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Patients with chest pain or other symptoms suggesting coronary ischemia require stratification based upon the probability of acute coronary syndrome (ACS) for proper treatment and disposition. This chapter discusses the features of low probability ACS, or possible ACS. By definition, patients classified into this group have no objective evidence of acute coronary ischemia or infarction—no characteristic electrocardiogram (ECG) ST-segment elevation or depression, and normal levels of cardiac markers.

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A key determination by the emergency physician is whether to pursue further evaluation for possible ACS. Currently 3% to 6% of patients thought to have noncardiac chest pain or a clear-cut alternative diagnosis will have a short-term adverse cardiac event.

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The clinical features of patients with possible ACS are the same as discussed in Chapter 17 “Chest Pain: Cardiac or Not.” High-risk historical features include chest pain with any of the following descriptors: radiating, occurs with exertion, described as pressure, similar to prior cardiac pain, or is accompanied by nausea or diaphoresis. However, even patients without high-risk features have some risk of ACS. Therefore absence of high-risk features should not solely be used to exclude ACS. Significant coronary disease is rare in patients <30 years old but youth does not completely eliminate ACS as a cause of acute chest pain. The physical exam should focus on excluding alternative diagnoses and detecting signs of cardiac failure.

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A previous negative cardiac test should not prevent an appropriate evaluation for ACS in a patient with a concerning history or ischemic ECG findings. Plaque rupture is a major cause of ACS and commonly occurs in lesions that were previously nonobstructive. Previous stress testing results cannot determine whether the patient's current symptoms represent new ischemia from a recent plaque rupture. In contrast, previous cardiac catheterization results can be of benefit in determining whether a patient should undergo stress testing after exclusion of myocardial infarction. It is unlikely that a patient with previously normal or near-normal coronary arteries has developed significant epicardial stenosis within 2 years of the procedure.

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The evaluation of patients with possible ACS can be conceptualized into a primary and secondary assessment. The primary evaluation must detect patients with ST-segment elevation that require emergent revascularization and distinguish between patients with definite ACS, possible ACS, and those with symptoms that are definitely not ACS. Alternative causes of chest pain should be considered (see Chapter 17).

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The primary evaluation should include a history, physical examination, ECG, chest radiography, and cardiac biomarkers if ACS remains in the differential diagnosis. Serial ECGs should be obtained in patients with ongoing symptoms. All available data should be used to create a composite picture for decision making. Some have calculated that when the pretest probability of ACS is ≤2%, further testing is not indicated. Others have suggested a threshold of <1%.

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At the conclusion of the primary evaluation, patients should be classified as having acute myocardial infarction (AMI), possible acute ischemia, or definitely not ischemia. Patients with possible acute ischemia are further stratified into high, intermediate, and low risk for adverse events based on the pattern of symptoms, clinical features, and ECG findings. ...

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