Chapter 21: Nuclear Cardiology Procedures in the Evaluation of Myocardial Viability

INTRODUCTION

It is estimated that 5.7 million Americans adults had heart failure (HF) in 2012 with an associated cost of $30.7 billion in that year alone.¹ The incidence of HF is high, with 870,000 new cases being reported annually and projections suggest a prevalence of >8 million adults in United States by 2030.¹ Mortality attributed to HF in 2011 was 58,309 in the United States, and its mention on death certificates was 284,388 (1 in 9 deaths).¹

Although survival of patients with diagnosed HF has improved over time, it remains a disease with poor outcome. Approximately 50% of HF patients will be deceased 5 years after diagnosis.²

Given the growing prevalence of ischemic HF (IHF) and its high mortality rate, the optimal management strategies for IHF has been the focus of research for the last several decades. Evidence, including randomized controlled trials, have accumulated and support the notion that patients with IHF may benefit from viability imaging to guide therapy and revascularization decisions.^3–9 In some, but not all studies, in patients with IHF, the presence of viability, demonstrated on noninvasive images, is a strong indicator that revascularization may improve survival.^7,8,10–13 This chapter will describe the concepts of viability, nuclear imaging methods of assessing viability, supportive data and finally patients who could benefit from viability assessment.

UNDERSTANDING THE CONCEPTS

Viable, stunning, and hibernating myocardium are important concepts that are often misunderstood. Misconceptions of these three entities sometimes can lead to incorrect clinical decision making.

Viable versus Nonviable Myocardium

Dysfunctional myocardium can be dichotomized into viable or nonviable myocardium. In the latter, the tissue is replaced by irreversible fibrosis and therefore cannot be reversed nor improved with revascularization. Conversely, viable myocardium is that with variable function, metabolism, and blood flow. In cases of dysfunctional but viable myocardium due to repeated episodes or persistent abnormal coronary flow, restoring coronary flow may result in metabolic recovery and recovery of function.^3,14 The main goal of viability imaging is to define the amount of viable myocardium in order to guide decision making to therapies which improve not only LV function but also clinical outcomes.^4–10

Stunning

Stunned myocardium refers to a post ischemic state where there is mismatch between function and flow. Although resting coronary flow has returned to normal, myocardial function remains impaired. The duration of stunning is dependent on the duration, severity, and size of the ischemic insult.^15–18 In stunned myocardium, metabolic alterations prevail over structural changes. Electron microscopy of stunned myocardium shows normal or just mildly degenerated cells.¹⁴ Observed metabolic derangement includes a decrease in calcium sensitivity of myofilaments.¹⁸ Metabolic changes can be complex and change over time. A GLUT 4 translocation to the sarcolemma and an increase in glucose uptake have been ...