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Circulation. 1995;91:46-53

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(Circulation. 1995;91:46-53.)
© 1995 American Heart Association, Inc.

Articles

Surgical Therapy for Coronary Artery Disease Among Patients With Combined Coronary Artery and Peripheral Vascular Disease

Charanjit S. Rihal, MD; Kim A. Eagle, MD; Mary C. Mickel, MS; Eric D. Foster, MD; George Sopko, MD, MPH; Bernard J. Gersh, MB, ChB, DPhil

From the Division of Cardiovascular Diseases and Internal Medicine, Mayo Clinic and Mayo Foundation, Rochester, Minn (C.S.R., B.J.G.); Department of Clinical Cardiology, Massachusetts General Hospital, Boston (K.A.E.); Department of Biostatistics, University of Washington, Seattle (M.C.M.); Albany (NY) Medical College (E.D.F.); and Cardiac Diseases Branch, National Heart, Lung, and Blood Institute, Bethesda, Md (G.S.).


*    Abstract
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 *Abstract
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Background Among patients with combined coronary artery and peripheral vascular disease, long-term benefits of surgical therapy compared with medical therapy for coronary artery disease are unknown.

Methods and Results Using prospectively collected data from the Coronary Artery Surgery Study registry, we performed a retrospective cohort analysis of 1834 patients (mean age, 56 years; 20% women) with both coronary artery and peripheral vascular disease and evaluated their long-term outcomes. Of these patients, 986 received (nonrandomly) coronary artery bypass graft surgery, and 848 were treated medically. Perioperative mortality was 4.2% (2.9% in the absence of peripheral vascular disease; P=.02). In a mean follow-up period of 10.4 years, 1100 deaths occurred (80% due to cardiovascular causes). For the surgical group, 4-, 8-, 12-, and 16-year estimated probabilities of survival were 88%, 72%, 55%, and 41%, respectively, and 73%, 57%, 44%, and 34%, respectively, for the medical group (P<.0001). Multivariate analysis demonstrated that type of therapy was independently associated with survival (P=.0001; {chi}2=15.34). Subgroup analysis suggested that benefits of surgical treatment on survival were limited to patients with three-vessel coronary artery disease and were inversely related to ejection fraction. Survival free of death or myocardial infarction was also significantly better among the surgical group. Type of therapy was significantly associated with occurrence of late events (P=.01; {chi}2=6.55). Subgroup analysis again demonstrated that beneficial effects of surgery were limited to patients with three-vessel coronary artery disease and were inversely related to ejection fraction.

Conclusions Surgical treatment provides long-term benefit for certain subgroups of patients with combined coronary artery and peripheral arterial vascular disease.


Key Words: bypass • peripheral vascular disease • coronary disease • surgery


*    Introduction
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 up arrowAbstract
 *Introduction
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down arrowDiscussion
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Among patients with peripheral atherosclerosis, complications of coronary artery disease are the leading causes of morbidity and mortality.1 2 Coronary angiography performed before peripheral revascularization operation has demonstrated an exceedingly high prevalence of severe, often inoperable coronary artery disease.3 4 5 Coronary artery disease accounts for most serious perioperative complications after noncardiac operations3 6 7 8 9 10 11 and for late morbidity and mortality.11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 If coronary artery bypass graft surgery (CABG) is performed before a peripheral vascular operation, the operative risk for noncardiac operations may be lower.3 30 31 32 33 34 35 36 Although it has been suggested that CABG may have important long-term survival benefits for patients with peripheral atherosclerosis,3 10 16 33 37 38 the decision to recommend CABG for patients with peripheral vascular disease remains complex and difficult. Because patients with peripheral atherosclerosis tend to be older and to have widespread vascular disease, end-organ damage, and poor risk factor profiles, their risk for complications after CABG is high.4 39 40 In particular, atheroembolism and neuropsychological morbidity may be substantial.41 42 43 44 Moreover, the presence of peripheral vascular disease is a marker of adverse long-term outcome after the operation.45 46 A randomized prospective trial of medical therapy compared with surgical therapy for coronary artery disease among patients with peripheral vascular disease has not been performed. Thus, the net long-term benefits or risks of coronary artery surgery among these high-risk patients are unknown.

The purposes of this study were (1) to evaluate the long-term outcome of patients with combined coronary artery and peripheral vascular disease who received CABG in comparison with medical management and (2) to provide specific recommendations about the performance of this procedure with regard to this group of patients. The prospectively collected clinical and laboratory data of the Coronary Artery Surgery Study (CASS) registry provided a valuable opportunity to study issues related to coronary artery revascularization on a large scale. We used the CASS registry to assess the comparative benefits of surgical and medical therapies for coronary artery disease among patients with peripheral atherosclerosis.


*    Methods
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 *Methods
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Experimental Design and Subjects
The CASS study design has been described previously.47 Patient enrollment was initiated in 1974 and terminated in 1979. At the 15 study sites, all patients whose primary indication for coronary angiography was coronary artery disease, either suspected or proved, were eligible for enrollment. Patients with significant valvular or cardiomyopathic diseases were ineligible.47 The CASS registry consists of patients whose initial therapy for coronary artery disease (either medical or surgical) was based on patient and physician preference.

The study reported here was a retrospective cohort analysis of the prospectively collected CASS data of all eligible patients in the CASS registry. For the purposes of this study, a patient was considered to have peripheral vascular disease if there was a clinical diagnosis of peripheral vascular disease (intermittent claudication or absent peripheral pulses) or cerebrovascular disease (transient ischemic attacks or stroke). Patients who did not have at least one operable coronary blood vessel and those with previous cardiac surgical treatment were excluded from our analysis.

Clinical Data
Technical details of diagnostic procedures and data recording have been described previously.47 Left ventricular volumes and ejection fractions were calculated with a single-plane adaptation of the area-length method of Dodge et al.48 Ongoing quality control checks were performed.47 An epicardial coronary artery was considered significantly diseased if it or a major branch of it had at least one 70% stenosis of its diameter. A 50% stenosis of the left main coronary artery was considered significant (counting as two vessels in the case of right dominant circulation or as three vessels in the case of left dominance). To be considered operable, a coronary artery had to have a >=50% stenosis with a normal-size distal vessel (at least one >=70% stenosis in the coronary arterial tree was required for consideration of surgery). A cardiac event was defined as either death (because 80% were due to cardiac causes) or myocardial infarction.

Data Analysis
Patients were divided into two groups based on initial treatment with CABG (surgical group) or medical therapy (medical group). For each CASS site, the number of days after angiography within which 95% of patients received surgical treatment (if within 1 year) was calculated. The surgical group consisted of patients who underwent operation within the specified number of days. The date of operation was the starting point for calculation of survival time. The medical group consisted of patients who did not have surgical treatment within the site-specific time period. For these patients, survival time was calculated from the date of enrollment plus the average number of days to operation at that CASS site. This adjusts for the discrepancy in exposure time between the medical and surgical groups.49 To eliminate the bias introduced by including all early deaths in the medical group, patients who died before reaching the time by which 95% of patients had undergone surgical treatment at that site were excluded from the analysis.49 Medical patients who subsequently underwent CABG remained in the medical group. Surgical 30-day mortality and morbidity were compared with 6860 CASS registry patients without peripheral vascular disease.

CASS registry patients were followed up annually through November 1982, with data obtained for 99% of them. In addition, a follow-up questionnaire was administered by mail between 1988 and 1991, with vital status obtained for 94% of the patients. Vital status through 1992 was also determined by searches of the National Death Index. Patients who were not identified as dead through follow-up or the National Death Index are assumed to be alive as of December 31, 1992.

Baseline characteristics of the medical and surgical groups were compared by the two-sample t test, the test for linear trend, or the {chi}2 test. Continuous variables are reported as mean±SD; categorical variables are listed as percentages. All probability values are two-tailed, and statistical significance was inferred at P<=.05.

The Kaplan-Meier product limit method was used to estimate survival and event-free survival probabilities.50 Survival analyses were censored at 16 years, but analyses of time to death or myocardial infarction were censored at 8 years because myocardial infarction data were collected only through 1982. All curves were stratified by number of diseased coronary arteries.

Several methods were used to compare survival curves between groups. Crossing of the curves early in the follow-up period, especially in the event-free analyses, violates the proportional hazards assumption of log-rank tests and Cox models. Therefore, the mean and median survival times of the two groups were also compared. The mean survival is equal to the area under the survival curve truncated at a specific time point (16 or 8 years). Median survival is the time at which the Kaplan-Meier estimate reaches 50%. Tests of these statistics do not make any assumptions about the relative shapes of the curves for the two groups. However, multivariate analysis is limited to adjustment for covariates by stratification.

Results of analyses stratified by number of diseased vessels or number of diseased vessels plus one other covariate were remarkably similar when log-rank tests or t tests comparing mean or median survival were used. All methods identified the same groups of patients as benefiting from CABG, with nearly identical P values. Therefore, only log-rank tests are presented.

Cox models were constructed to analyze the independent effect of type of initial treatment on survival and event-free survival.51 First, significant predictors of survival were allowed to enter a stepwise Cox regression model. Interactions between significant variables were also considered. The treatment variable was then added to determine whether it provided additional predictive power. If the treatment variable was significant, then its interactions with the other covariates were allowed to enter the model. Only the interaction between treatment and ejection fraction was significant in any of the models. Because including this interaction also improved the proportionality of the adjusted hazards, it was retained even when it was only marginally significant. Adherence to the assumption of proportional hazards was verified for all models in which treatment was found to be an independent predictor of survival.


*    Results
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*Results
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Baseline Characteristics
Among the 24 958 patients enrolled in the CASS registry, 3271 were identified to have peripheral vascular (n=1409) or cerebrovascular disease (n=326) or both (n=99). Of these patients, 368 were excluded because of prior cardiovascular operation, 1074 were excluded for lack of operable coronary arteries (44 were excluded for both reasons), and 39 were excluded because of missing survival data. Thus, the study consisted of a cohort of 1834 patients (mean age, 56 years; 20% women). Of these, 986 underwent CABG (mean age, 56±8 years; 20% women) and 848 received medical treatment (mean age, 55±9 years; 20% women).

In general, the surgical patients had more severe symptoms (78% compared with 58% Canadian Cardiovascular Society class III or IV) and were more likely to have unstable angina (Table 1Down). Similarly, a greater proportion of the surgical group required concomitant medical treatment with ß-blockers and nitroglycerin.


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  		Table 1. Baseline Characteristics on Enrollment in the CASS Registry of 1834 Patients With Peripheral Vascular Disease

At cardiac catheterization, the surgical patients had evidence of more severe epicardial coronary artery disease (three-vessel disease in 56% compared with 34% in the medical group) and significantly lower mean left ventricular ejection fraction (surgical group, 0.54±0.18; medical group, 0.58±0.16) (P<.0001) (Table 2Down).


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  		Table 2. Cardiac Catheterization and Coronary Angiographic Findings Among 1834 Patients in the CASS Registry With Peripheral Vascular Disease

Perioperative Morbidity and Mortality
For the 986 patients with peripheral vascular disease undergoing CABG, 30-day mortality was 4.2%, significantly higher than the 2.9% mortality among patients without peripheral atherosclerosis (P=.02 compared with 6860 CASS registry patients without peripheral vascular disease) (Table 3Down). Eleven patients who survived from 32 to 197 days postoperatively but died without being dismissed from the hospital are also included in these calculations. A trend toward higher rates of perioperative myocardial infarction or stroke was observed (Table 3Down).


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  		Table 3. Perioperative1 Complications for 7846 CASS Registry Patients Undergoing Coronary Artery Bypass Graft Surgery

Late Survival
In late follow-up (mean, 10.4±5.4 years), 1100 of the 1834 patients (60%) died, with the cause of death determined for 1051. The majority of these deaths (80%) were due to cardiovascular causes (myocardial infarction, congestive heart failure, sudden death, and stroke), and a small proportion (20%) were due to noncardiovascular causes.

The Kaplan-Meier product limit estimated probabilities of survival are shown in Fig 1Down. The overall curve is adjusted for the number of diseased vessels. Surgical therapy for coronary artery disease was strongly and significantly associated with enhanced long-term survival among patients with peripheral atherosclerosis. For the surgical group, the 4-, 8-, 12-, and 16-year estimated probabilities of survival were 88%, 72%, 55%, and 41%, respectively, compared with 73%, 57%, 44%, and 34% for the medical group (P<.0001; log-rank statistic=32.6).



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  		Figure 1. Kaplan-Meier estimated probability of survival among 1834 patients with peripheral vascular disease enrolled in the Coronary Artery Surgery Study registry. Survival curve is adjusted for number of diseased coronary arteries.

To further define which subgroups of patients benefited from surgical therapy, patients were stratified according to number of diseased epicardial vessels. Among those with zero or one-vessel coronary artery disease (n=431; Fig 2Down), surgical treatment was not associated with improved long-term survival (16-year probability of survival, 58% compared with 59%; P=.76).



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  		Figure 2. Kaplan-Meier estimated probability of survival among 1834 patients with peripheral vascular disease enrolled in the Coronary Artery Surgery Study registry, stratified to zero or one-vessel (top), two-vessel (middle), and three-vessel (bottom) coronary artery disease.

Among patients with two-vessel coronary artery disease (n=555; Fig 2Up), a trend toward better late survival with surgical therapy was observed: the estimated probability of survival at 4, 8, 12, and 16 years for the surgical group was 87%, 72%, 55%, and 43%, respectively, compared with 84%, 66%, 50%, and 37%, respectively (P=.16), for the medical group. Among patients with two-vessel coronary artery disease with >=70% proximal left anterior descending stenosis, a trend favoring improved survival among surgically treated patients was observed but did not reach statistical significance (P=.08).

The benefits of surgical therapy were clearly evident among patients with three-vessel coronary artery disease in whom CABG was associated with markedly improved late survival (n=848; Fig 2Up). Among this group of patients, 4-, 8-, 12-, and 16-year estimated probability of survival was 85%, 64%, 47%, and 31%, respectively, compared with 57%, 40%, 27%, and 18%, respectively, for the medical group (P<.0001; log-rank statistic=45.1). Because the data were analyzed according to the intention-to-treat principle, this large difference occurred despite late CABG operations among the medical group.

Separate survival curves were constructed for men and women to evaluate any possible differences. As in the entire cohort and the male subgroup, the benefit of surgical treatment in women was limited to those with three-vessel disease. Separate survival curves stratified according to type of peripheral vascular disease were also constructed (1508 lower extremity; 425 cerebrovascular). In both groups, a long-term survival advantage favoring surgery was found among patients with three-vessel coronary artery disease but not among those with one- or two-vessel coronary artery disease. Only 6% of the surgical group did not have angina, precluding meaningful comparison to medically treated patients without angina.

Predictors of Survival
Multivariate Cox proportional hazards analyses were performed to select covariates independently associated with late survival. A large number of clinical and angiographic variables were considered ("Appendix"). Clinical variables such as age, congestive heart failure score, and diabetes mellitus were strongly associated with survival. The angiographic variables of left ventricular ejection fraction, number of diseased coronary vessels, and left ventricular score were also strongly associated with survival. When added to the model displayed in Table 4Down, type of therapy for coronary artery disease was very highly significantly correlated with survival (P=.001; {chi}2=15.34). A significant interaction between type of therapy and ejection fraction was also found (Table 4Down). Whereas it is not possible to give an overall instantaneous hazard ratio for the entire group, ratios can be calculated for specific ejection fraction strata. As shown in Table 4Down, instantaneous hazard ratios for medical therapy are inversely related to ejection fraction (1.82, 1.30, and 0.93 for ejection fractions of 25%, 50%, and 75%, respectively). Thus, patients with an ejection fraction of 25% had an 82% higher probability of death at any given point during the follow-up period if treated medically. The benefits of surgical treatment disappeared at ejection fractions >=75%. Importantly, this risk remains after the covariates listed in Table 4Down are controlled for.


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  		Table 4. Multivariate Cox Proportional Hazards Survival Analysis Among Patients in the CASS Registry With Peripheral Vascular Disease

Similar multivariate Cox models were constructed for subgroups stratified according to the number of diseased coronary vessels (full models not shown). Among patients with zero or one-vessel and those with two-vessel coronary artery disease, therapy was not associated with survival (P=.89, {chi}2=0.02 and P=.88, {chi}2=0.02, respectively). However, among patients with three-vessel coronary artery disease, choice of therapy was highly correlated with outcome (P<.0001; {chi}2=16.55). As with the main group, a significant interaction between type of therapy and ejection fraction was found (instantaneous hazard ratios for medical therapy of 2.33, 1.53, and 1.00 for ejection fractions of 25%, 50%, and 75%, respectively).

Cardiac Event–Free Survival
To compare the effects of medical and surgical therapies in reducing nonfatal and fatal cardiac end points, a cardiac event was defined as death (80% due to heart disease) or myocardial infarction. Although large numbers of cardiac events occurred in both patient groups (Fig 3Down), patients in the surgical group had significantly fewer cardiac events, with 4- and 8-year estimated probabilities of freedom from events of 79% and 63%, respectively, compared with 66% and 51%, respectively, for those in the medical group (P<.0001; log-rank statistic=32.3).



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  		Figure 3. Kaplan-Meier estimated probability of freedom from cardiac event among 1821 patients with peripheral vascular disease enrolled in the Coronary Artery Surgery Study registry. Cardiac event defined as death or myocardial infarction.

As was the case for overall survival, the benefits of surgical therapy over medical therapy depended on the extent of coronary artery disease (Fig 4Down). Among patients with zero or one-vessel disease, no difference between groups was observed. Among patients with two-vessel coronary artery disease, a trend toward fewer cardiac events in the surgical group was noted (P=.0516). Differences in cardiac events attained significance only among patients with three-vessel coronary artery disease (Fig 4Down). Among these patients, the 4- and 8-year estimated probabilities of freedom from cardiac events were 77% and 57%, respectively, significantly better than the 52% and 37%, respectively, for the medical group (P<.0001; log-rank statistic=39.7). No sex differences in surgical benefit were found.



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  		Figure 4. Kaplan-Meier estimated probability of freedom from cardiac event among 1821 patients with peripheral vascular disease enrolled in the Coronary Artery Surgery Study registry, stratified to zero or one-vessel (top), two-vessel (middle), and three-vessel (bottom) coronary artery disease. Cardiac event was defined as death or myocardial infarction.

Predictors of Cardiac Event–Free Survival
Multivariate Cox proportional hazards cardiac event–free survival analysis was also performed (Table 5Down). Variables of left ventricular function and severity of coronary artery disease were strongly associated with cardiac event–free survival. When added to the model depicted in Table 5Down, the choice of therapy for coronary artery disease was also significant (P=.01; {chi}2=6.55). Although the interaction between type of therapy and ejection fraction was of borderline statistical significance, it was retained because it improved the proportionality of the adjusted hazard ratios (instantaneous hazard ratios for medical therapy of 1.57, 1.26, and 1.00 for ejection fractions of 25%, 50%, and 75%, respectively).


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  		Table 5. Multivariate Cox Proportional Hazards Cardiac Event–Free1 Survival Analysis Among Patients in the CASS Registry With Peripheral Vascular Disease

Among patients with zero or one-vessel disease, the type of therapy was not significant (P=.72; {chi}2=0.13), nor was it significant for patients with two-vessel disease (P=.10; {chi}2=2.69). The influence of therapy on cardiac event–free survival was significant only for the subgroup with three-vessel coronary artery disease (P=.003; {chi}2=8.86). A consistent interaction between ejection fraction and type of therapy was again observed (instantaneous hazard ratios for medical therapy of 2.00, 1.42, and 1.00 for ejection fractions of 25%, 50%, and 75%, respectively).


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
*Discussion
down arrowReferences
 
Among patients with coronary artery disease and peripheral vascular disease, the comparative long-term benefits of surgical therapy compared with medical therapy for coronary artery disease are unknown. During longitudinal observation of patients with peripheral vascular disease, cardiac events are the leading causes of morbidity and mortality in both the short term and long term.13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Whereas previous CABG is associated with a better prognosis,3 10 16 30 31 32 33 34 35 36 37 the risks of performing the procedure are formidable.4 39 40 Thus, objective assessment of long-term risks and benefits is crucial to clinical decision making. To address these issues, we undertook the study reported here.

Present Study
Our major finding, and most important conclusion, is that CABG clearly had beneficial effects on long-term outcome, despite the higher initial risk of perioperative morbidity and mortality. The beneficial effects of bypass operations were concentrated in the higher-risk anatomic subgroups: overall survival and cardiac event rates were improved only among the patients with three-vessel coronary artery disease. Among patients with zero or one-vessel disease, no objective benefit to surgical therapy was demonstrated (surgical therapy may have improved anginal symptoms; however, this was not assessed). Trends favoring surgical therapy were noted for patients with two-vessel disease.

Clinical Implications
Patients with the combination of coronary artery disease and peripheral vascular disease are frequently encountered in clinical practice. They tend to have multiple coronary risk factors and manifest widespread vascular disease with significant end-organ damage.3 Clinical decision making is complex and challenging.1 Our data shed light on the role of CABG in this group of patients. CABG in appropriately selected patients has important short- and long-term benefits, albeit at a higher immediate risk.

Our study was not designed to address whether CABG or percutaneous transluminal coronary angioplasty should be performed specifically to decrease the risk of a subsequent noncardiac operation. Such a question can be answered only with a prospective randomized clinical trial. Our data help define subgroups with long-term benefits from coronary revascularization. Also, these data complement our previous contention that if independent indications for coronary revascularization exist, the clinical need for major noncardiac (especially major vascular) operations may influence the timing of the coronary revascularization procedure.1

Strengths and Limitations
Our study has several important strengths and limitations. Clinical studies comparing two or more treatment strategies derive their greatest value from a prospective randomized design. Although the present study was nonrandomized, data collection and follow-up were performed in a systematic and standardized fashion with strict quality control checks. All studies of this type are unavoidably affected by selection bias, as evidenced by the clinical and angiographic differences between the surgical and medical groups. Although on the basis of standard clinical and angiographic criteria, the surgical group appeared to be "sicker" at enrollment, the possibility of confounding by unrecognized factors cannot be ruled out. A prospective randomized design would minimize any such differences. Use of multivariate Cox models allowed us to control simultaneously for several clinical and angiographic variables while assessing the relation of therapy to outcome. The high hazard ratios associated with medical therapy support our primary conclusion.

Another important limitation of our study bears emphasizing: significant advances have occurred in both medical and surgical therapies for coronary artery disease since the years of enrollment in CASS. It can be argued that the generalizability of these data to patients receiving cardiac care in the 1990s is limited. However, with this database, we have been able to obtain 16-year follow-up after initial enrollment and therapy, a perspective that enhances the value of the data.

Conclusions
On the basis of our data, we conclude that long-term outcomes among patients with combined coronary artery disease and peripheral vascular disease were improved by coronary revascularization. Of importance, the salutary late effects of myocardial revascularization were not offset by the higher initial risk for patients undergoing surgical treatment. The demonstrable benefits of CABG were concentrated among the higher-risk anatomic subset of patients with three-vessel coronary artery disease and were inversely proportional to ejection fraction. We believe these data will be useful in guiding therapy among this high-risk subset of patients with both coronary artery and peripheral vascular disease.


*    Footnotes
 
Reprint requests to C.S. Rihal, MD, HGH-McMaster Clinic, Hamilton General Hospital, 237 Barton St E, Hamilton, Ontario, Canada L8L 2X2.

Variables considered in the Cox stepwise proportional hazards analyses were age, sex, current smoking at enrollment, any history of smoking, hypertension, diabetes mellitus, chronic pulmonary disease, renal disease, history of neoplasia, number of associated medical conditions, previous myocardial infarction, ß-blocker use, history of congestive heart failure, congestive heart failure score (an index of congestive heart failure [range, 0 to 4] based on four items: history of congestive heart failure, use of digitalis, use of diuretics, and presence of rales on physical examination), Canadian Cardiovascular Society class, probable or definite angina, unstable angina, functional impairment, number of diseased vessels, number of operable vessels, CAGE 50 (number of coronary artery segments with >=50% stenosis), left main coronary artery disease, left ventricular ejection fraction, and left ventricular score (left ventricular wall motion score, a measure of left ventricular function, the sum of wall motion scores for each of the standard five segments of a right anterior oblique left ventriculogram [normal, 1; moderate hypokinesis, 2; severe hypokinesis, 3; akinesia, 4; dyskinesia, 5; aneurysm, 6]).

Received October 19, 1993; accepted August 2, 1994.


*    References
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up arrowResults
up arrowDiscussion
*References
 

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