<200 U/L 2-300 U/L 3-400 U/L 4-500 U/L >500 U/L

Fig. 1. Proportion of patients who had positive (black bars) and negative (white bars) rest MPI and levels of (A) CK-MB and (B) CK elevations. Of 104 patients who had creatine kinase myocardial band (CK-MB) level <8 ng/mL, 61% had negative images, whereas of 192 patients who had CK-MB >8 ng/mL, only 27% had negative images (sensitivity 83%). (Modifiedfrom Kontos MC, Fratkin MJ, Jesse RL, et al. Sensitivity of acute rest myocardial perfusion imaging for identifying patients with myocardial infarction based on a troponin definition. J Nucl Cardiol 2004;11:12-9; with permission.)

who have ACS but also provides a validated measurement of the ischemic risk area. The size of a perfusion defect is of significant clinical importance, as patients who have larger defects have a worse long-term prognosis [27,28]. The most important determinant of infarct size is the ischemic risk zone or the amount of myocardium in jeopardy [29]. MPI is the only technique among those commonly available that can determine the ischemic risk zone [30,31]. In studies in which post-MI patients had MPI before discharge, defect size correlated well with other outcome predictors, including left ventricular ejection fraction [32,33], regional wall motion index [32], end-systolic volume [33], and peak CK levels [34].

Even in the absence of ischemic ECG changes, the ischemic risk area can be large. We found that the ischemic risk area ranged from 0% to 62% of the left ventricle, with a mean risk area of 18% + 11%. Even patients who had normal ECGs at the time of presentation had risk areas similar to those of patients who had abnormal but non-ischemic ECGs (16% + 12% versus 19% + 12%, P — .25) (Fig. 3) [35]. One explanation for this finding is that MI in these patients is often caused by occlusion of the left circumflex coronary artery, and the myocardial territory supplied by it is "silent" on a surface ECG [36]. In a small study that included patients who had ST-segment depression, Christian and colleagues [37] performed


















Fig. 2. Outcomes associated with results of acute rest MPI. Patients who had positive rest MPI (dark bars) had significantly (P < .0001) more MI; MI or revascularization (MI/R); and MI, revascularization, or significant coronary artery disease (>70% stenosis) (MI/R) or significant disease than patients who had negative rest myo-cardial perfusion imaging (white bars). (Modified from Kontos MC, Jesse RL, Schmidt KL, et al. Value of acute rest sestamibi perfusion imaging for evaluation of patients admitted to the emergency department with chest pain. J Am Coll Cardiol 1997;30:976-82; with permission.)

early MPI in 14 patients who did not have ST-segment elevation and later underwent coronary angiography. The culprit vessel was the circumflex coronary artery in six (43%) patients. In another study of 79 patients presenting with a nonischemic ECG and acute MI, we found that the left circumflex coronary artery was the infarct-related artery in 42% of the cases [35].

In patients who have left circumflex occlusions, the absence of ischemic ECG changes does not predict small MIs [36]. O'Keefe and colleagues [38] reported that the risk area was not significantly different in patients who had left circumflex occlusion associated with ST-segment elevation, left circumflex occlusion without ST-elevation, and right coronary artery occlusion. Consistent with these results, we found that the ischemic area at risk was similar in patients in whom the infarct-related artery was the left circumflex (18% + 10%, median 19%), the right coronary (18% + 13%, median 17%), or the left anterior descending artery (18% + 10%, median 19%) [35].

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