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Patients with chest pain or other symptoms suggesting coronary ischemia require clinical stratification into groups, 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 ECG ST-segment elevation or depression, and normal levels of cardiac markers. Patients with diagnostic ECG or cardiac marker levels, or those with other high-risk features, are discussed in Chapter 53, Acute Coronary Syndromes: Acute Myocardial Infarction and Unstable Angina.

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Of all patients with possible ACS, 5% to 15% ultimately prove to have ACS.1–4 Unfortunately, the rate of discharge from the ED for patients with ACS remains approximately 4%.5 Patients with ACS who are discharged home from the ED have worse clinical outcomes and higher mortality compared with those patients who are initially hospitalized.5 The clinical data readily available to the emergency physician, such as historical features, examination findings, and ECG results, alone are not sufficient to exclude ACS among most patients, as 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.6,7 Therefore, most patients with possible ACS should undergo further cardiac testing.

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In the U.S., chest pain accounts for 6 million ED visits per year.8 Although approximately two thirds of patients undergo an evaluation for coronary disease, only 15% to 25% will ultimately be diagnosed with ACS.9 Of patients with undifferentiated chest pain, 7% will have ECG findings consistent with acute ischemia or infarction, and 6% to 10% of those in whom cardiac markers are ordered will have initially positive results.10 The remaining patients who do not have diagnostic ECG changes or initially positive cardiac marker results have possible ACS and require additional testing to confirm or exclude ACS. Diagnosis of this condition leads to an annual cost of approximately $10 billion to $12 billion in the U.S.11–13

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The ED assessment of a patient with possible ACS requires continual reassessment to properly reclassify the patient as likely ACS or unlikely ACS.

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ACS is a constellation of signs and symptoms resulting from an imbalance of myocardial oxygen supply and demand. There are three general ACS classifications: unstable angina, non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI). Unstable angina is a type of ACS with no elevation of biomarkers and no pathologic ST-segment elevation, resulting in ischemia but not infarction. Acute myocardial infarction (AMI) occurs when myocardial tissue is devoid of oxygen and substrate for a sufficient period of time to cause myocyte death. NSTEMI is characterized by biomarker elevation and no pathologic ST-segment elevation. STEMI is characterized by ST-segment elevation and biomarker elevation (STEMI) although biomarker elevation is not required at onset to make this diagnosis. Detailed ...

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