Clinical Practice/Controversy: Emergency Department Evaluation of the Lower-Risk Patient: Whom Can You Send Home?

Introduction

Of the more than 8 million visits to emergency departments (EDs) in the United States for chest pain or other potential ischemic symptoms, only a small minority will yield a diagnosis of an acute coronary syndrome (ACS). Less than 5% of patients so affected will prove to have ST-elevation myocardial infarction (MI), and only approximately one fourth will receive the final diagnosis of non–ST-elevation ACS (NSTE-ACS). Identifying patients who can be discharged early, with minimal additional testing, while also ensuring that patients at high risk for cardiac events are appropriately triaged to receive more advanced evaluation and management, remains one of the most common and challenging clinical scenarios that cardiologists, emergency medicine physicians, and primary care providers face on a daily basis.

Chest and abdominal pain are the two most common symptoms prompting emergency room visits in the United States (see Chapter 6 ), with a volume that has remained relatively stable over the past decade. However, practice in the ED and inpatient evaluation of these patients has evolved considerably. Compared with a decade ago, more patients hospitalized with chest pain now undergo advanced imaging studies, such as echocardiography, computed tomography, or cardiac magnetic resonance imaging, as evidenced by a shift in the frequency of such imaging from only 3.4% in 1999 to 15.9% of patients in 2008. Patients presenting with chest pain were much more likely to be hospitalized or transferred to another institution or to die than patients with other chief complaints such as abdominal pain. Despite the fact that the rate of admission, transfer, or death has declined over the past decade, from 42.5% in 1990 to 35.2% in 2008, the burden of evaluating patients with chest pain in the ED and efforts to address the typically high rate of complications continue to create tremendous demands upon the health care system.

The clinical presentation of patients with suspected MI and the key considerations in the general approach to their evaluation are discussed in Chapter 6 . The principles behind the optimal use of cardiac troponin (cTn) are addressed in Chapter 7 . Other biomarkers are discussed in Chapter 8 , and the use of imaging is described in Chapter 9 . This chapter reviews specific strategies and algorithms integrating each of the elements of clinical, laboratory, and imaging data to identify those patients for whom the probability that an ACS is the cause of their symptoms is deemed to be very low. An efficient approach to evaluating such “low-probability” patients should minimize the time until diagnosis, reduce the need for additional testing, and limit the duration of hospitalization, while avoiding erroneous discharge of the patient with an ACS. Older studies suggest that as many as 2% of patients with acute MI may be erroneously discharged from the ED. In the current era, an acceptable “miss” rate for MI generally is viewed as less than 1%.

Defining the “Low-Probability Patient”

Before defining the specific population of patients who can be safely discharged from the ED, it is worthwhile making the distinction between the labels of “low probability” and “low risk,” because these two terms often are interchanged freely in discussing chest pain and ACS. In characterizing such patients, it is preferred to specify “low probability” for the presence of ACS, rather than “low risk,” which most commonly is employed in estimating the likelihood of subsequent cardiovascular events (see also Chapter 6 ). Risk stratification is a critical step in the evaluation of patients with documented ACS and incorporates many of the same clinical characteristics used for diagnosis. However, the risk estimates and clinical implications in this patient population are much different from those in patients undergoing evaluation for suspected ischemic symptoms. For example, a patient may have a high probability for ACS and therefore merit hospitalization but be at moderate or low risk for subsequent cardiovascular events (see Figure 6-2 ). Risk stratification in patients with documented ACS is reviewed in detail in Chapter 11 .

Considerations for Defining Probability and Ruling Out ACUTE Coronary Syndrome

The evaluation of the patient with suspected ischemic symptoms integrates, at a minimum, the patient’s comorbid conditions, history and presentation, and electrocardiographic findings. Most patients, except those with the very lowest probability for ischemia, also will have at least one biomarker of necrosis (i.e., cTn) measured in their evaluation. Decisions regarding subsequent noninvasive testing in general, and which modality in particular is most appropriate, remain controversial. The most challenging aspect of identifying patients with a low probability for ACS is the absence of a single “gold standard” test for this clinical entity. Troponin assays identify myocardial injury but not the underlying cause (see Chapter 7 ). The diagnosis of MI is made on the basis of the clinical scenario, testing results, and ultimately, medical judgment.

Depending on clinical characteristics and electrocardiography findings, most patients can be classified into groups of very low, low, intermediate, or high probability (see also Chapter 6 ). This first, immediate estimation of probability is important, because the value of all subsequent testing is dependent on the pretest probability of disease. Typically, additional testing is most useful in those patients with intermediate pretest probability ( Figure 12-1 ). In patients with a high pretest probability, even a “negative” subsequent test would not be reassuring, because a higher-than-acceptable false-negative rate is likely. Conversely, in the very-low-probability patients, a positive test result is much more likely to be a false positive than to represent the identification of “true” disease. In practical terms, this circumstance might correspond to measuring cTn in a 25-year-old woman with a history of 3 days of chest discomfort relieved with an antacid. Alternatively, an initially normal cTn level would not be entirely reassuring in a 75-year-old patient with diabetes and vascular disease presenting with typical chest discomfort.

In the initial management of patients with suspected ischemia, it often is more important to rule out unstable ischemic syndromes than to definitively “rule in” the diagnosis of ACS. Assessing pretest probability of disease is as important in excluding such disease as in its ultimate diagnosis. In deciding whether a test is appropriate to rule out a disease, one must consider the underlying prevalence of the disease under consideration and the specificity of the test to exclude disease. For example, as discussed further on, a computed tomography (CT) angiogram would be useless to exclude ACS in a patient with documented coronary disease. Similarly, measuring high-sensitivity cTn in a patient with end-stage renal disease and severe left ventricular hypertrophy will yield results that can be challenging to interpret.