Adjunctive physiologic and intravascular imaging modalities as described in Chapters 24 and 25 enable operators to 1) direct ischemia-guided revascularization; 2) diagnose microvascular disease, coronary vasospasm, diffuse endothelial dysfunction, myocardial bridging or any combination of these entities; 3) characterize plaque morphology and severity; and 4) optimize acute and long-term percutaneous coronary intervention results. Herein we describe emerging clinical applications of in vivo physiologic and imaging modalities in the settings of stable ischemic heart disease, symptomatic myocardial ischemia without obstructive epicardial atherosclerosis, and acute coronary syndromes.
STABLE ISCHEMIC HEART DISEASE
Physiologic Evaluation of Intermediate Stenoses
As discussed in Chapter 24, fractional flow reserve (FFR) can accurately assess the hemodynamic significance of intermediate coronary lesions (40%-80% diameter stenosis) and guide coronary revascularization in patients with single vessel, multi-vessel, and left main disease. Physiologic assessment can also help simplify and determine the overall interventional approach for complex bifurcation lesions by converting them into a predominantly single lesion. In addition, FFR can be used to assess the need for intervention of jailed side-branches.
Considered the gold standard for invasive physiologic assessment, FFR has been preferred over other coronary indices such as hyperemic stenosis resistance (HSR), coronary flow reserve (CFR) and the instantaneous wave-free ratio (iFR) because it has a normal reference value of 1.00, is reproducible and relatively easy to perform, and has robust literature supporting its use for safely deferring and guiding revascularization.1-4 While HSR may be even more accurate in predicting the hemodynamic significance of an epicardial lesion, it requires both pressure and flow sensors for measurement and currently does not have substantial data supporting its clinical use.5,6
In patients with contraindications to adenosine use, non-hyperemic indices such as whole cycle distal pressure to aortic pressure ratio (Pd/Pa) or iFR may be preferable for intermediate lesion assessment. Although predictive of FFR with a diagnostic accuracy of 75% to 85%, Pd/Pa has a narrower gradient window and smaller signal-to-noise ratio, resulting in less robustness for ischemia prediction. For example, a pressure wire drift of ±2 mm Hg resulted in 31% of misclassified study lesions using Pd/Pa, compared to 21% for FFR (P < 0.001).7
Another non-hyperemic index, iFR measures the difference between distal and aortic coronary pressure during a specific mid to late diastolic period of the cardiac cycle, known as the wave-free period, when the resting resistance is relatively constant and low. Several studies have suggested that iFR has a diagnostic accuracy of 75% to 85% for identifying a hemodynamically significant FFR.8-11 The VERIFY study, however, reported a weaker correlation between iFR and FFR and recommended against its use for clinical decision making.12 Due to the variability of reported correlations between FFR, iFR, and Pd/Pa, the RESOLVE study was undertaken with analysis in a core laboratory to help settle questions surrounding the diagnostic accuracy of iFR and ...