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INTRODUCTION AND EPIDEMIOLOGY

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About 8 million patients with chest pain present to a U.S. ED each year.1 Of these, 50% to 70% are placed into an observation unit or admitted to the hospital, yet only about 10% are eventually diagnosed with an acute coronary syndrome.2,3,4,5 Still, 2% to 5% of patients with acute myocardial infarctions are missed on initial presentation and discharged from the ED.2 We discuss the features and approach that help differentiate acute coronary syndrome from other causes of chest pain. The chapters titled "Acute Coronary Syndromes" and "Low Probability Acute Coronary Syndromes" discuss management of these specific syndromes.

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Acute chest pain is the recent onset of pain, pressure, or tightness in the anterior thorax between the xiphoid, suprasternal notch, and both midaxillary lines. Acute coronary syndrome includes acute myocardial infarction and acute ischemia (unstable angina). Acute myocardial infarction is defined by myocardial necrosis with elevation of cardiac biomarkers and is classified by ECG findings as ST-segment elevation myocardial infarction or non-ST-segment elevation myocardial infarction. Unstable angina is a clinical diagnosis defined by chest pain or an equivalent (neck or upper extremity pain) from inadequate myocardial perfusion that is new, occurring with greater frequency, less activity, or at rest. Patients with unstable angina do not have pathologic ST-segment elevation on ECG or cardiac biomarker elevation, but they are at risk of eventual myocardial damage absent recognition and treatment.

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PATHOPHYSIOLOGY

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The chest wall, from the dermis to the parietal pleura, is innervated by somatic pain fibers. Neurons enter the spinal cord at specific levels corresponding to the skin dermatomes. Visceral pain fibers are found in internal organs, such as the heart, blood vessels, esophagus, and visceral pleura. Visceral pain fibers enter the spinal cord and map to areas on the parietal cortex corresponding to cord levels shared with somatic fibers. Stimulation of visceral or somatic afferent pain fibers results in two distinct pain syndromes. Pain from somatic fibers is usually easily described, precisely located, and often experienced as a sharp sensation. Pain from visceral fibers is generally more difficult to describe and imprecisely localized. Patients with visceral pain are more likely to use terms such as discomfort, heaviness, pressure, tightness, or aching. Visceral pain is often referred to an area of the body corresponding to adjacent somatic nerves, which explains why pain from an acute myocardial infarction may radiate to the neck, jaw, or arms. Factors such as age, sex, comorbid illnesses, medications, drugs, and alcohol may interact with psychological and cultural factors to alter pain perception and communication.

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CLINICAL FEATURES

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RISK ASSESSMENT

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Patients with abnormal vital signs, concerning ECG findings (if available initially), a history of prior coronary artery disease, multiple atherosclerotic risk factors, or any abrupt, new, or severe chest pain or dyspnea should ...

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