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.