Review
computed tomography (SPECT) or dobutamine echocardiography, was associated with an increased probability of survival, this finding was not found to be statistically significant after adjustment for multiple baseline factors for mortality.29
However, the analyzed
subgroup represented fewer than 50 % of patients from the study and there was no randomization of the patients. The assessment of viability was performed by two commonly used techniques. However, neither of them is considered a gold standard of viability evaluation and, since there was no randomization and double-blinded control, we cannot conclude with certainty that the result of viability studies did not influence the final treatment strategy.
Currently, positron emission tomography (PET), SPECT, dobutamine stress echocardiography, and magnetic resonance imaging (MRI) are among the most common methods of assessment of viability in ICM.
Positron Emission Tomography
The extent of myocardial viability using PET may be evaluated by comparison of myocardial perfusion and metabolism in segments with balanced reduction of function and perfusion. Myocardial perfusion is evaluated with a flow tracer using either 13
F-fluorodeoxyglucose (FDG) uptake. Positive 18 F-FDG
The American Heart Hospital Journal
above-mentioned radionuclide tracer is more than 50–60 % in the dysfunctional myocardium.33,34
Thallium-201 is a radioactive substance that has cellular uptake similar to that of potassium. Thallium uptake by the myocardial cell is partially an active process involving the sodium/potassium adenosine triphosphatase (Na/K ATPase) pump and requires both myocardial cell integrity and good perfusion for delivery to different areas of the heart.
In 1977, it was suggested that stress-induced thallium defects could normalize (‘redistribute’) on images repeated in several hours.35
This was considered to be useful in the
differentiation of transient perfusion abnormalities related to ischemic tissue versus infarct. Redistribution after four hours can overestimate the extent of myocardial necrosis. Reinjection of a smaller dose of thallium-201 immediately before the redistribution images are taken improves the detection of viable myocardium.36
This method has been
Viability studies performed with the use of technetium-99m sestamibi and technetium-99m tetrafosmin are reported to be comparable with studies performed using thallium-201.34,39
shown to detect viability in 50–70 % of territories that would be necrotic on standard thallium redistribution imaging.36–38
N-ammonia or
rubidium-82. Metabolism is evaluated with cellular 18
uptake indicates cellular viability. The presence of a mismatch pattern between cellular viability and myocardial perfusion is considered to show areas of the heart where hibernating myocardium is present and myocardial function may be improved if the blood supply is restored.
PET is considered to be the gold standard for the evaluation of myocardial viability.30
Studies have demonstrated the
importance of FDG-PET for morbidity and mortality benefit in patients undergoing revascularization. Revascularization of patients with viable myocardium is reported to improve patient prognosis in 55 % and left ventricular ejection fraction in 27 % of patients with ICM.31 Revascularization decisions incorporating myocardial viability evaluation have significantly improved mortality and prognosis ofpatients with ICM.32
Single-photon Emission Computed Tomography
Myocardial viability may be assessed by analysis of myocardial uptake of thallium-201, technetium-99m sestamibi, or technetium-99m tetrofosmin by SPECT. Myocardium is considered viable if the uptake of the
108 Viability Studies—Comparison of Techniques
Although FDG-PET scanning is considered to be the gold standard for assessing myocardial viability, its widespread use is limited by the high cost of the imaging system and tracers. Bonow et al. evaluated the compatibility of viability assessment by both FDG and thallium-201-based methods.41
Four hundred thirty-two
myocardial segments were analyzed from comparable transaxial tomograms. One hundred sixty-six segments were found to be irreversible on the standard four-hour redistribution using thallium. FDG uptake occurred in 121 of these regions (73 %). When using the thallium reinjection method only 78 of the 166 regions (47 %) showed thallium activity. This is significantly lower than FDG uptake images obtained by PET. However, in subgroup analysis the areas with severe thallium reuptake defect after reinjection (<50 % of peak activity) showed uptake in 51 % of the segments, which is consistent with results obtained from FDG uptake in the same group. Irreversible defects with mild or moderate reduction in thallium activity represent viable myocardium in a majority of cases as was confirmed by FDG uptake analysis. The superiority of PET viability studies was also shown by Alexanderson et al.41 Their analysis showed that PET has a 30 % higher viability detection rate when compared with thallium SPECT redistribution.
Winter 2011
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