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.
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.
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
The ED assessment of a patient with possible ACS requires
continual reassessment to properly reclassify the patient as likely
ACS or unlikely ACS.
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 provided in Chapter 53, Acute Coronary Syndromes: Acute Myocardial Infarction and Unstable Angina.
Both unstable angina and AMI are characterized by the final common denominator
of decreased blood delivery to the myocardium. For the vast majority
of patients, this is caused by intraluminal obstruction within the
coronary arteries, usually associated with atherosclerosis. Atherosclerotic
plaques can slowly enlarge, leading to progressive exertional angina.
Or, more commonly, plaques become unstable and rupture, leading
to coronary artery thrombosis. If the thrombus is completely occlusive,
STEMI occurs, but partial occlusion may result in intermittent angina,
rest angina, or NSTEMI. Importantly, plaque rupture is unpredictable
and commonly occurs in plaques previously demonstrated to be “nonocclusive.” Plaque
rupture without vessel occlusion has been found to be associated
with unstable angina and even stable angina symptoms14 (see Chapter 53, Acute Coronary Syndromes: Acute Myocardial Infarction and Unstable Angina for further discussion).
In addition to plaque rupture, other less common causes of decreased myocardial
blood flow include spontaneous coronary thrombosis, coronary vasospasm, congenital
coronary artery malformation, and aortic valvular disease.
The distinction between NSTEMI and unstable angina is based upon
elevated cardiac markers of necrosis in the case of NSTEMI. Troponin
I and troponin T are the most specific cardiac markers available.
Release of these biomarkers requires cellular death and breakdown.
These biomarkers do not reach detectable thresholds for at least
6 hours after infarction. Thus, patients presenting soon after infarction
may have normal biomarker results and initially be categorized as
having a possible ACS. Patients with evolving myocardial
infarctions (MIs) represent approximately 4% of patients
undergoing serial cardiac markers and generally have other high-risk
features of ACS such as ST-segment depression.15,16
Historical features of patients with possible ACS are discussed
in Chapter 52, Chest Pain: Cardiac or Not.
The history can be seen as a tool, or diagnostic test, and is ultimately
used as part of the clinician’s global assessment. However,
the history alone cannot be used to diagnose, or exclude, ACS and
must be used along with other information. Obtain detailed information,
including symptom quality, location, duration, severity, associated
symptoms, precipitating and relieving factors, and similarity to
prior episodes. Consider other noncardiac but life-threatening causes
of chest pain (see Tables 52-3 and 52-4 in Chapter 52, Chest Pain: Cardiac or Not).
Among patients with possible ACS, historical features can be
categorized as low risk, probable low risk, probable high risk,
and high risk (Table 55-1).17 However,
even patients in the low-risk category have some risk of ACS, thus,
placement in this grouping should not solely be used to exclude
Table 55-1 Chest Pain Descriptors
and Risk for Acute Coronary Syndrome
| Save Table
Table 55-1 Chest Pain Descriptors
and Risk for Acute Coronary Syndrome
|Risk Category||Feature||Positive LR (95% CI)|
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