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The exercise treadmill test: Estimating cardiovascular prognosis

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ABSTRACTAbnormal hemodynamic responses to exercise treadmill testing may indicate an increased risk of coronary events and death, even if evidence of ischemia is absent. Exercise duration and the blood pressure, heart rate, and heart rhythm responses to exercise have prognostic significance.

KEY POINTS

  • Of the prognostic factors, exercise duration is the one most strongly associated with risk of coronary events and death, independent of age, sex, or known presence and severity of coronary artery disease.
  • A decrease in blood pressure with exercise can reflect severe coronary artery disease or left ventricular systolic dysfunction.
  • A heart rate that does not increase adequately during exercise or does not recover rapidly after exercise is associated with an increased risk of death.
  • Exercise training may help to improve the prognosis of patients with an abnormal hemodynamic response to exercise caused by poor general health.


 

References

Two 55-year-old men without a history of coronary artery disease both undergo an exercise treadmill test to evaluate atypical angina. Both men have normal results on electrocardiography (ECG) at rest. The results of their exercise treadmill tests are shown in Table 1. Which one of the following statements is correct?
  • 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.

PROGNOSTIC VARIABLES

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.

Exercise duration

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.

Blair SN, et al. Physical fitness and all-cause mortality: a prospective study of healthy men and women. JAMA 1989; 262:2395–2401. Copyright © 1989, American Medical Association. All rights reserved.
Figure 1. Age-adjusted, all-cause mortality rates per 10,000 person-years of follow-up by physical fitness categories in 3,120 women and 10,224 men in the Aerobics Center Longitudinal Study.

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.

Myers J, et al. Exercise capacity and mortality among men referred for exercise testing. N Engl J Med 2002; 346:793–801. Copyright © 2002, Massschusetts Medical Society. All rights reserved.
Figure 2. Relative risks of death from any cause among subjects with various risk factors who achieved an exercise capacity of less than 5 metabolic equivalents (METs) or 5 to 8 METs, as compared with subjects whose exercise capacity was more than 8 METs. Error bars are 95% confidence intervals for the relative risks. BMI = body mass index; COPD = chronic obstructive pulmonary disease.
Exercise duration has also been shown to provide risk stratification in subsets of patients with coronary artery disease defined anatomically. The Coronary Artery Surgery Study (CASS), which analyzed 30 variables in 4,083 patients with symptomatic coronary artery disease, found that survival at 4 years was 100% in patients with three-vessel coronary disease and preserved left ventricular function who had good exercise capacity, ie, who were able to keep walking to level 5, which is 12 minutes or more.12

Exercise duration is such a good prognostic indicator that it is included in risk scores for exercise treadmill testing.13,14

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