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Resuscitating the Exercise Stress Test TAKE-HOME MEDICAL GRAND ROUNDS POINTS FROM LECTURES BY CLEVELAND CLINIC AND VISITING FACULTY Resuscitating the exercise stress test MICHAEL S. LAUER, MD We are asking the wrong question. Co-director, Coronary Intensive Care Unit; Department of Cardiology, Cleveland Clinic Instead of trying to use exercise testing for diagnosis, we should use it to determine prog- II ABSTRACT nosis. With coronary disease endemic in our society, many, if not most, patients referred for By incorporating additional data, clinicians exercise stress testing have some degree of can transform the exercise stress test from a coronary disease. Exercise stress testing should poor diagnostic test to a good prognostic not be used to diagnose coronary disease, but one. These additional data include indices of to predict cardiac events and mortality. These how the patient's heart rate and blood findings should then be used to guide treat- pressure respond to exercise. Computerized ment and to educate patients about risk fac- measures of electrocardiographic ST- tors. segment changes that adjust for heart rate We are looking for the wrong lesions. also show promise. An exercise ECG is considered abnormal if it shows 1 mm or more of horizontal or LD DOESN'T NECESSARILY MEAN obsolete downsloping ST-segment depression 60 to 80 when it comes to medical technology. msec after the J point. However, ST-segment A case in point is the exercise stress test, abnormalities appear only when the patient used for decades as a test for coronary artery has hemodynamically obstructive lesions; Clinicians are disease. These days, many physicians are stenoses of 50% may not cause stress-induced using the turning away from the traditional exercise ischemia. stress test with echocardiography (ECG), cit- We are ignoring other important infor- exercise stress ing its lack of sensitivity, and instead are mation. Non-electrocardiographic variables test to ask the ordering thallium imaging and other costly, such as exercise capacity and heart-rate and high-tech imaging tests. blood-pressure changes during exercise can wrong question Nevertheless, I believe we can resusci- contribute valuable information. These vari- tate the exercise stress test if we refine the ables are particularly useful when combined way we interpret the results, supplement into a measure called the chronotropic index. them with additional heart rate and hemo- We are using the wrong methods. We are static information—and use them to answer beginning to show that computerized analyses the right question. of ECGs, adjusted for heart rate, may be more accurate and valuable than visual inspections. • EXERCISE TESTING OFTEN MISSES DISEASE • IMPROVING EXERCISE TESTING Study after study has shown that exercise test- Measuring exercise capacity ing that relies on visual interpretations of To improve our interpretation of exercise exercise ECG can miss many cases of coronary tests, one of the factors we should measure is artery disease. Sensitivity can be as low as exercise capacity (essentially, physical fit- 45%, though specificity is somewhat higher, ness). Exercise capacity and improvement in at around 85%.i-3 exercise capacity are powerful and indepen- There are four possible reasons the test dent predictors of survival and survival free of performs so poorly. cardiac mortality and morbidity.4-6 278 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 66 • NUMBER 5 MAY 1999 Downloaded from www.ccjm.org on September 25, 2021. For personal use only. All other uses require permission. TABLE 1 Levels of exercise and metabolic equivalents consumed LEVEL ESTIMATED METS* ACTIVITY Mild 2-3 Baking, slow dancing, writing, playing golf with a cart Moderate 3-5 Gardening, playing the drums, swimming slowly Vigorous 6 Dancing 8 Playing field hockey 10 Jogging at 10 minutes/mile 12 Playing squash *1 metabolic equivalent (MET) is the amount of oxygen consumed during awake rest, about 3.5 mL/kg/min FROM FLETCHER GF, BALADY G, FROELICHER VF, ET AL. EXERCISE STANDARDS. A STATEMENT FOR HEALTH-CARE PROFESSIONALS FROM THE AMERICAN HEART ASSOCIATION WRITING GROUP. CIRCULATION 1995; 91:580-615. Exercise capacity is measured in METs, or to exercise were more likely to suffer acute metabolic equivalents, the amount of oxygen coronary events than patients with ST-seg- consumed at rest by an awake patient (TABLE 1). ment depression and normal heart-rate Younger people have a markedly higher func- responses 10 tional capacity than older people, and men a Ellestad named the phenomenon slightly higher capacity than women. chronotropic incompetence. The association In a study that followed 3,400 patients for between chronotropic incompetence and 2 years, we found that low exercise capacity heart disease has been repeatedly confirmed in and impaired cardiac perfusion as shown on other studies,11 and chronotropic incompe- thallium scans were both strongly predictive tence has been shown to be associated with of mortality.7 myocardial scarring.12 Exercise capacity is only one of the factors When a patient taking an exercise test Exercise that should go into an assessment of cardiac fails to reach 85% of his or her age-predicted capacity is health. It does not fill in the whole picture, maximum heart rate, the test is often classified and in addition, it may be inaccurate for indi- as "nondiagnostic." However, we believe the only one of vidual patients because it is generally estimat- finding should be considered much more omi- the factors in ed from published tables rather than measured nous and in most cases, chronotropic incom- directly by gas-exchange analyses.8 petence should be considered "evidence of an cardiac health adverse prognosis." Recognizing chronotropic incompetence Unfortunately, simple use of the percent A second factor that should be measured is the of target heart rate achieved is problematic. patient's heart-rate response to exercise. This Estimated peak heart rates are highly variable was suggested more than 20 years ago, after a in each age group, making it difficult to apply healthy 51-year-old man suffered sudden car- them to individuals. Resting heart rate and diac death shortly after passing his exercise physical fitness also strongly affect the stress test with flying colors. The only abnor- patient's ability to reach a predicted peak mality had been that his heart rate had not heart rate. risen above 110 beats per minute during exer- cise. An autopsy showed severe two-vessel Calculating the chronotropic index coronary disease with 80% stenosis of the left To solve this problem, Wilkoff has created a anterior descending and circumflex arteries.9 simple measure now called the chronotropic This event prompted cardiologist Myrvin index that predicts mortality independent of H. Ellestad and several colleagues to review age, resting heart rate, and metabolic work.13 the records of 2,700 patients. They found that Before exercise, a person has a certain patients with attenuated heart-rate responses metabolic reserve, which is the difference CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 66 • NUMBER 5 MAY 1999 279 Downloaded from www.ccjm.org on September 25, 2021. For personal use only. All other uses require permission. MEDICAL GRAND ROUNDS Calculating the chronotropic index • • N HEALTHY PERSONS, heart rate increases Percent metabolic _ (METsstaee - METsrest) ^ - X U with exercise in a predictable fashion. reserve used (METspeak - METsrest) Persons whose heart rate fails to increase to the expected level have a higher risk of cardiac mor- The second step is to calculate the heart rate bidity and mortality. The chronotropic index can reserve, the difference between the patient's esti- show whether a patient's heart-rate response to mated peak heart rate (220 minus the patient's exercise is healthy or unhealthy. age) and resting heart rate. The percent heart rate The first step is to calculate the patient's meta- reserve used at a particular stage of exercise is: bolic reserve. One metabolic equivalent (1 MET) is the amount of oxygen a person consumes at rest, Percent heart rate = (HRstafie - HRrest) ^ typically about 3.5 mL/kg/min. The difference reserve used (HRpeak - HRrest) between the peak and resting MET levels is called the metabolic reserve. To calculate the chronotropic index, divide the The estimated MET level at any stage of exer- percent heart rate reserve used by the percent cise (METsstaKe) is displayed automatically by com- metabolic reserve used. If the ratio is less than 0.8, puterized treadmills. Using this figure, the clini- the patient's heart rate is lower than expected dur- cian can calculate how much of the metabolic ing exercise, and the patient has a high risk of car- reserve the patient has used. diac morbidity and mortality. between the level of oxygen consumption at was the true cause of the mortality. In a the peak of strenuous exercise (exercise capac- prospective study of 231 consecutive patients, ity) and resting oxygen consumption. During we adjusted the chronotropic index for exercise, this reserve is used up. Analogously, myocardial ischemia detected by ECG and for Alow the heart rate reserve can be defined as the other possible confounding factors. We found chronotropic difference between the estimated peak heart that a low chronotropic index and failure to rate and the resting heart rate. This reserve is reach 85% of the target heart rate both index predicts also used up as exercise progresses. remained predictive of mortality, independent mortality At each stage of exercise, we can calculate of ischemia.16 both the percent of the metabolic reserve used Confirming this finding is our recent arti- independent and the percent of the heart rate reserve used, cle in the Journal of the American Medical of age, resting and we can plot these values on a line graph. Association,17 in which we show that a low The slope of this line is called the chronotrop- chronotropic index and failure to reach 85% heart rate, and ic index.
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