Chapter 15

### INTRODUCTION

In the past two decades, a great body of literature has established the use of nuclear imaging for risk stratification in patients with known or suspected coronary artery disease (CAD). The earlier studies have been reinforced, more recently, with the use of single photon emission computed tomography (SPECT) and electrocardiographic (ECG)-gated SPECT imaging. This chapter will review the use of stress radionuclide SPECT perfusion imaging for risk stratification in a general population and among patients with chronic CAD. Risk stratification prior to major non-cardiac surgery is discussed in Chap. 16 and risk assessment after therapeutic intervention is detailed in Chap. 17.

### RISK ASSESSMENT

Risk stratification is of crucial importance for the practice of contemporary medicine. Appropriate management of known coronary disease should include the assessment of the individual risk of future cardiac events, including cardiac death and myocardial infarction (MI).1 Extending the paradigm of non-invasive cardiac testing beyond the detection of disease is especially important, as risk assessment permits patient management decisions to be formulated on an evidence-based approach. Patients who are identified as being at a high risk for subsequent cardiac events should be considered for aggressive management, including cardiac catheterization and revascularization procedures that may improve their outcome. Conversely, the management focus in patients with low future event rate should be shifted toward risk factor modification and aggressive medical therapy,1,2 reserving invasive procedures for patients who fail medical management. Additional testing in this low-risk group should also be avoided, thereby minimizing cost. A risk assessment, outcomes-based model strives for improved patient outcome and avoidance of complications from unnecessary procedures, and is cost-effective.

Risk strata are often defined in many ways but when related to CAD events, specifically non-fatal MI and cardiac death, an annual event rate of <1% per year is accepted as a low-risk group of patient. In a similar fashion, those patients with an annual event rate of >3% are considered a high-risk cohort, with those between 1% and 3% considered at intermediate risk.3

Clinical parameters may be used to define risk such as those based on cardiac risk factors4 and the characterization of symptoms, such as chest pain5 or the presence of dyspnea.6 However, risk assessment based on clinical findings and resting ECG only is limited. Exercise testing can also help, especially when examining the patient's functional capacity.7 Exercise-induced ECG changes and risk indices, such as the Duke treadmill score, also have substantial prognostic value.8 Unfortunately, using clinical data and the Duke treadmill score, most patients (55%) with suspected CAD would fall in an intermediate-risk group9 necessitating additional risk stratification.

Coronary angiography, considered the "gold standard" for the diagnosis of CAD, does not provide information about the physiologic significance of atherosclerotic lesions, especially in borderline lesions (50–70% stenosis). In fact, disparity often exists when comparing this anatomic finding with the physiologic data obtained with SPECT.10 More importantly, angiographic data do not provide a clear marker of risk of adverse events, especially ...

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