Chapter 51: Low-Probability Acute Coronary Syndrome

INTRODUCTION AND EPIDEMIOLOGY

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 acute coronary syndrome (ACS; either infarction or ischemia including unstable angina pectoris). The evaluation of those with possible 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 discharged home from the ED have worse 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 are observed or admitted for further 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, and ST-segment elevation myocardial infarction. A detailed discussion regarding these diagnoses is provided in Chapter 49, “Acute Coronary Syndromes.”

Patients presenting soon after infarction may have normal biomarker results and initially be categorized as having possible ACS. However, patients with evolving myocardial infarction generally have other high-risk features of ACS such as ST-segment depression, and they develop positive injury biomarker patterns over time.^6,7

CLINICAL FEATURES

HISTORY AND COMORBIDITIES

The history is the base tool to assess patients with possible ACS, but it alone cannot exclude the condition. Key data are symptom quality, location, duration, severity, associated symptoms, precipitating and relieving factors, and similarity to prior episodes known to be from ACS. Consider other noncardiac but life-threatening causes of chest pain (see Chapter 48, “Chest Pain”).

Among patients with possible ACS, historical data allow patients to be placed into categories of low risk, probable low risk, probable high risk, and high risk. However, even patients with low-risk features or in a low-risk category have a residual risk of ACS.⁸ Lowest-risk features include pleuritic, positional, reproducible, or sharp/stabbing pain. Another low-risk feature is pain that is not exertional or located in a small inframammary area. Higher-risk features include chest pressure (positive likelihood ratio [LR+] 1.3), pain similar to or worse than prior cardiac pain (LR+ 1.8), and associated nausea/vomiting ...