It has been estimated that over 7 million patients present annually to emergency departments (ED) with symptoms suggestive of coronary artery disease (CAD).1 In those patients without CAD, admission to the hospital is not necessary and may strain bed resources for the patients in need of acute care. However, patients with acute coronary syndrome (ACS) have a high morbidity and mortality and should not be discharged. If the symptoms are classic and accompanied by diagnostic electrocardiographic (ECG) changes (new ST-segment elevation, ST-segment depression, etc.), the identification of ACS is straightforward and admission and therapy is warranted. However, in the absence of diagnostic ECG changes either with or without classic symptoms, the decision to admit or discharge the patient is far more difficult and complex. The clinical history and ECG itself (if non-diagnostic) do not serve as sufficient discriminators for optimal clinical decision-making. Thus, many patients who do not have ACS are hospitalized unnecessarily and those with ACS are discharged inappropriately. The use of acute rest myocardial perfusion imaging (ARMPI) has been developed to assist in the triage decision. The current American College of Cardiology (ACC)/American Heart Association (AHA) guidelines recommend ARMPI as a class I indication in patients with chest pain presenting to the emergency room. This chapter will evaluate the role of ARMPI in the management of ED patients, and contrast with other emerging technologies such as echocardiography (ECHO) and computerized tomography.
Pathophysiology of Acute Coronary Syndrome
The ACS comprises a spectrum of presentations including ST elevation myocardial infarction, non-ST elevation myocardial infarction, and unstable angina (UA). The hallmark of ACS is the rupture or erosion of vulnerable atherosclerotic plaque(s) with associated occlusive or non-occlusive thrombus formation, resulting in myocardial ischemia and/or injury. The evolution is very complex with interaction of the vessel wall, coagulation system, and inflammatory agents, culminating in ACS.2–4 The clinical result is quite variable from spontaneous resolution of the thrombus and healing of the plaque to extensive myocardial infarction, cardiogenic shock, and sudden cardiac death.
Clinical Presentation and Triage
Of patients presenting to the ED with chest pain, about 95% do not have ECG evidence of evolving Q-wave myocardial infarction, and only approximately 20% will ultimately have evidence of UA or non-Q-wave myocardial infarction.5 The initial triage of these chest pain patients involves three important goals as prioritized in Fig. 21-1. The first goal is to identify ACS for admission and appropriate therapy. The second is to identify patients with CAD, at high risk for short-term cardiac events, who should be admitted. The third goal is to identify patients suitable for discharge and outpatient evaluation. Many patients fall into this category. If myocardial ischemia is considered to be unlikely, patients should then be evaluated for non-cardiac causes of chest pain for appropriate therapy or discharge from the ED.