Instantaneous wave-free ratio derived from coronary computed tomography angiography in evaluation of ischemia-causing coronary stenosis: Feasibility and initial clinical research

The instantaneous wave-free ratio (iFR) closely related to fractional flow reserve (FFR) is a adenosine-independent physiologic index of coronary stenosis severity. We sought to evaluate whether iFR derived from coronary computed tomographic angiography (iFR(CT)) can be used as a novel noninvasive method for diagnosis of ischemia-causing coronary stenosis. We retrospectively enrolled 33 patients (47 lesions) with coronary artery disease (CAD) and examined with coronary computed tomographic angiography (CTA), invasive coronary angiography (ICA), and FFR. Patient-specific anatomical model of the coronary artery was built by original resting end-diastolic CTA images. Based on the model and computational fluid dynamics, individual boundary conditions were set to calculate iFR(CT) as the mean pressure distal to the stenosis divided by the mean aortic pressure during the diastolic wave-free period of rest state. Ischemia was assessed by an FFR of up to 0.8, while anatomically obstructive CAD was defined by a stenosis of at least 50% by ICA. The correlation between iFR(CT) and FFR was evaluated. The receiver operating characteristic (ROC) curve was used to select the cut-off value of iFR(CT) for diagnosis of ischemia-causing stenosis. The diagnostic performances of iFR(CT), coronary CTA, and iFR(CT) plus CTA for ischemia-causing stenosis were compared with ROC curve and Delong method. On a per-vessel basis, iFR(CT) and FFR had linear correlation (r = 0.75, p < 0.01). ROC analysis identified an optimal iFR(CT) cut-off value of 0.82 for categorization based on an FFR cut-off value 0.8, and the diagnostic accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of iFR(CT) were 78.72%,70.59%, 83.33%,70.59%, and 83.33%, respectively. Compared with obstructive CAD diagnosed by coronary CTA (AUC = 0.60), iFR(CT) yielded diagnostic improvement over stenosis assessment with AUC increasing from 0.6 by CTA to 0.87 (P < 0.01) and 0.90 (P < 0.01) when iFR(CT) plus CTA. In conclusion, iFR(CT) is a promising index improving diagnostic performance over coronary CTA for detection of ischemia-causing coronary stenosis.