## INTRODUCTION AND EPIDEMIOLOGY

This chapter discusses the features of low-probability or possible acute coronary syndrome (ACS), which includes patients who have chest pain or another equivalent ischemic symptom but no objective evidence of acute coronary ischemia or infarction—that is, no characteristic ECG ST-segment elevation or depression and normal levels of cardiac markers. Patients with diagnostic ECG or cardiac marker levels or with other high-risk features are discussed in chapter 49, "Acute Coronary Syndromes."

Of ED 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.1 The remaining patients who do not have diagnostic ECG changes or initially positive cardiac marker results have low-probability or possible ACS. The evaluation of those with possible or actual ACS costs approximately $10 billion to$12 billion each year in the United States.2

Of all patients with possible ACS, 5% to 15% ultimately prove to have ACS.3 The rate of discharge from the ED for patients with ACS remains approximately 4%. Patients with ACS who are discharged home from the ED have worse clinical outcomes and higher mortality compared with patients who are initially hospitalized. The clinical data readily available to the emergency physician, such as historical features, examination findings, and ECG results, cannot exclude ACS among most patients, because 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.4,5 Therefore, most patients with possible ACS undergo further observation and testing.

## PATHOPHYSIOLOGY

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 discussion is in chapter 49.

The distinction between NSTEMI and unstable angina is based on elevated cardiac markers of necrosis in the case of NSTEMI. Troponin I and troponin T are the most specific cardiac markers of cell injury or death available. These biomarkers may not reach detectable thresholds for up to 6 hours after infarction. Patients presenting soon after infarction may have normal biomarker results and initially be categorized as having possible ACS. Patients with evolving myocardial infarctions represent approximately 4% ...

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