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Figure 16. Flow chart outlining a cost-effective strategy for the evaluation of coronary artery disease (MPI-myocardial perfusion imaging; GXT-graded exercise stress test).

weeks of the acute event. The ability to perform vasodilator stress imaging safely two to four days following acute infarction with greater separation of low and high risk patients compared with submaximal exercise imaging makes this an attractive strategy. Patients identified as low risk can be considered for early discharge whereas those at high risk can be referred for early angiography and possible revascularization.

Unstable Angina

Patients with an acute non-ST segment elevation coronary syndrome (unstable angina or non-Q wave myocardial infarction) who have recurrent angina, ST segment depression or elevated troponin should be considered for early coronary angiography. Patients with unstable angina without ST segment depression or biochemical markers of myocardial injury who respond to initial medical treatment can be effectively risk stratified with perfusion imaging. In the latter group, there is no reduction in hard cardiac events with early coronary angiography and revascularization compared with

Figure 17. 50 year old male with previous inferior MI. A GXT demonstrated mild ST segment depression at a high workload without angina. The MPI study was done for risk stratification. A large area of severely decreased uptake is seen in the inferior wall at stress (arrows). This area normalizes at rest. Subsequently, angiography demonstrated an occlusion of the RCA which was dilated and stented.

Figure 17. 50 year old male with previous inferior MI. A GXT demonstrated mild ST segment depression at a high workload without angina. The MPI study was done for risk stratification. A large area of severely decreased uptake is seen in the inferior wall at stress (arrows). This area normalizes at rest. Subsequently, angiography demonstrated an occlusion of the RCA which was dilated and stented.

a non-invasive strategy reserving angiography for patients with recurrent angina or evidence of inducible ischemia. The presence of jeopardized viable myocardium identifies patients at high risk for cardiac death or non-fatal infarction whereas the absence of a reversible defect identifies patients at low risk.

Pre-operative Cardiac Risk Assessment

Pre-operative MPI can help identify patients at high risk for perioperative cardiac complications by identifying jeopardized viable myocardium.

Patients undergoing vascular reconstructive surgery including repair of an abdominal aortic aneurysm have a high prevalence of CAD and are at an increased risk of perioperative cardiac ischemic complications including death and non-fatal myocardial infarction. Age greater than 70 years, history of myocardial infarction, cardiac failure, angina pectoris and diabetes help stratify patients into low, intermediate (one to two factors) and high risk (three or more factors). Patients considered by clinical assessment to be at intermediate risk can be further risk stratified with dipyridamole MPI based on the extent and severity of reversible defects. Patients at high clinical risk may be referred for coronary angiography although we prefer an initial strategy of MPI in both intermediate and high risk patients reserving angiography for those with extensive inducible ischemia.

The negative predictive value of a normal scan is approximately 99% while approximately 30% of intermediate risk vascular surgical patients with scintigraphic evidence of inducible ischemia will have a perioperative cardiac ischemic complication. Perioperative beta-blockers reduce cardiac death and non-fatal infarction in high-risk patients. Alternatively patients considered to have a high probability of multivessel CAD and left ventricular dysfunction by non-invasive assessment can selectively be referred for coronary angiography and possible myocardial revascularization (CABG or PTCA) with a view to improving their long-term survival.

Clinical Role in Defining Myocardial viability

Impaired left ventricular function is an important determinant not only of symptoms but also of survival. Regional LV dysfunction may result from (a) a transmural myocardial infarction (b) a non-transmural (subendocardial) myocardial infarction (c) a transient period of myocardial ischemia ("stunning") causing a wall motion abnormality for a variable length of time after the transient ischemic insult or (d) a prolonged period of resting myocardial hypoperfusion ("hibernation"). It has been suggested that hibernating myocardium may actually represent the effects of repetitive stunning i.e., patients with hibernating myocardium have reduced coronary flow reserve without reduced resting coronary blood flow.

If the myocardium in the abnormally contracting region is viable i.e., has sustained metabolic activity, left ventricular dysfunction in patients with CAD may be potentially reversible with restoration of coronary blood flow for those patients with stunned or hibernating myocardium. On the other hand regional left ventricular dysfunction due to myocardial infarction (scar) is irreversible after revascularization. Recovery of regional left ventricular dysfunction may occur (a) after thrombolytic therapy or PTCA for acute myocardial infarction (b) after exercise induced ischemia (c) in patients who have experienced unstable angina and (d) in the setting of chronic "silent" ischemia (when overt or compensated cardiac failure may be the dominant clinical finding) after CABG or PTCA.

There are a number of non-invasive techniques available for the assessment of myocardial viability (Table 4). Metabolic imaging with 18F-FDG is often considered the gold standard for assessing myocardial viability. 18F-FDG uptake in a region of decreased perfusion (mismatch pattern) supports a determination of myocardial viability. Presently, few centers have the capacity to perform PET studies.

201Tl and 99mTc-sestamibi myocardial perfusion imaging assess perfusion and cell membrane integrity and therefore provide information regarding myocardial viability. Both defect reversibility and severity are important markers of viable myocardium (Fig. 18). With quantitative analysis of regional tracer activity, a threshold of 201Tl or 99mTc-sestamibi activity greater than 50% of maximal regional activity as a criterion for viability has a positive predictive accuracy of approximately 70% and a negative predictive accuracy of 90% for improved regional left ventricular function after revascularization. The predictive accuracy of myocardial perfusion imaging for functional recovery can be improved by considering the level of regional tracer activity as a continuum and documenting whether there is inducible ischemia.

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