Acute coronary syndromes (i.e. unstable angina and non-ST-elevation myocardial infarction) represent an acute phase in the natural history of chronic CAD, which may either progress to myocardial infarction and death or return to the chronic stable phase of CAD. Patients with unstable angina are separated into low-, intermediate-, or high-risk groups on the basis of history and physical examination, 12-lead ECG, and serum cardiac markers. Low-risk patients can be treated on an outpatient basis, intermediate-risk patients can be treated in a monitored hospital bed, and high-risk patients are typically admitted to an intensive care unit. Little evidence exists regarding the safety of early exercise testing in unstable angina. One review of this topic found three studies including 632 patients with stabilized unstable angina who had a 0.5% death or myocardial infarction rate within 24 hours of their exercise test.19
Generally, exercise testing appears useful and safe in the evaluation of low-risk patients (normal or unchanged ECG during an episode of chest discomfort and normal cardiac markers) with unstable angina on an outpatient basis. In this group of patients, testing can be performed when patients have been free of symptoms of active ischemia or heart failure for a minimum of 8 to 12 hours. Intermediate-risk patients can be tested after 2 to 3 days, although carefully selected patients can be evaluated earlier. In general, as in stable angina, the exercise treadmill should be the standard mode of stress testing in patients with a normal resting ECG who are not taking digitalis.1
The Research on Instability in Coronary Artery Disease (RISC) study group examined the use of predischarge symptom-limited bicycle exercise testing in 740 men admitted with unstable angina or non-Q-wave myocardial infarction and found that the major independent predictors of 1-year infarction-free survival in multivariable regression analysis were the number of leads with ischemic ST-segment depression and peak exercise workload achieved.20 Moreover, the Fragmin During Instability in Coronary Artery Disease (FRISC) study found in 766 unstable angina patients who had both a troponin T level test and a predischarge exercise test that the combination of a positive troponin T and exercise-induced ST depression stratified patients into groups with a risk of death or myocardial infarction that ranged from 1% to 20%.21 Important exercise variables include not only ischemic parameters such as ST depression and chest pain but also parameters that reflect cardiac workload.
There is a growing body of evidence concerning the role of early exercise testing in emergency department chest pain centers. The rationale for its use in this setting is to provide rapid and efficient risk stratification and management for chest pain patients who possibly have acute coronary disease. Several studies have demonstrated that exercise testing improves the efficiency of management of low-risk and carefully selected intermediate-risk patients and may lower costs without compromising safety. However, exercise testing in emergency department chest pain centers should only be performed after the exclusion of high-risk features or other indications for hospital admission, and only as part of a carefully constructed management protocol.19
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