Published online before print July 22, 2002, doi: 10.1161/01.CIR.0000024413.15949.ED
(Circulation. 2002;106:666.)
© 2002 American Heart Association, Inc.
Clinical Investigation and Reports |
Prediction of Long-Term Prognosis in 12 169 Men Referred for Cardiac Rehabilitation
From Toronto Rehabilitation Institute (T.K., D.J.M., J.K., R.J.S.), Faculty of Medicine (T.K.), Faculty of Physical and Health Education (T.K., R.J.S.), and Department of Public Health Sciences (J.B., P.C., R.J.S.), University of Toronto, Ontario, Canada; and George Washington University (L.F.H.), Washington, DC.
Correspondence to T. Kavanagh, MD, Marina Cardiac Research Department, 5385 Yonge St, Toronto, ON M2N 5R7, Canada. E-mail terence.kavanagh{at}utoronto.ca
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Abstract |
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Background— Predicting the risk of cardiac and all-cause death in patients with established coronary heart disease is important in counseling the individual and designing risk-stratified rehabilitation and secondary prevention programs. Cox proportional hazards and Kaplan-Meier survival curves were thus completed on initial assessment data obtained from patients referred to an outpatient cardiac rehabilitation center.
Methods and Results— A single-center prospective observational design took peak cardiorespiratory exercise test data for 12 169 male rehabilitation candidates aged 55.0±9.6 years (7096 myocardial infarctions [MIs], 3077 coronary artery bypass grafts [CABGs], and 1996 documented cases of ischemic heart disease [IHD]). A follow-up of 4 to 29 years (median, 7.9) yielded 107 698 man-years of experience. Entry data were tested for associations with time to cardiac and all-cause death. We recorded 1336 cardiac deaths (953 MI, 225 CABG, and 158 IHD) and 2352 all-cause deaths. A powerful predictor of cardiac and all-cause mortality was measured peak oxygen intake (
O2peak). For the overall sample, values of <15, 15 to 22, and >22 mL/kg per minute yielded respective multivariate adjusted hazard ratios of 1.00, 0.62, and 0.39 for cardiac and 1.00, 0.66, and 0.45 for all-cause deaths. For the separate diagnostic categories, apart from O2peak, the only other significant predictors of cardiac death common to all 3 were smoking and digoxin, and for all-cause death, age, smoking, digoxin, and diabetes.
Conclusions— Exercise capacity, as determined by direct measurement of O2peak, exerts a major long-term influence on prognosis in men after MI, CABG, or IHD and can play a valuable role in risk stratification and counseling.
Key Words: exercise • prognosis • coronary disease • survival
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Introduction |
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Previous authors have discussed the predictive value of exercise test data in subjects with established heart disease. However, conclusions were generally based on submaximal tests or on an oxygen intake (
O2) predicted from treadmill stages or times, with a limited follow-up period.1–7 We therefore examined the prognostic importance of maximal cardiopulmonary exercise testing determined at entry to a large outpatient cardiac rehabilitation program, with the subjects followed over a median of 7.9 (range, 4 to 29) years. |
Methods |
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Study Population
A total of 13 131 men were referred to an outpatient cardiac program between September 1, 1968, and December 31, 1994. Patients with noncoronary diagnoses (n=808) and in whom there were technical problems in respiratory gas collection (n=154) were excluded. This left a sample of 12 169 men (7096 myocardial infarctions [MIs], 3077 coronary artery bypass grafts [CABGs], and 1996 documented cases of ischemic heart disease [IHD]), referred 13.4±3.9 weeks after the event.
Follow-Up
As approved by the Institutional Committee on Human Experimentation, a questionnaire covering demographics and present health status was mailed to all patients. If an envelope was returned marked "address unknown," the new address was sought through contacts such as family members, employer, family physician, and cardiologist. If additional mailings elicited no response, a final search was made through a computer program (Pro-Phone) that lists all published Canadian telephone numbers and addresses.
Death certificates were examined for decedents resident in Ontario. For 56 decedents no longer resident in Ontario, cause of death was determined on the basis of our clinical interpretation of responses from a physician, spouse, or family.
Cardiorespiratory Testing
Cardiorespiratory testing has been described previously.8 The patients performed a progressive test (16.7 W/min) on an electrically braked cycle ergometer until subjective exhaustion, plateauing of oxygen intake, or clinical contraindications to continue.9 All tests were directly supervised by a physician, with the same senior team of technical assistants in attendance throughout the period 1968 to 1994.10 Patients continued with their usual medications on the day of the test.
Statistical Methods
SAS Software version 8.2 was used for all statistical analyses. Univariate analysis examined clinically relevant variables, including cardiovascular drugs, and those that were statistically significant (P<0.05) were entered into a multivariate Cox Proportional Hazards model. This procedure was carried out for the group as a whole and for each of the diagnostic categories. Survival time was defined as from the time of the exercise test to the time of cardiac or all-cause death for each diagnostic category (MI, CABG, or IHD). Patients with a diagnosis of MI or IHD who subsequently underwent coronary revascularization were included up to the time of their procedure and then censored. Age and total cholesterol were treated as continuous variables, peak oxygen intake as a 3-level categorical and then a continuous variable, and all other variables as binary data. Exertional hypotension was defined as a drop in systolic blood pressure below resting value or failure to rise more than 10 mm Hg with 2 successive increases in workload (33.4 watts). Wald 
2 was calculated, yielding 95% CIs and probability values for the relative risk of individual items after adjusting for other variables.
Kaplan-Meier survival curves were generated, providing a univariate expression of survival in relation to cardiac death, cardiac death for the 3 categories of O2peak, and all-cause death. To determine whether a O2peak estimated from peak work rate11 possesses the same prognostic power as measured O2peak, we tested the differences in the areas under the receiver operating characteristic curve using the z statistic of Hanley and McNeil.12
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Results |
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The median follow-up time was 7.9 (range, 4 to 29) years. We were unable to contact 583 patients (4.8% of our sample). During follow-up, 1336 cardiac deaths (953 MI, 225 CABG, and 158 IHD) and 2352 all-cause deaths (11.0% and 19.3%, respectively, of the original sample) were recorded.
Clinical and Physiological Characteristics
Subjects were in their mid-50s, with body dimensions representative of sedentary men of the same age group (Table 1). Over one half of all patients were myocardial infarction survivors, and approximately one half were taking a ß-blocker or a platelet inhibitor (most commonly aspirin).
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The average O2peak values were typical of postcoronary men in their mid-50s. The peak heart rates, respiratory exchange ratios, and Borg ratings of perceived exertion indicate that values were at or close to the individuals’ maximal oxygen intake. Complex ventricular arrhythmias and significant ST-segment depression (
0.2 mV) occurred in 30% and 15% of patients, respectively.
Predictors of Mortality
For the total group, and taking a peak O2 of <15 mL/kg per minute (4.3 METs) as baseline, multivariate analysis showed that values of 15 to 22 (4.3 to 6.3 METs) and >22 mL/kg per minute (>6.3 METs) yielded, respectively, a 38% and 61% reduction in the risk of cardiac death over the follow-up period (Table 2).
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ß-Blockers and platelet inhibitors also exerted a protective effect (risk reductions of 13% and 31%, respectively). Significant adverse predictors of cardiac death (P<0.01) were age, diabetes, continuing smoking, exertional hypotension, ST-segment depression, and taking various cardioactive drugs (Table 2).
Results for patients with MI, CABG, and IHD show that only O2peak, smoking, and digoxin had significant effects common to all 3 diagnostic categories (Table 3).
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In terms of all-cause deaths, O2peak, platelet inhibitors, and ß-blockade were significant favorable predictors for the group as a whole (Table 4); only O2peak, smoking, diabetes, digoxin, and age were significant (P<0.0001) predictors common to all 3 diagnostic categories.
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Kaplan-Meier Curves
The Kaplan-Meier curves for all diagnostic categories show the 15-year probability of surviving a cardiac death was 81% (Figure 1), and death from all causes was 69% (Figure 2). The 15-year values for the individual diagnostic categories were similar, as follows: MI, 79% and 68%; CABG, 83% and 70%; and IHD, 85% and 74%.
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Grouping data by initial O2peak, there was a clear distinction of prognosis between the 3 initial categories of aerobic fitness. For cardiac deaths, the 15-year survival rates for patients with O2peak values of <15, 15 to 22, and >22 mL/kg per minute were 
65%, 81%, and 88% (log-rank test, P<0.001) (Figure 3). Considering deaths from all causes, the corresponding figures were 48%, 69%, and 80% (log-rank test, P<0.001).
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When treated as a continuous variable, a 1-mL/kg per minute increment in O2peak could be equated with a 9% improvement in prognosis (hazard ratio, 0.91) or a 2.6-year advantage in prognosis for the lower half of the sample and a gain of 1.0 year in the upper half of the sample.
Survival prospects were substantially worsened by exertional hypotension (15-year survival from cardiac deaths, 67.7% versus 81.6%, P<0.001; from all-cause deaths, 53% versus 70%, P<0.001) and by continued smoking (15-year survival from cardiac deaths, 75% versus 83%, P<0.001; from all-cause deaths, 61% versus 72%, P<0.001).
Finally, receiver operating curve analysis showed that the values for areas under the curves for measured and predicted VO2peak were, respectively, 0.66 (0.64 to 0.69) and 0.66 (0.67 to 0.69). There was no significant difference between the areas under the curve (Z=0.135; P=0.893).
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Discussion |
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To our knowledge, this study contains the largest number of coronary patients investigated for the predictive value of cardiopulmonary exercise testing with the longest follow-up. The data demonstrate that the primary factor influencing prognosis in coronary heart disease patients is the individual’s effort tolerance, as assessed objectively by the measurement of
O2peak. This substantiates the conclusions previously formed by Vanhees et al, 13 who examined the predictive power of measured O2peak in a smaller sample over a shorter median follow-up. A more recent study involving a large group of men referred for exercise testing, some normal and some suffering from cardiovascular disease, also found exercise capacity, estimated on the basis of treadmill performance, to be a more powerful predictor of mortality than other established risk factors.7 Observations in healthy men have also demonstrated that a 1-MET (3.5 mL/kg per minute) advantage in O2peak (7 versus 6 METs) is associated with a halving of mortality rate (1.5% to 0.75%) over an 8-year follow-up.14 Moreover, a similar benefit is seen if an unfit but otherwise healthy group of adults is persuaded to become fit.15
In chronic heart failure patients, a O2peak of 14 mL/kg per minute has been recognized as a cutoff below which the risk of death is significant and cardiac transplantation becomes a preferred option.16–18 Myers and Gullestad,19 on the other hand, noted that whereas patients who achieved >14 mL/kg per minute had an 82% survival rate and those with 14 mL/kg per minute or less had a survival rate of 60%, this 20% difference was similar when using any cutoff value between 10 and 17 mL/kg per minute. Chomsky et al20 found a poorer prognosis only in those with an aerobic power of <10 mL/kg per minute. The O2peak cutoff points selected for our patient categorization were determined by the characteristics of our sample (mean O2peak, 20.2±5.1 mL/kg per minute) and the levels customarily used in prognostic stratification of patients referred for cardiac rehabilitation (<4, 4 to 6, and >6 METS).21,22
In our sample, the estimated 15-year prognosis dropped substantially from the least-fit group (average O2peak, 13.0 mL/kg per minute, 35% mortality) to the middle group (average O2peak, 18.6 mL/kg per minute, 19% mortality). In an average (not median) 6-year follow-up of a smaller sample, Vanhees et al13 found a progressive decline in all-cause mortality with aerobic power from a group with a O2peak of 15.5 mL/kg per minute to the quintile with the highest aerobic fitness (32.3 mL/kg per minute). In their sample, the largest difference in prognosis was seen between the 2 highest quintiles of fitness.
Dorn et al,23 reporting on a long-term follow-up of postmyocardial infarction patients who completed an exercise program, found a reduction of 8% to 14% in all-cause death for every 1 MET increment in work capacity. The 9% improvement in prognosis associated with a 1 mL/kg per minute advantage of O2peak in our cohort suggests that cardiac rehabilitation patients who make only modest gains in cardiorespiratory fitness could nevertheless obtain significant prognostic and functional benefits. Such gains may be particularly relevant for patients in the lowest of our 3 categories, ie, O2peak <15 mL/kg per minute.
The O2peak may be serving in part as a measure of residual pump function.20 However, others have suggested that prognosis bears little relationship to more direct measures of cardiac performance.16,19,24 We thus suspect that over a long-term follow-up, those with a low initial aerobic power progress more quickly to the point where pump function becomes inadequate. Another possibility is that an individual’s peak O2 influences the proportion of aerobic power used during heavy activities and thus the vulnerability to catecholamine-induced arrhythmias. Exertional hypotension is also a significant factor in prognosis25 and again an indication of impaired pump function, although here we are looking at the ability of the individual to augment stroke volume during exercise in the face of a poorly perfused myocardium.
For the exercise test laboratories that do not have the facilities for collecting and analyzing respiratory gases, our finding that on receiver operating curve analysis the predicted O2peak has the same prognostic power as measured O2peak seems reassuring. However, others have shown that predicted values overestimate O2, particularly in cardiac patients,26 and this is confirmed in our data. For instance, 55% of patients whose measured O2peak was <15 mL/kg per minute had a predicted measure of 15 to 22 mL/kg per minute, whereas only 19% of those with a predicted value of <15 mL/kg per minute had a measured O2peak of 15 to 22 mL/kg per minute. Similarly, 33% of patients with a measured O2peak ranging from 15 to 22 mL/kg per minute were predicted to have their measurements in the range of >22 mL/kg per minute, whereas only 7% of those with a predicted value of 15 to 22 mL/kg per minute had their measured O2peak in the higher category of >22 mL/kg per minute. Caution is urged, therefore, in the use of predicted values for risk stratification. Obviously, our findings need to be evaluated using other data, particularly from the chronic heart failure population.
For interest, we have derived a simple multivariate risk score using the coefficients from the Cox model and the 4 highest performing variables in Table 3, as follows: O2 (in mL/kg per minute) <15=2, 15 to 22=1, >22=0; digoxin, yes=2, no=0; and smoking, yes=2, no=0. Low risk is defined as <2, moderate risk as 3 or 4, and high risk as >5. We used this grouping to predict 5-, 10-, and 15-year survival probabilities as well as to calculate Kaplan-Meier survival curves (Figure 4). We note that the patient who has a low level of fitness and is a smoker is at much greater risk of cardiac death than one who is free from these risk factors. Because both factors are amenable to exercise rehabilitation, this emphasizes its value. However, the cumulative risk score needs to be validated on a separate data set.
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Our follow-up lacks studies in women, and, although risk factors in women resemble those in men (although some differ in magnitude of effect),27 it nevertheless remains important to determine whether peak aerobic power carries similar prognostic weight in women, given that they seem less vulnerable to exercise-induced sudden death.28,29 We also have a preponderance of white professional or managerial-class patients. Thus, our findings need replication in samples of differing ethnic and socioeconomic status. Finally, the length of the follow-up, although a unique and desirable feature of our study, also exposes the observations to potential contamination from secular trends in behavior and environmental factors; these may have influenced the shape of the mortality curves. For instance, during the period from 1968 to 1994, the average life expectancy of the Canadian male has increased substantially.30
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Conclusions |
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Whether a cardiac patient is referred for rehabilitation after MI, CABG, or the onset of IHD, the most important single predictor of both cardiac and all-cause deaths is the
O2peak as measured by cardiorespiratory testing. Even a small exercise-induced gain in aerobic power should thus make a major difference not only in functional capacity but also in survival prospects. |
Acknowledgments |
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This work was supported by a grant from Canadian Cardiac Rehabilitation Foundation. The authors are indebted to the Human Performance Laboratory staff, including Dr N. Davidson, Dr A. Elzawi, Dr B. Evans, Dr V. Matosevic, Dr A. Qureshi, P. Baskey, RN, R. Belliard, K. Bernsdorff, J. D’Elia, S. Qureshi, MSc, Barb Smith-Perrin, and E. Strohm, MSc; to M. Bhagi for her help with data analysis; and to J. Will for administrative assistance and manuscript preparation. We also thank Dr Victor Froelicher, MD, Stanford University, for his advice about the final preparation of this manuscript.
Received April 18, 2002; revision received May 16, 2002; accepted May 16, 2002.
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References |
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K Woolf-May, S Bird, and P D MacIntyre Physical activity levels during phase IV cardiac rehabilitation in a group of male myocardial infarction patients * Commentary Br. J. Sports Med., March 1, 2005; 39(3): e12 - e12. [Abstract] [Full Text] [PDF]
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A. S. Leon, B. A. Franklin, F. Costa, G. J. Balady, K. A. Berra, K. J. Stewart, P. D. Thompson, M. A. Williams, and M. S. Lauer Cardiac Rehabilitation and Secondary Prevention of Coronary Heart Disease: An American Heart Association Scientific Statement From the Council on Clinical Cardiology (Subcommittee on Exercise, Cardiac Rehabilitation, and Prevention) and the Council on Nutrition, Physical Activity, and Metabolism (Subcommittee on Physical Activity), in Collaboration With the American Association of Cardiovascular and Pulmonary Rehabilitation Circulation, January 25, 2005; 111(3): 369 - 376. [Abstract] [Full Text] [PDF]
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M. Borjesson and M. Dellborg Exercise testing post-MI: still worthwhile in the interventional era Eur. Heart J., January 2, 2005; 26(2): 105 - 106. [Full Text] [PDF]
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F. X. Kleber The predictive value of cardiorespiratory fitness Eur. Heart J., August 2, 2004; 25(16): 1374 - 1375. [Full Text] [PDF]
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J. A. Laukkanen, S. Kurl, R. Salonen, R. Rauramaa, and J. T. Salonen The predictive value of cardiorespiratory fitness for cardiovascular events in men with various risk profiles: a prospective population-based cohort study Eur. Heart J., August 2, 2004; 25(16): 1428 - 1437. [Abstract] [Full Text] [PDF]
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