Review ArticleNon-invasive quantification of coronary vascular dysfunction for diagnosis and management of coronary artery disease
Introduction
The last several decades have seen significant advances in the evaluation and treatment of coronary artery disease. Multiple non-invasive methods for the diagnosis of coronary atherosclerosis have matured and are in wide clinical practice. Improving risk factors and primary1,2 and secondary prevention strategies3 have resulted in a decline in cardiac mortality and myocardial infarction.4,5 The focus of these diagnostic methods and treatment strategies has been on the identification and treatment of atherosclerotic lesions of the epicardial coronary arteries, particularly obstructive, ischemia causing lesions. While there is no doubt that this approach has been tremendously successful, it must be noted the coronary arterial circulation extends from large epicardial conduit arteries through resistance arterioles (i.e., the microvasculature) to the intra-myocardial capillary bed. Dysfunction of the microvasculature and pre-obstructive disease of the epicardial arteries can not only cause typical anginal symptoms but also may be harbingers of adverse prognosis. In the following paragraphs, we will discuss current approaches to the quantification of coronary vascular function and the evidence supporting its potential diagnostic and prognostic implications with a particular focus on ischemic heart disease.
Section snippets
Diagnosis of Obstructive Coronary Artery Disease
In addition to clinical history and symptoms, stress testing modalities are an integral part of the standard evaluation for epicardial coronary atherosclerotic disease among intermediate risk patients.6 Addition of imaging to identify stress-induced perfusion abnormalities substantially improves sensitivity for diagnosing obstructive stenosis. A recent meta-analysis has demonstrated excellent diagnostic performance of both SPECT and PET myocardial perfusion imaging (radionuclide MPI, R-MPI)7
Quantification of Myocardial Blood Flow (MBF) and Flow Reserve
As discussed above, two important limitations of myocardial perfusion imaging arise from semi-quantitative interpretation: the underestimation of the extent of ischemia when all three coronary territories are affected and inability to identify patients with non-obstructive stenosis. Quantitative assessment of MBF offers the opportunity to add important information to semi-quantitative assessments of R-MPI and potentially overcome these limitations. Both absolute stress MBF and flow reserve,
Quantitative MBF for Diagnosis of Obstructive CAD
A number of studies have demonstrated that among relatively young patients with modest coronary risk factor burdens and predominantly single-vessel CAD, a relationship exists between MBF or flow reserve and percent diameter stenosis on angiography (Figure 2).52, 53, 54, 55 These studies demonstrate that myocardial vasodilator capacity is relatively preserved for lesions with <50% stenosis. With increasing severity of stenosis beyond this level, there is progressive worsening of CFR. This
Prognostic Implications of Coronary Vascular Dysfunction
Because quantitative measures of coronary vascular function integrate the fluid dynamic effects of atherosclerosis throughout the coronary arterial tree including epicardial stenoses with early changes to endothelial and/or smooth muscle function, quantitative myocardial flow reserve may be a superior measure of overall vascular health that provides unique information about clinical risk. Five studies have demonstrated that PET measures of myocardial flow reserve improve cardiac risk assessment
Medical Treatment of Coronary Vascular Dysfunction
Despite growing understanding of the pathophysiologic basis and prognostic significance of coronary vasomotor dysfunction, the treatment implications of this condition remain uncertain. Several standard treatments which have been proven to reduce risk in persons with atherosclerosis including statins91, 92, 93, 94, 95, 96, 97, 98 and multiple classes of antihypertensive medications99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117 have been shown to
Implications for Angiography and Revascularization
While the use of CFR to identify candidates for medical therapy remains untested, even less certainty exists about how to incorporate CFR into decision making for angiography and revascularization. As discussed above, addition of CFR to other high-risk findings on stress testing may improve identification of patients with high-risk coronary anatomy (i.e., left main or three-vessel coronary disease).56,57 However, because diffuse atherosclerotic changes and microvascular dysfunction can also
Conclusions
Despite major progress in the evaluation and treatment of coronary artery disease, focus has largely been on epicardial atherosclerosis. Methods to evaluate coronary vasodilator function are rapidly maturing and are complementary to current standard of care. These tools offer the potential to identify earlier stages of atherosclerotic coronary disease as well as to improve risk stratification and selection and titration of medical and revascularization therapies.
Acknowledgments
The work was funded in part by Grants from the National Institutes of Health (RC1 HL101060-01, T32 HL094301-01A1).
Disclosures
Dr Murthy owns equity in General Electric. Dr Di Carli receives research funding from Toshiba.
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