MYOCARDIAL VIABILITY

Left ventricular function is a major predictor of outcome in patients with coronary artery disease. Acute ischemia, postischemic dysfunction (stunning), myocardial hibernation, or a combination of these 3 are a mong the reversible forms of myocardial dysfunction. In myocardial stu nning, dysfunction occurs despite normal myocardial perfusion, and fun ction recovers spontaneously over time. In acute ischemia and hibernat ion, there is regional hypoperfusion. Function improves only after rev ascularization. Evidence of myocardial viability usually relies on the demonstration of uptake of various metabolic tracers, such as thalliu m (thallous chloride Tl 201) or fludeoxyglucose F 18, by dysfunctional myocardium or by the demonstration of contractile reserve in a dysfun ctional region. This can be shown as an augmentation of function durin g the infusion of various sympathomimetic agents. The response of vent ricular segments to increasing doses of dobutamine may indicate the un derlying mechanism of dysfunction. Stunned segments that have normal p erfusion show dose-dependent augmentation of function. If perfusion is reduced as in hibernating myocardium, however, a biphasic response us ually occurs: function improves at low doses of dobutamine, whereas hi gher doses may induce ischemia and, hence, dysfunction. But in patient s with severely impaired perfusion, even low doses may cause ischemia. Myocardial regions with subendocardial infarction or diffuse scarring may also have augmented contractility during catecholamine infusion d ue to stimulation of the subepicardial layers. In these cases, augment ation of function after revascularization is not expected. Because the underlying mechanism, prognosis, and therapy may differ among these c onditions, it is crucial to differentiate among dysfunctional myocardi al segments that are nonviable and have no potential to regain functio n, hibernating or ischemic segments in which recovery of function occu rs only after revascularization, and myocardial stunning in which func tion is expected to recover spontaneously. Because combinations of all of these disorders may occur, even in the same segments, caution shou ld be used in interpreting the imaging results.