Myocardial Viability

Disease Background.
A key issue for imaging is to determine whether a given portion of the heart is viable. This means looking at areas of the heart that are not functioning properly and determining whether tissue is still alive and can recover if the blood supply is restored by revascularization. This is a biochemical question. Biochemists and biologists have shown that glucose is a protective substrate to the heart when blood flow is limited. FDG PET helps to determine viability, because those areas of the myocardium that are viable will have glucose metabolism. On the other hand if the myocardial muscle is dead, it will not have any glucose metabolism. The patient whose myocardial muscle demonstrates no glucose metabolism will not benefit from having blood supply re-established to the muscle. Such a patient would need medical therapy or a heart transplant. About 35% of coronary artery disease patients who receive bypass surgery or angioplasty to revascularize the heart do not show improvement in cardiac function because the affected tissue is not reversible (i.e., is dead).

Case Example.
A 57-y-old patient with a previous heart attack was evaluated by echocardiography (echo), which showed that the patient´s left ventricular ejection fraction (percentage of blood ejected from the heart during cardiac cycle) was compromised at 35% (normal >55%) and that wall motion was abnormal. An FDG PET cardiac study was requested to evaluate for viable, reversible myocardium. The PET image on the left in Figure 20 was obtained by using 13N ammonia in a study of blood perfusion to the heart. 13N ammonia has been approved by the U.S. Food and Drug Administration for imaging blood flow in the heart. The donut-like structure is the heart muscle, and the chamber it encloses is the left ventricle. The defect (arrow) seen toward the right side of the donut is an area of compromised blood flow. The image on the right in Figure 20 is the FDG PET glucose metabolism study, and it clearly shows FDG metabolism in the same area that is compromised with regard to blood flow. This patient, therefore, would be likely to benefit from revascularization (bypass surgery to restore blood to a portion of the heart). This is referred to as a mismatch pattern (i.e., low blood flow with high glucose metabolism).

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FIGURE 20. Case example, myocardial viability.

Why Did FDG PET Help?
FDG PET showed that the patient had viable myocardial tissue, which, if blood flow could be restored, could return the function of the heart closer to normal. The patient, therefore, could avoid a heart transplant by undergoing bypass surgery instead. This patient underwent bypass surgery. The ejection fraction returned to 50% and the wall motion to normal levels.

Key Management Issues.

Summary of Evidence for FDG PET in Myocardial Viability Assessment.
Myocardial viability studies with FDG PET should be performed in patients with ischemic heart disease and impaired left ventricular function who are potential candidates for coronary revascularization (Table 21).

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TABLE 21 FDG PET in Myocardial Viability: Results of Literature Search

Presence of myocardial viability as determined by FDG PET predicts functional improvement, improved daily life activity levels, and improved survival after revascularization.

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