate of FFR developed as an alternative to wire-based intracoronary physiology. One advantage of angio-based FFR systems is allowing the generation of a pullback curve and discrimination of the physiological contribution of each single lesion as well as diagnosis of diffuse disease. The value of QFR to assess the functional results of PCI has been tested in the prospective Angio-Based Fractional Flow Reserve to Predict Adverse Events After Stent Implantation (HAWKEYE) study. Seven hundred fifty-one vessels in 602 patients undergoing angiographically satisfactory second-generation drug eluting stent (DES) implantation were analyzed. At the end of the procedure, the operator acquired projections for QFR computation performed offline by an independent core laboratory. Receiver operating characteristic (ROC) curve analysis identified a post-PCI QFR best cut-off of <0.90 (area under the curve 0.77; 95% confidence interval [CI]: 0.74-0.80; p< 0.001). After correction for potential confounding factors, post-PCI QFR<0.90 was associated with a 3-fold increase in risk for the vessel-oriented composite endpoint at 2 years (HR: 2.91; 95% CI 1.63-5.19; p< 0.001). Further, a very important finding of the HAWKEYE study was the demonstration that QFR could discriminate among different coronary artery disease (CAD) patterns. In vessels with suboptimal functional result, the site of the QFR drop was in-stent in 13% of the cases, while a focal drop outside the stent was identifiable in 32% of the cases. Thirty-four percent of vessels showed diffuse disease, while in 21% a combination of the aforementioned possibilities was present. It is important to note that QFR analyzability depends on quality of angiography and it is feasible in around 80% of the cases. Moreover, QFR is not applicable in specific lesion subsets such as left main, bifurcation and ostial lesions.

Virtual PCI The inherent limit of post-PCI physiology is to add measurement and consequent actions after the end of a procedure that has been deemed successful by the same operator who is performing it. In addition, it is associated with the increase in procedural time and costs. Thus, a broad application of post-PCI physiology, although clinically meaningful, is implausible.