pharmacologic stress imaging
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Although neither as sensitive nor as specific as invasive procedures, exercise stress testing has become a standard means of identifying and grading coronary artery disease in people with typical and atypical angina pectoris as well as in those in certain critical occupations (airline pilots, firefighters). It has been found useful for risk stratification in survivors of myocardial infarction (MI), and in planning and monitoring rehabilitation after MI, coronary bypass surgery, and balloon angioplasty. It is also used to assess the safety of exercise programs for people at risk of coronary artery disease because of age or personal or family history. The Master two-step test, based on repeated ascent and descent of a step-stool, has been superseded by more sophisticated and reproducible methods. Standard exercise testing employs graded physical exertion on an electrically driven treadmill with variable inclination and speed. Alternative methods include a mechanical stair-climbing machine, a stationary bicycle ergometer, and (for those with certain physical disabilities) an arm-exercising (hand-crank) machine. Various protocols and end-points are used to measure the outcome of stress testing. Workloads are measured in metabolic equivalents (MET), 1 MET being the amount of oxygen consumed at bed rest (3.5 mL/kg/min). In maximal (symptom-limited) stress testing, the subject continues to exercise at increasing levels of challenge until chest distress, significant hypertension or hypotension, certain arrhythmias, fatigue, gait problems, or severe dyspnea occurs. The Bruce protocol, a standard maximal exercise treadmill protocol, begins with a treadmill speed of 1.7 mph and a grade of 10°, designed to achieve a workload of 4.6 MET, and increases both speed and grade every 3 minutes. In submaximal (pulse-limited) stress testing, the subject continues exercising until reaching a target heart rate based on age, health history, and physical condition (unless the test must be stopped earlier because of symptoms). A stress test session usually lasts 6-10 minutes. Elevation or depression of ST segments by more than 1 mm during exercise is strongly suggestive of coronary artery disease. Other suggestive changes are T-wave inversion, arrhythmia, a fall in systolic blood pressure, and a marked rise in diastolic blood pressure. Exercise stress-testing protocols achieve 85-90% accuracy in identifying patients without coronary artery disease. About 5% of asymptomatic adults have positive stress test results, but only one third of these have angiographically demonstrable coronary artery disease. False positive test results occur more frequently in women. Large studies have shown that delayed return to resting heart rate, reduced chronotropic index (fraction of cardiac reserve used during exercise), and reduced age-specific functional capacity (in MET) are all potent predictors of cardiovascular mortality, regardless of etiology. Exercise stress testing is contraindicated in acute MI, severe congestive heart failure, severe hypertension, hemodynamically significant valvular disease or arrhythmia, active thromboembolic disease, and extreme obesity. As an alternative to exercise, pharmacologic challenge may be performed by intravenous infusion of dobutamine, an adrenergic agonist that increases heart rate and blood pressure in a fashion similar to physical exertion, or of dipyridamole or adenosine, which dilate normal coronary arteries but do not increase blood flow through vessels narrowed by atherosclerosis. In addition to continuous ECG monitoring, the cardiac effects of stress or pharmacologic challenge can be assessed by myocardial scintigraphy after intravenous injection of thallium 201; cineangiography after injection of technetium 99m, with or without multiple-gated acquisition bloodpool imaging; or single-photon emission computed tomography.