Myocardial Ischemia, Fluorodeoxyglucose, and Severity of Coronary Artery Stenosis: The Complexities of Metabolic Remodeling in Hibernating Myocardium
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To the Editor:
Myocardial imaging of perfusion (with either 13N-ammonia or 82Rb) and of metabolism with fluorodeoxyglucose (FDG) is a validated method for differentiating viable myocardium from necrosis or scar.1,2 Early in its clinical application, FDG was injected during exercise stress to intensify the difference between normal and ischemic myocardium. This approach was based on the following general principles of myocardial metabolism. (1) Normally perfused, nonischemic myocardium metabolizes primarily fatty acids. (2) Underperfused, ischemic myocardium metabolizes glucose. (3) Fasting suppresses glucose metabolism in normally perfused myocardium (but has no effect on anaerobic glucose metabolism in ischemic myocardium). (4) Catecholamines released during exercise stress further suppress exogenous glucose metabolism in normal myocardium but do not alter anaerobic glucose metabolism in ischemic myocardium.3–5 In principle, “hot-spot” imaging of myocardial ischemia would therefore identify “significant stenosis” of a coronary artery independently and separately from myocardial perfusion or anatomic stenosis severity.
However, those of us who enthusiastically supported FDG imaging quickly abandoned this approach 17 years ago. Hot-spot imaging during exercise was deemed not reliable because heart metabolism was, in fact, more complex than the generalized concepts outlined above. The switching of metabolic substrates in the heart is a complex process and cannot be reliably assessed by FDG under fasting and exercise conditions. Consequently, FDG uptake in the heart under these conditions does not reliably differentiate between ischemic and nonischemic myocardium.
Although FDG imaging did not prove diagnostically useful under exercise conditions, at rest it was able to differentiate between ischemic, viable myocardium and necrotic, scarred myocardium. Not surprisingly, this approach is used widely for deciding on revascularization procedures. The understanding of myocardial metabolism has now evolved with a better molecular and genetic understanding of these early and perhaps erroneous concepts.
There is no question that imaging with the glucose …