THE NONINVASIVE ASSESSMENT OF MYOCARDIAL VIABILITY

The major objective of noninvasive imaging for detection of myocardial viability is to assist in the improved selection of patients with coronary artery disease and severe left ventricular dysfunction who would benefit most from revascularization. The techniques most commonly used to identify viable myocardium are thallium-201 (Tl-201) scintigraphy, positron emission tomography (PET) using a flow tracer in combination with a metabolic tracer, technetium-99m (Tc-99m) sestamibi imaging, and dobutamine echocardiography. On stress Tl-201 scintigraphy, asynergic regions showing normal thallium uptake, an initial defect with delayed redistribution at 3-4 h, late redistribution at 24 h, or defect reversibility after reinjection of a second dose of Tl-201 at rest all suggest preserved viability. The greater the final uptake of Tl-201 in areas of regional myocardial dysfunction preoperatively, the greater the improvement in ejection fraction after coronary, revascularization. Demonstration of uptake of fluoro-18 deoxyglucose (FDG) in regions of diminished blood flow on PET imaging also correlates well with improved systolic function after revascularization. Tc-99m sestamibi may also be useful for assessment of myocardial viability, particularly after thrombolytic therapy for acute myocardial infarction. Dobutamine echocardiography has good positive predictive value for viability determination, but absence of systolic thickening in an akinetic zone in response to intravenous infusion of the drug may still be associated with viable myocardium in 25-50% of segments. Of all the techniques cited above, quantitative resting Tl-201 scintigraphy may be the best approach for distinguishing between viable and irreversibly injured myocardium.