Cardiac CT for the Evaluation of Acute Coronary Syndrome in the Emergency Department

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Introduction

More than 8 million Americans each year present to the emergency department (ED) with acute chest pain. This poses a diagnostic challenge to physicians, who must distinguish patients with life-threatening conditions, including acute coronary syndrome (ACS), aortic dissection, and pulmonary embolism, from those who have chest pain related to more benign causes.

Most patients with chest pain are discharged with a diagnosis of noncardiac origin. Only 12% to 15% of patients with chest pain have ACS, which includes diagnoses such as unstable angina, non-ST elevation myocardial infarction (NSTEMI), and ST elevation myocardial infarction (STEMI).

Routine workup for chest pain is estimated to cost approximately $8 billion each year and often requires admission of patients who ultimately have a non–life-threatening or noncardiac cause of chest pain. Despite advances in diagnostic tools, about 2% to 4% of patients are misdiagnosed and discharged home with ACS, and these patients have twice the mortality rate as those who are admitted and treated with the standard of care. These deficiencies highlight the need for improved algorithms to evaluate patients with acute chest pain. Noninvasive imaging has the potential to rule out ACS in select patients to prevent admission and additional cardiac testing.

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Standard Approach in Diagnosing Acute Coronary Syndrome

ACS is a heterogeneous syndrome ranging from unstable angina to myocardial infarction (MI; NSTEMI or STEMI), which is usually caused by a ruptured atherosclerotic plaque and thrombus formation that significantly narrows or occludes a coronary artery. Symptoms arise when the oxygen demand of myocardium exceeds supply delivered by coronary vessels, causing reversible myocardial ischemia (unstable angina) or infarction (NSTEMI or STEMI).

The most common clinical presentation is chest pain, which is relatively nonspecific for ACS. Chest pain can also represent other life-threatening conditions, such as aortic dissection, pulmonary embolism, tension pneumothorax, cardiac tamponade, and mediastinitis, or it can represent more common non–life-threatening conditions such as gastroesophageal reflux disease, musculoskeletal pain, and pneumonia.

On admission, the initial history and physical examination are helpful in identifying factors to increase suspicion of or exclude ACS as the cause of chest pain. Chest pain from myocardial ischemia or MI is often gradual in onset and is described as chest discomfort, and radiation to the left arm or lower jaw is more specific to ACS. Other nonspecific symptoms include diaphoresis, nausea and vomiting, dyspnea, syncope, and palpitations. Caution should be taken with patients who are older, female, or diabetic because they may present with atypical symptoms such as weakness, dyspnea, and abdominal pain, and consequently this population is at higher risk for missed diagnosis and delay in care. Pain that responds to nitroglycerin is often thought to have a cardiac cause (or esophageal spasm), but this is not a reliable indicator to distinguish cardiac pain from noncardiac chest pain. Although the presence of cardiac risk factors or known coronary artery disease (CAD) increases the risk for ACS, it is a poor predictor in an acute setting because chest pain itself has a much higher predictive value for ACS than the presence of cardiac risk factors.

All patients undergo 12-lead electrocardiography to assess for evidence of ischemia or infarction, such as new ST elevation (the most specific finding for ACS) or depression in two anatomically contiguous leads, Q waves >30 ms, and inverted T waves in precordial leads. Ischemic changes may be masked in patients who have baseline electrocardiographic abnormalities from hypertrophy, bundle branch block, or ventricular pacing. In most patients with ACS, the electrocardiogram (ECG) is normal or nondiagnostic, so it is recommended that the patient be placed on telemetry with serial ECGs obtained every 5 to 10 minutes to assess for ongoing ischemia in suspected cases of ACS.

Serum biomarkers are also drawn to assess for myocardial injury. Many serum biomarkers have been studied in the setting of ACS (e.g., troponin, creatine kinase-muscle/brain [CK-MB], natriuretic peptides, C-reactive protein [CRP], myeloperoxidase), but cardiac troponin is the most commonly used biomarker in EDs because it is widely available and is highly sensitive and specific for myocardial injury. However, myocardial injury is not specific to ACS. Troponin levels can be elevated in the setting of trauma, renal failure, sepsis, hypertensive emergency, pulmonary embolism, and other conditions. In the setting of ACS, troponin level elevation may not be detected until 4 to 6 hours after the onset of symptoms, so current guidelines recommend that serial troponin levels be checked every 6 to 8 hours to improve detection of ACS. High-sensitivity troponin assays are being developed to detect infarction earlier in ACS, but this will increase the number of false-positives and potentially lead to unnecessary additional testing.

After obtaining the history, physical, initial ECG, and cardiac biomarkers, the need for further studies—noninvasive or invasive imaging—is determined by risk stratification.

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Patient Stratification in the Evaluation of Acute Coronary Syndrome

From a noninvasive imaging perspective, patients can be divided into three groups based on their risk for ACS and need for further evaluation and workup. The first group of patients consists of those who have high-risk or definite ACS based on the physical examination, ECG, and cardiac biomarkers. These patients are admitted and referred for invasive coronary angiography and possible percutaneous or surgical intervention. Noninvasive imaging is unnecessary in these patients because it will not change the need for catheterization.

The second group of patients consists of those who have a clear noncardiac and non–life-threatening diagnosis, with a normal ECG and cardiac biomarkers, in whom ACS can be confidently excluded. These patients can safely be discharged from the ED without the need for observation or cardiac imaging.

The third group of patients is the largest and consists of those who fall in between the two other groups. These patients often have nonspecific or atypical chest pain, normal or nondiagnostic ECG, and normal cardiac biomarkers. This group usually represents the patient population at a low to intermediate risk for ACS, and they require further evaluation to include or reasonably exclude ACS. Patients with unstable angina usually fall into this group because, by definition, they have normal cardiac biomarkers and frequently have normal or nondiagnostic ECGs. Patients in this group are usually admitted to an observation unit for serial ECGs and cardiac biomarkers; additional tests with stress testing or noninvasive imaging techniques are performed, depending on patient characteristics.

Multiple scoring systems have been developed to assess clinical risk after an ACS; they are primarily based on the physical examination, history, electrocardiographic findings, and biomarkers. Two widely used scoring systems include the TIMI (Thrombolysis In Myocardial Infarction), which assesses the risk of 14-day mortality and MI after ACS, and GRACE (Global Registry of Acute Coronary Events), which assesses index hospitalization mortality and 6-month mortality after ACS. Although these risk-scoring systems focus on outcomes and are not designed to diagnose ACS, they are often used as part of risk stratification to guide management in individuals with suspected ACS. Patients who are at a higher risk of mortality will often be immediately referred for invasive coronary angiography.