Key points
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History, physical examination, 12-lead ECG, and serum markers are useful, but not sufficient, for diagnosing ACS and for risk stratification in the ED.
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MPI can be useful in the decision-making process in patients with suspected ACS in the ED and can be cost-effective if used in suitably selected patients.
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The logistics of ED imaging are not insurmountable but require coordination between ED staff and the nuclear laboratory and a good method of communication between the two. In most institutions, this is performed 8 to 12 hours daily and on weekdays only.
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Normal perfusion on rest MPI in a patient with chest pain in the ED predicts a low risk for cardiovascular events, especially if obtained during the episode or soon after resolution.
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A normal rest MPI in patients at intermediate or high clinical risk of ACS should be followed up with an outpatient stress MPI.
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An abnormal rest MPI in ED patients can occur in patients with ischemia (but no infarction) who have normal serum troponin levels and no acute ST/T changes.
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A stress rather than a rest MPI can be considered in patients suspected of having ACS in the ED if they are pain free and have two sets of negative cardiac markers and no acute ischemic ECG changes.
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A perfusion defect on rest MPI in patients with chest pain in the ED and with no history of prior MI should prompt admission to the hospital with plans for early coronary angiography. Such an abnormal perfusion pattern can represent myocardial ischemia or necrosis. The presence of a regional wall motion/thickening abnormality does not help in the differentiation but normal regional function favors ischemia.
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A normal rest MPI can be observed in patients with ischemia if the tracer injection is performed long after the resolution of chest pain. The precise window of opportunity is not well defined but some have suggested 2 hours. We believe that for better sensitivity, this window should be as narrow as possible, and preferably less than 1 hour.
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Rest-BMIPP (β-methyl- p -[ 123 I]iodophenyl-pentadecanoic acid) may be useful for imaging patients presenting to the ED after symptom resolution. This tracer however is not yet approved for clinical use in the United States.
Background
Evaluating patients presenting to the ED with chest pain suspicious for ACS continues to be a challenge to both emergency physicians and consulting cardiologists. The initial questions posed are:
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Is emergent treatment in the ED warranted?
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What is the disposition of the patient: admit to the inpatient or observation unit or discharge home?
This is obviously not an idle question; millions of patients in the United States, and many more across the globe, have such a presentation. The decision-making in these situations has enormous personal, social, and economic implications. An overly conservative strategy of admitting all patients suspected of having ACS places an excessive economic burden on the health care system and stretches the availability of inpatient beds. On the other hand, ED physicians are acutely aware of the small but medicolegally significant proportion of patients who are inappropriately discharged home while having true ACS only to develop MI or sudden cardiac death shortly afterwards.
An ECG should be performed within 5 minutes of presentation and interpreted immediately to help stratify patients with ST elevation to immediate coronary angiography with PCI or, alternatively, fibrinolytic therapy. The ECG can also be helpful in confirming the diagnosis of ACS and in risk stratification in patients without ST elevation. However, a normal ECG (or one with nonspecific findings) is not uncommon in patients presenting to the ED with chest pain or pain-equivalent symptoms and should not be used to exclude the diagnosis of ACS. Similarly, an elevated cardiac troponin assay at presentation to the ED signifies a high-risk patient, but since several hours are required after symptom onset for cardiac troponins to become detectable in the serum, its absence is not reassuring. Furthermore, even patients with normal serial cardiac markers can be at high risk of future MI since rest (unstable) angina does not necessarily result in elevation of serum biomarkers.
In Figure 14-1 , we show a suggested flowchart for the evaluation of patients presenting to the ED with chest pain and/or chest pain–equivalent symptoms such as dyspnea or jaw pain. In our experience, after the screening ECG is interpreted and STEMI is excluded, a detailed history is the most important factor that can help triage the patient. A clinical assessment is formed that incorporates findings on the history and physical examination, ECG, laboratory tests (including troponins), and other available pertinent results such as chest radiography. Patients judged to have an MI or determined to be at high risk for short-term adverse events are admitted to the hospital for further treatment and evaluation that may include coronary angiography. Alternatively, patients judged to have noncardiac chest pain or who are at low risk can be safely discharged home to follow-up with their primary care physicians who can consider further work-up as deemed appropriate.
For patients in the intermediate-risk group, MPI could help in the decision-making process. The following cases illustrate the usefulness of MPI in these patients and the pitfalls that should be avoided.
Case 14-1
Chest Pain With Negative Biomarkers ( Figure 14-2 )
A 43-year-old woman with hypertension developed chest pain radiating to the left arm, described as squeezing in nature, while driving to work. She does not smoke tobacco but has a significant family history for premature CAD. She was pain free when she arrived in the ED. The physical examination was benign. There were no acute ischemic changes on the ECG. Initial cardiac markers were negative. A rest MPI was performed ( Figure 14-2 , A ) and revealed normal perfusion and LV function. The tracer was injected 2 hours after onset of the pain. The patient was discharged home. She underwent regadenoson MPI 2 days later (she could not walk on a treadmill due to a recent history of lower extremity trauma). The stress images were also normal ( Figure 14-2 , B ). She was treated with risk factor modifications.
Comments
The majority of patients presenting to the ED with chest pain have a normal or nondiagnostic ECG. When management is directed by clinical examination, ECG, and initial cardiac markers, an important percentage of patients with ACS (including MI) are erroneously discharged home.
Approximately 50% are admitted to the hospital, but more than 50% of them will not have ACS as their final diagnosis. Multiple observational and randomized studies have demonstrated the usefulness of rest MPI in reducing unnecessary hospitalizations and cost in patients with suspected ACS, compared to routine care. The true value of a rest MPI is apparent when it is completely normal. In a patient with active chest pain presenting to the ED, a negative scan rules out the diagnosis of MI and has a very high negative predictive value for subsequent cardiac events over the next few months. In the large Emergency Room Assessment of Sestamibi for the Evaluation of Chest Pain (ERASE chest pain) trial, which randomized 2475 patients with chest pain or other symptoms suggestive of ACS with a normal or nondiagnostic ECG to usual care versus care directed by Tc-99m sestamibi or tetrofosmin rest MPI, hospitalization in patients not having ACS was significantly decreased (42% vs. 52%, p < .001). There was no difference on triage of patients with ACS (including those having MI) and no difference in 30-day outcomes. In this study, a rest MPI added an average of 30 minutes to the evaluation but reduced unnecessary hospitalizations without adversely affecting outcomes or reducing indicated hospitalizations. A cost-effectiveness analysis of a multicenter study by Heller et al. suggested a mean cost savings per patient of $4,258; in a smaller randomized study, Stowers et al. found a median hospital cost saving per patient of $1,843 in the MPI group.
Case 14-2
Abnormal Rest MPI After Subsidence of Chest Pain ( Figure 14-3 )
A 34-year-old woman with no prior cardiac history or risk factors for CAD presented to the ED within 1 hour of onset of chest pain with associated nausea. She did not have a family history of premature CAD and was not on any medications. Her physical examination was unrevealing except for an elevated blood pressure of 153/92 mm Hg. The ECG showed nonspecific T-wave changes but no ST shifts and no Q waves. Initial troponin was negative. Rest MPI performed during the episode of chest pain revealed a moderate-sized perfusion defect in the distribution of the LCX with normal LVEF ( Figure 14-3 , A, B ). The patient was admitted to the cardiac care unit and treated for ACS. Serial cardiac markers showed an increase in the troponin I and CK-MB levels. Coronary angiogram ( Figure 14-3, C ) showed an occlusion at the ostium of the second obtuse marginal branch of the LCX, which was successfully treated with a drug-eluting stent.
