[Myocardial viability. Study of viability by myocardial scintigraphy].

UNLABELLED

FUNDAMENTAL PRINCIPLES: Myocardial scintigraphy is a metabolic approach to myocardial viability visualizing the localization, the extent and to some degree the quantity of non-functional yet viable myocardial tissue. Potential for functional recovery cannot be ascertained directly from the scintigram but can be inferred from commonly observed behavior after blood flow has been restored. Myocardial scintigraphy is thus fundamentally different from other functional exploration methods such as echocardiography or nuclear magnetic resonance imaging which can detect residual contractile capacity unmasked by inotropic stimulation. It must be remembered however that such 'forced' contractility may not necessarily be expresses spontaneously after revascularization and that, however detected, truly viable myocardium may not recover normal contractility after reperfusion when associated with non-transmural infarction or diffuse fibrosis. PET AND THALLIUM 201 SCANS: Positron emission tomography (PET) is the gold standard. Accomplished after administration of an isotope labeled substance (18-fluoro-deoxyglucose, FDG), the PET scan visualizes metabolic activity in viable myocardium. Special equipment is however required and facilities are limited, particularly in France. Thallium 201 scans can be acquired with conventional gamma cameras and protocols have been widely developed with nearly equivalent performance in certain situations of doubtful residual viability after post-infarction thrombolysis or angioplasty. It must be noted however that in such cases, search for homolateral or contralateral ischemia may be the main objective rather than the detection of residual viability. A 3-step thallium 201 scintigraphy protocol with stress, 4-hr redistribution then imaging after reinjection is usually sufficient to document ischemia or viability warranting revascularization. The problem is quite different for patients with major myocardial dysfunction and histological remodeling due to hypokinetic dilated cardiomyopathy. In such types of myocardium, chances of recovering inotropic capacity are quite limited and detecting viable tissue would be technically difficult; however with a proper protocol (without stress, resting images late after injection), thallium 201 scintigraphy can be helpful.

PERFORMANCE

Data in the literature shows that isotopic techniques lack specificity by overestimating the extent of viable tissue capable of recovering contractility. Actually this could be seen as an advantage since the consequences of missing even a small chance for revascularization warrant risking an ineffective procedure for a patient whose only alternative is heart transplantation. This situation explains why 18-FDG PET exploration should be performed even if the thallium scintigram leaves very little room for hope of recovering viable myocardium in patients with terminal disease.

PERSPECTIVES

Isotopic exploration of the myocardium is a moving field and routine practice can expect to benefit from research conducted in pioneer centers. The future offers two main perspectives: the development of metabolic tracers giving more precision than thallium 201 (for example isotope-labeled fatty acids); and technical advances in conventional gamma cameras more adapted to the physical characteristics of 18 FDG used for PET scans. Scintigraphy is an indispensible tool for metabolic exploration of the myocardium. Only nuclear magnetic resonance spectroscopy may provide comparable results.