- Patient B is more likely than patient A to develop coronary artery disease.
- Patient B has a worse cardiovascular prognosis than patient A.
- Patient A’s exercise ECG results are falsely positive, whereas patient B’s results are truly positive.
- On the basis of their blood pressures during exercise, patient A has a higher risk of stroke than patient B.
EXERCISE TESTING FOR DIAGNOSIS AND PROGNOSIS
When we perform a stress test such as the treadmill test, we are asking two questions: does the patient have coronary artery disease (ie, what is the patient’s diagnosis) and is he or she likely to die or suffer a coronary event soon (ie, what is the patient’s prognosis).1,2
A stress test used diagnostically is considered to have a positive result if the patient develops signs and symptoms of ischemia during stress, ie, ST-segment depression and angina.1 The diagnostic accuracy of exercise testing is commonly assessed separately from its prognostic accuracy. Unfortunately, diagnostic accuracy can be assessed only in the minority of patients who subsequently undergo coronary angiography—the gold standard for comparison.
In contrast, the prognostic accuracy of a stress test can be assessed in a much larger group of patients, using clinical outcomes as the comparison standard; only those who undergo early revascularization and those who are lost to follow-up are excluded from this group.
Although the stress-induced markers of ischemia used in diagnosis—ST-segment depression and angina—have prognostic value as well, other variables are more powerful predictors of outcome. In this article I will discuss those other prognostic variables and how to interpret them.
Variables measured during exercise treadmill testing that predict outcome are actually indicators of general fitness and function of the autonomic nervous system:
- Exercise duration
- Exercise hypotension
- Exercise hypertension
- Chronotropic incompetence
- Heart rate recovery
- Ventricular ectopy.
In the Bruce protocol used in exercise stress testing, the test begins with the treadmill set to a low speed (1.7 miles per hour) and a 10% incline, and every 3 minutes the speed and angle of incline are increased. Other protocols are similar. The test continues for a maximum of 27 minutes (usually attainable only by well-trained individuals) or until the patient quits or develops signs or symptoms of ischemia or an arrhythmia. Average time for a middle-aged adult is 8 to 10 minutes.
Because the longer the patient goes, the harder he or she must work, exercise duration—the number of minutes the patient can continue in the protocol—is a good measure of his or her functional capacity. Another way to measure functional capacity is to measure oxygen uptake during exercise, which can be converted to metabolic equivalents (METs): 1 MET = 3.5 mL O2/kg/min. However, most laboratories estimate functional capacity from exercise duration in a specific exercise protocol (eg, the Bruce protocol) based on published nomograms.
Remarkably, the longer the patient can keep going on the treadmill, the less likely he or she is to die soon of coronary artery disease—or of any cause. In fact, of the prognostic variables measured during exercise treadmill testing, exercise duration is the strongest.1,2 Its prognostic value has been demonstrated in healthy subjects being screened for coronary artery disease (Figure 1)3–6 and in patients being evaluated for suspected or known coronary artery disease (Figure 2).7–10 The independent prognostic value of exercise duration has been demonstrated in men,3,4,7,8 women,4–7,9 and the elderly.11 Although functional capacity decreases with age and generally is lower in women than men, exercise duration retains its prognostic value after adjusting for age and sex.
Exercise duration is such a good prognostic indicator that it is included in risk scores for exercise treadmill testing.13,14