Key points
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Acute coronary syndrome encompasses a wide spectrum of clinical presentations, including UA, non–ST elevation MI, and ST elevation MI.
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Patients with ST elevation MI are preferably treated with primary PCI or thrombolytic therapy.
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Rest MPI can be used to assess infarct size, area at risk, and the impact of various interventions to modify infarct size.
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Rest MPI can be used to identify culprit vessels if the tracer is injected during pain.
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Stress MPI has no role during an acute presentation.
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Stress MPI is clearly helpful in risk stratification of stabilized patients after the first 24 to 48 hours after acute onset.
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Such patients include those who were treated conservatively, those who were treated with thrombolytic therapy, or those who were treated with PCIs but had either less than ideal angiographic result or incomplete revascularization.
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Vasodilator MPI can be used 24 to 48 hours after acute presentation in the absence of angina, heart failure, or serious arrhythmia within the past 24 hours.
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Depending on patient selection, the stress MPI abnormalities could be mild, absent, or severe and extensive.
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The MPI findings could be used as effective tools to further triage patients toward coronary interventions or optimal medical management.
Background
The ACSs encompass UA, NSTEMI, and STEMI. In 2006, 1,365,000 patients were discharged from U.S. hospitals with a diagnosis of ACS. Of the total discharges, 810,000 patients had an MI, with nearly one third being STEMI. Patients with STEMI are treated emergently, and preferably with PCI or with thrombolytic (fibrinolytic) therapy if PCI is not readily available. Patients with STEMI who are treated with thrombolytic therapy or conservatively and who have high-risk clinical features or who are unstable should undergo coronary angiography with intent to revascularize. Stable patients who received thrombolytic therapy or who were treated conservatively can undergo stress MPI for risk stratification and particularly to assess LV function, infarct size, and residual ischemia in the infarct and remote zones.
The use of clinical information, biomarkers, and ECG results, as well as risk stratification models (such as the TIMI or GRACE risk score or the PURSUIT model), can be used in the early risk stratification of patients with UA or NSTEMI. Rest MPI can also be used in the evaluation of these patients in the emergency department (ED) and will be discussed in Chapter 14 . Patients with UA or NSTEMI with high-risk features, such as elevated cardiac biomarkers, ST segment depression, hemodynamic instability, reduced LV function, signs or symptoms of HF, recurrent or persistent angina despite optimal medical therapy, new or worsening mitral regurgitation, sustained ventricular arrhythmias, PCI within 6 months, prior CABG, high-risk TIMI or GRACE score, or high-risk findings on noninvasive testing, are preferably treated with an invasive strategy that includes coronary angiography and revascularization with either PCI or CABG.
Stress MPI can be used in the evaluation and risk stratification of patients at low to intermediate risk who have UA or NSTEMI, those with STEMI who have received thrombolytic therapy, and patients who were treated with coronary interventions with either less than optimal results by angiography or those who have had incomplete revascularization. Stress MPI should be performed in patients who have been free of ischemic symptoms, serious arrhythmias, or HF for a minimum of 12 to 24 hours. High-risk features on stress MPI carry > 3% annual mortality rate and include a resting LVEF < 35%, exercise LVEF < 35%, large reversible perfusion defect, multiple reversible perfusion defects of moderate size, large fixed perfusion defect with LV dilation or with increased lung uptake (if imaged with thallium-201), or a Duke treadmill risk score of ≤−11. Rest MPI can also be used for infarct sizing in patients who are post-MI. Appropriateness criteria for stress MPI in patients with ACS are listed in Table 9-1 .
| SCORE | |
|---|---|
| Appropriate Indications | |
| Possible ACS (stress MPI) | |
| No ischemic ECG changes, LBBB, or paced rhythm + low risk TIMI score + borderline, equivocal, or minimally elevated troponin | 9 |
| No ischemic ECG changes, LBBB, or paced rhythm + low risk TIMI score + negative troponin | 8 |
| No ischemic ECG changes, LBBB, or paced rhythm + high risk TIMI score + negative troponin | 8 |
| No ischemic ECG changes, LBBB, or paced rhythm + high risk TIMI score + borderline, equivocal, or minimally elevated troponin | 8 |
| Possible ACS (rest MPI) | |
| No ischemic ECG changes, LBBB, or paced rhythm + initial negative troponin + recent or ongoing chest pain | 7 |
| Elevated troponin without additional evidence of ACS | 7 |
| UA/NSTEMI (within 3 months) | |
| No prior coronary angiography + hemodynamically stable + no recurrent angina + no signs of CHF (to evaluate for inducible ischemia) | 9 |
| STEMI (within 3 months) | |
| No prior coronary angiography + hemodynamically stable + no recurrent angina + no signs of CHF (to evaluate for inducible ischemia) | 8 |
| Inappropriate Indications | |
| Definite ACS | 1 |
| STEMI within 3 months + PCI with complete revascularization + no recurrent symptoms | 2 |
| STEMI within 3 months + hemodynamic instability, signs of cardiogenic shock or mechanical complications | 1 |
| ACS + post-revascularization + asymptomatic (evaluation prior to hospital discharge) | 1 |
| Before participation in cardiac rehabilitation | 1 |
Although some of the guidelines recommend exercise treadmill testing as the stress test of choice and reserve stress testing with MPI for those with uninterruptable ECG or those who cannot exercise, exercise treadmill testing as a stand-alone test is rarely used in real practice. Vasodilator stress MPI has been the modality of choice in patients with MI as it allows early stress testing compared to exercise stress testing.
Case 9-1
Rest Imaging With Tracer Injection During Chest Pain ( Figure 9-1 )
A 56-year-old African American woman with history of HTN and GERD presented to the ED with 2 weeks of intermittent retrosternal chest pains that initially occurred with exertion but progressed to occurring at rest. The initial ECG and cardiac markers in the ED were unremarkable. She was injected with 33 mCi of Tc-99m sestamibi at rest while she was still having chest pain. The rest SPECT images with attenuation correction revealed abnormal perfusion in the LAD territory with an LVEF of 65% and normal wall motion ( Figure 9-1, A ). Subsequent coronary angiography showed a 99% proximal LAD stenosis involving the first diagonal branch with right to left collaterals ( Figure 9-1, B, C ). The RCA and LCX had mild disease ( Figure 9-1, B ). The LAD was stented with a drug-eluting stent ( Figure 9-1, C ).
