From the beginning, the application of exercise stress testing for the diagnosis of obstructive CAD has been focused on the presence and extent of ST-segment deviations during and immediately after exercise. This is obtained by serial recordings of the 12-lead electrocardiograph (ECG), which is often aided by computer analysis.
Exercise ECG testing has modest diagnostic utility, mostly in patients with an intermediate pretest likelihood for having disease. It is now recognized that the early reported sensitivities of exercise ECG testing were affected by a verification bias. In other words, the performance of coronary angiography was influenced by the results of the exercise test. This verification bias leads to overestimation of sensitivity and underestimation of specificity. Recent data suggest that the true sensitivity of exercise testing is only about 50%. Despite this limitation, exercise ECG testing remains a useful prognostic test. An index derived from the exercise ECG test that incorporates exercise time, magnitude of ST-segment deviation, and angina, also known as the Duke Treadmill Score, has proved to be a powerful prognosticator of events. The Duke Treadmill Score is calculated as follows:
Duke Treadmill Score = Exercise time -
where Max ST deviation is the maximum ST-segment deviation (elevation or depression) noted in any of the 12 ECG leads, compared with baseline. The treadmill angina index is defined as having a value of 0 if no angina occurs, 1 if angina occurs during exercise but is not test-limiting, or 2 if test-limiting angina occurs. Exercise time is based on the Bruce Protocol.
Using the Duke Treadmill Score, patients may be divided into categories of low risk (score greater than or equal to +5), intermediate risk (score less than 5 but greater than or equal to -10), and high risk (score less than -10). In the original study, the 5-year survival rates among patients categorized as having low, intermediate, and high risk were 97%, 91%, and 72%, respectively. Of note, the exercise treadmill score provided prognostic information independent of coronary angiography findings. The ability of the score to predict risk has been validated in many different subpopulations, including women. The annual cardiac death rate has been reported to be very low (0.3% to 1.2% per year) in patients with low-risk scores. Recent studies suggest, however, that exercise
Figure 3-1. Normal stress echocardiography response. Images obtained at end-diastole (ED) and end-systole (ES) at rest and immediately after exercise stress from the parasternal long axis (LAX), short axis (SAX), and apical four-chamber (AP4) and two-chamber (AP2) windows. Notice the decrease in ES left ventricular cavity size after stress.
capacity is the most important factor in the Duke Treadmill Score.2
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