Rest-injected 201thallium in the evaluation of myocardial viability.

In recent years 201thallium scintigraphy at rest has been used for evaluating myocardial viability in patients with chronic ischemic coronary artery disease and left ventricular dysfunction. Based on the assumption that reversible myocardial dysfunction arises from chronic hypoperfusion (hibernation), resting 201thallium scintigraphy is performed by acquiring two sets of images, one early after tracer injection and a second following 3 to 4 hours to allow for the redistribution process to take place. However, redistribution of 201thallium following injection at rest rarely occurs, and in many studies it does not significantly contribute to the identification of reversibly dysfunctional myocardium. In fact, current interpretation of resting 201thallium scintigraphy is based on the measurement of regional tracer uptake on the redistribution images, using a fixed threshold value (most commonly from 50 to 65% of maximal uptake) that arbitrarily identify viable (presumably reversible) and nonviable (presumably irreversible) dysfunctional myocardium. The practical implication of this approach is relevant as it implies that analysis of a single set of images is adequate for viability information. As with other nuclear techniques, sensitivity of 201thallium scintigraphy for predicting functional recovery following revascularization is very high, but specificity is suboptimal, reflecting the identification of substantial residual tracer uptake in territories that will remain dysfunctional at rest following successful revascularization. Inadequate timing of follow-up functional evaluation, ongoing degeneration of hibernating myocytes, admixture of necrotic and normal myocardium in dysfunctional areas are among factors that likely explain this discrepancy. It remains to be evaluated in future studies whether revascularization of these areas that contain viable myocardium but where resting function does not change, may also contribute to the beneficial effects of revascularization in patients with left ventricular dysfunction.