MYOCARDIAL VIABILITY- ASSESSMENT AND CLINICAL RELEVANCE

Abstract

The assessment of myocardial viability is an important indication for noninvasive imaging in patients with coronary artery disease and chronic ischemic left ventricular dysfunction. Left ventricular function is a major determinant of survival in coronary artery disease. The goal of myocardial viability assessment is to differentiate patients with potentially reversible from irreversible left ventricular dysfunction9. In patients with chronic coronary artery disease and left ventricular dysfunction, there exists an important subpopulation in which revascularization may significantly improve regional or global left ventricular function, as well as symptoms and hence therapeutic and prognostic benefits in selected patients. The underlying pathophysiology involves reversible myocardial dysfunction (hibernation or stunning) which may exist independently or may coexist within the same patient. Stunned myocardium refers to the state of persistent regional dysfunction after a transient period of ischemia followed by reperfusion, most commonly present in acute coronary syndromes. Hibernating myocardium refers to a condition of chronic sustained abnormal contraction due to chronic underperfusion in patients with coronary artery disease in whom revascularization causes recovery of function. These states of potentially reversible left ventricular dysfunction commonly have preserved cell membrane integrity and metabolic activity to maintain cellular functions in the absence of normal myocyte contractility secondary to resting ischemia. Stunned myocardium improves its function early post revascularization, whereas hibernating myocardium may need longer time to fully recover in function. Furthermore, exercise capacity improved in patients with viable myocardium, and long term prognosis appeared favorable if patients with viable myocardium underwent revascularization. Viable myocardium has unique characteristics and these form the basis for the different imaging modalities that are currently available for the assessment of myocardial viability. A number of diagnostic techniques have emerged for differentiating viable from non viable myocardium in dysfunctional regions. These include evaluation of regional perfusion, cell membrane integrity, and metabolism using nuclear techniques with various radionuclide tracers; contractile reserve using dobutamine echocardiography or magnetic resonance imaging. More conventional approaches of identifying scarred and necrotic myocardium including presence of occluded coronary artery, regional contractile dysfunction, Q waves on electrocardiogram have been shown to be less accurate. New modalities include use of metabolic tracers with single photon emission tomography (SPECT), precise quantitative metabolic evaluation with positron emission tomography (PET), assessment of microvascular integrity with contrast echocardiography and use of magnetic resonance imaging (MRI). Most of these techniques are reasonably accurate in predicting myocardial viability.