ATLANTA--In choosing the appropriate test, keeping clinical context in mind and understanding what the test indicates and what it does not indicate are key elements a physician must remember in selecting the correct imaging modality for the testing of myocardial viability, said Christopher Hanson, MD, professor of medicine and radiology at Thomas Jefferson University Hospital in Philadelphia, during an imaging symposium at the American College of Cardiology (ACC) annual conference on Monday.
“We are living through an epidemic of coronary artery disease (CAD), seeing increased instances of heart failure and also seeing a larger cohort of revascularized patients than we did 15 to 20 years ago,” said Hanson.
Citing myocardial viability as a “clinically important problem,” Hanson noted that there is not a clear definition of viability. One component of viability is when the myocardium is still alive, but not necessarily able to be improved with revascularization. A second definition of viability is that the myocardium may be recoverable by way of revascularization, and the prognosis of the patient can be expected to improve.
In selecting the appropriate test for viability, Hanson explained, “There are a large number of non-ischemic causes of heart failure and they overlap. It’s quite common to find myocardium with mild to moderate CAD and you have to be able to sort that out [on the image] and find out which form of therapy is most appropriate for the patient.”
In searching for viability, Hanson said that physicians should be looking for “hibernating myocardium” or resting ischemia. “For viable myocardium,” he explained, “think of the rules of two.” The test must determine first if the myocardium is alive, and second, if it is resting ischemia and still viable. Therefore, Hanson noted that physicians will need to test for the differentiation between the two conditions. Depending on how the test is done, he said that some modalities may provide both answers.
In the assessment of viability, thallium is the standard way to determine viability, said Hanson. “It has the best kinetics, and it is a potassium analog,” he explained. By adjusting the protocols for thallium, such as delaying imaging reinjection, more answers can be found during testing, he noted.
Metabolic agents can also be used, said Hanson, who noted that FDG, a glucose analog, will show viability and it may also show resting ischemia, depending on how the imaging test is completed. “The problem with FDG is that under certain conditions, especially for diabetics, the uptake is different because the heart prefers to metabolize fatty acids. The way around that is to force the myocardium to consume fatty acids as its primary source of energy.”
In terms of what to look for, Hanson said that the function of the heart’s contractions implies viability. If you are looking at wall motion, function tends to get worse with ischemia,” he said.
Once an imaging test is read, treatment options can include revascularization or medical therapy; however, in viable myocardium, “there is a dramatic difference in survival between revascularization and medical therapy.”
After the physician reads the appropriately chosen test, the right care decision can be made, noted Hanson.
“The focus for the treatment of any patient, for any condition, is what effect it’s going to have on quality of life and survival,” Hanson concluded.