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Vorarlberg Institute for Vascular Investigation and Treatment (C.H.S., S.A., T.M., P.L., H.D.), Feldkirch 6800, Austria; and Department of Medicine, Academic Teaching Hospital Feldkirch (C.H.S., S.A., T.M., G.H., H.D.), Feldkirch 6800, Austria
Address all correspondence and requests for reprints to: Dr. Heinz Drexel, Carinagasse 47, A-6800 Feldkirch, Austria. E-mail: vivit{at}lkhf.at.
| Abstract |
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Objective: The objective of this study was to investigate the impact of the MetS (Adult Treatment Panel III criteria) and insulin resistance (as estimated by the homeostasis model assessment index) on the incidence of vascular events.
Design and Patients: This was a prospective cohort study enrolling 750 consecutive patients undergoing coronary angiography for the evaluation of coronary artery disease.
Setting: The study was performed at a tertiary care clinical research center.
Main Outcome Measure: The main outcome measure was the incidence of vascular events over 2.3 yr.
Results: Both the MetS and insulin resistance predicted vascular events after controlling for non-MetS risk factors [hazard ratio (HR), 2.74 (95% confidence interval, 1.714.39; P < 0.001) and 1.51 (1.241.84; P < 0.001), respectively]. After additional adjustment for insulin resistance, the MetS remained significantly predictive of vascular events [HR, 2.69 (1.574.64); P < 0.001], and conversely, insulin resistance remained significantly predictive of vascular events despite adjustment for the MetS [standardized HR, 1.41 (1.141.75); P = 0.002]. Additional adjustment for the presence of type 2 diabetes revealed that both the MetS [adjusted HR, 2.57 (1.474.51); P = 0.001] and homeostasis model assessment of insulin resistance [standardized adjusted HR, 1.37 (1.091.73); P = 0.007] significantly predicted vascular events independent from diabetes status.
Conclusions: Both the MetS and insulin resistance are strong and mutually independent predictors of vascular risk among angiographed coronary patients.
| Introduction |
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Insulin resistance is a key player in the pathophysiology of the MetS and has even been postulated as its underlying cause (12). In general, the stigmata of the MetS are significantly associated with insulin resistance (13, 14). However, because the ATP III definition of the MetS does not include a measurement of insulin resistance, patients with the MetS are not necessarily insulin resistant (13, 15). Cardiovascular risk increases in parallel to insulin resistance both among patients with diabetes (16) and among nondiabetic patients (17, 18). Despite the established role of insulin resistance in the pathophysiology of the MetS and as a predictor of cardiovascular disease, it is still uncertain to what extent insulin resistance per se accounts for the increased vascular risk conferred by the MetS as defined by ATP III criteria.
Data on the predictive power of the MetS in patients already affected by cardiovascular disease are scarce (19, 20). It has been demonstrated (20) that the MetS increases cardiovascular risk in women with coronary artery disease (CAD). However, the impact of the MetS on cardiovascular risk in the much larger population of men with established CAD remains unknown. Furthermore, there is a paucity of data about the prospective impact of the MetS in patients with diabetes. Therefore, we prospectively evaluated the impact of the MetS and insulin resistance on future vascular events among angiographed coronary patients enrolling in a large cohort that included men as well as women and both patients with and without type 2 diabetes mellitus.
| Subjects and Methods |
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At baseline, coronary angiography was performed with the Judkins technique. Stenoses of 50% or more were considered significant, and coronary arteries were defined as angiographically normal in the absence of any visible lumen narrowing at angiography, as described previously (22, 23).
From the 750 patients included in the analyses, 164 had diabetes mellitus according to World Health Organization (WHO) criteria (24), and 586 did not have diabetes. Among patients with diabetes, 42.7% were not receiving any antidiabetic medication, and 34.8%, 32.3%, 24.4%, and 1.2% were receiving, alone or in combination, sulfonylurea, biguanides, insulin, and
-glucosidase inhibitors, respectively. Overall, 64.7% of our patients were taking aspirin, 31.3% statins, 2.9% fibrates, 12.1% calcium antagonists, 47.1% ß-blocking agents, 36.0% angiotensin-converting enzyme inhibitors, and 3.9% angiotensin II-blocking agents. No patient was receiving thiazolidinediones.
According to ATP III criteria (11), the MetS was diagnosed in the presence of any three of the following: waist circumference greater than 102 cm in men and greater than 88 cm in women, triglycerides of 150 mg/dl (1.7 mmol/liter) or more, HDL cholesterol less than 40 mg/dl (1.0 mmol/liter) in men and less than 50 mg/dl (1.3 mmol/liter) in women, blood pressure of 130/85 mm Hg or higher, or fasting glucose of 110 mg/dl (6.1 mmol/liter) or higher. We defined a MetS score as the number of these traits in a given patient.
To measure insulin resistance, we used the homeostasis model assessment (HOMA) (25), which has been shown to be a reliable estimate of insulin resistance both among nondiabetic patients and patients with type 2 diabetes (26). Patients with diabetes who were receiving insulin treatment (n = 40) were excluded from the calculation of insulin resistance.
Prospective study
During a follow-up period of 2.3 ± 0.4 yr we recorded fatal and nonfatal cardiovascular end points, including coronary death (fatal myocardial infarction, sudden cardiac death, and mortality from congestive heart failure due to CAD), fatal ischemic stroke, nonfatal myocardial infarction, nonfatal ischemic stroke, and need for coronary artery bypass grafting, percutaneous coronary intervention, or noncoronary revascularization.
Statistical analysis
Differences in baseline characteristics were tested for statistical significance with the Mantel-Haenszel
2 test for trend and the ordered Jonckheere-Terpstra test for categorical and continuous variables, respectively. The Wilcoxon-Gehan statistic was used to compare differences in the cumulative incidence rates of vascular events. Adjusted hazard ratios (HR) for the incidence of vascular events were derived from Cox proportional hazards models. For these calculations, continuous variables were z-transformed, and the trend over the categories of the MetS score was analyzed by introducing the MetS score as an ordinal variable in the Cox proportional hazards models. Results are given as the mean ± SD if not denoted otherwise. All statistical analyses were performed with the software package SPSS 12.0 for Windows (SPSS, Inc., Chicago, IL).
| Results |
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The baseline demographic data of our 750 patients were characteristic for a cohort undergoing coronary angiography for the evaluation of CAD, with a mean age of 62.6 ± 10.4 yr, a preponderance of male gender (n = 509; 67.9%), and a high prevalence of type 2 diabetes mellitus (n = 164; 21.9%). Baseline angiography revealed CAD in 615 patients (82.0%), and 456 patients (60.8%) had significant coronary stenoses of 50% or more.
Quantitative aspects of MetS and insulin resistance
From the total study cohort, 280 patients (37.3%) had three or more MetS traits and therefore fulfilled the criteria for the MetS. Table 1
summarizes the baseline characteristics of our patients with zero through five markers of the MetS. The prevalence of type 2 diabetes mellitus increased with an increasing MetS score from 0% in patients with no MetS risk factor through 57.1% in patients with five MetS risk factors (Ptrend < 0.001). BMI, the prevalence of angiographic CAD, and, concordant with ATP III criteria for the diagnosis of the MetS, waist circumference, triglycerides, and systolic as well as diastolic blood pressures significantly increased, and HDL cholesterol significantly decreased through the categories of the MetS score (Ptrend < 0.001 for all). Low-density lipoprotein (LDL) cholesterol and the LDL peak particle diameter decreased with an increasing MetS score. HOMA insulin resistance increased significantly (Ptrend < 0.001) with an increasing number of MetS risk factors (Fig. 1
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Incidence of vascular events
During a mean ± SD follow-up time of 2.3 ± 0.4 yr, we recorded 95 vascular end points (encompassing 26 coronary deaths, three fatal ischemic strokes, 10 nonfatal myocardial infarctions, 10 nonfatal ischemic strokes, 15 coronary artery bypass graftings, 14 percutaneous coronary interventions, and 17 revascularizations at the carotid or peripheral arteries). The incidence of vascular events was higher in men (n = 509) than in women (n = 241; 16.9% vs. 8.9%; P = 0.006), it was higher in patients with type 2 diabetes (n = 164) than in nondiabetic patients (n = 586; 21.9% vs. 12.2%; P = 0.003), and it was higher among patients with significant coronary artery stenoses of 50% or more at baseline (n = 456) than in patients without such lesions (n = 294; 20.4% vs. 4.7%; P < 0.001).
Event-free survival was significantly lower (P < 0.001) in patients with the MetS than in patients without the MetS (Fig. 2
). In Cox regression analysis, adjusting for age, gender, smoking, BMI, and LDL cholesterol, the MetS proved independently predictive for the incidence of vascular events [adjusted HR, 2.74 (95% confidence interval, 1.714.39); P < 0.001].
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Figure 3A
depicts the results of important subgroup analyses. The MetS proved significantly predictive for vascular events among both men and women [adjusted HR, 2.45 (1.474.11; P = 0.001) and 5.03 (1.4617.03; P = 0.010), respectively]. Furthermore, the MetS was significantly predictive for vascular events among patients with type 2 diabetes as well as among nondiabetic patients [adjusted HR, 4.51 (1.2915.75; P = 0.018) and 2.37 (1.324.25; P = 0.004, respectively]. Among patients with significant coronary stenoses of 50% or more, the MetS was a significant predictor of vascular events [adjusted HR, 2.42 (1.483.97); P < 0.001]. The adjusted HR for patients without significant CAD [2.28 (0.5010.34); P = 0.285] was not significantly different from the HR for those with significant CAD (for interaction of MetS x CAD, P = 0.982). However, because of the low absolute number of end points in patients without significant CAD, the confidence interval was wide. In subgroup analyses with respect to both gender and the presence of significant coronary stenoses of 50% or more at baseline, the MetS proved significantly predictive for vascular events among men with significant stenoses [n = 350; adjusted HR, 2.18 (1.283.72); P = 0.004]; among women with significant stenoses (n = 106), the adjusted HR was 3.95 (0.9615.87; P = 0.053).
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The incidence of vascular events increased with an increasing MetS score. For patients with zero through five MetS risk factors, the respective incidence rates were 8.8, 12.0, 10.7, 14.1, 25.0, and 32.3% (Ptrend < 0.001). Compared with patients without any MetS risk factor, the HRs adjusted for non-MetS risk factors were 2.03 (0.686.06), 2.10 (0.696.38), 3.04 (0.999.33), 7.34 (2.4721.85), and 14.37 (4.2149.12)] for patients with one through five MetS risk factors (Fig. 3B
; through the categories of the MetS score Ptrend < 0.001).
Independent effects of MetS and insulin resistance on the incidence of events
Like the MetS, HOMA insulin resistance proved significantly predictive of vascular events in our cohort of coronary patients [standardized HR controlled for age, gender, smoking, BMI, and LDL cholesterol, 1.51 (1.241.84); P < 0.001]. Interestingly, however, after additional adjustment for HOMA insulin resistance, the MetS remained significantly predictive for the incidence of vascular events, with an adjusted HR of 2.69 (1.574.64; P < 0.001). Conversely, insulin resistance added incremental prognostic value to the clinical diagnosis of the metabolic syndrome; in the regression model described above, it proved significantly (P = 0.002) predictive for the incidence of vascular events even after additional adjustment for the presence of the metabolic syndrome, with a standardized adjusted HR of 1.41 (1.141.75).
Additional adjustment for the presence of type 2 diabetes revealed that both the MetS [adjusted HR, 2.57 (1.474.51); P = 0.001] and HOMA insulin resistance [standardized adjusted HR, 1.37 (1.091.73); P = 0.007] significantly predicted vascular events independent from diabetes status. Consistently, the trend toward an increased incidence of vascular events with an increasing number of MetS traits remained significant after additional adjustment for HOMA insulin resistance and diabetes status (Ptrend < 0.001), with adjusted HRs of 1.53 (0.504.67), 1.02 (0.313.33), 2.03 (0.636.55), 4.45 (1.3814.32), and 7.49 (1.9728.49) for patients with one through five MetS traits.
In subgroup analyses with respect to diabetes status, HOMA insulin resistance proved significantly predictive of vascular events among patients with type 2 diabetes [standardized adjusted HR, 1.56 (1.182.06); P = 0.002]; among nondiabetic patients, the increase in vascular risk associated with HOMA insulin resistance was not significant [standardized adjusted HR, 1.30 (0.861.98); P = 0.210]. However, the impact of HOMA insulin resistance on the incidence of vascular events was not significantly different between patients with type 2 diabetes and nondiabetic patients (for interaction of HOMA insulin resistance x diabetes status, P = 0.383). When fasting insulin was used as a measure of insulin resistance among nondiabetic patients, the results were similar to those obtained with HOMA insulin resistance; the association between fasting insulin and the incidence of vascular events in nondiabetic patients did not reach statistical significance [standardized adjusted HR, 1.39 (0.931.93); P = 0.116], whereas the MetS remained significantly predictive of vascular events in nondiabetic patients after adjustment for fasting insulin [HR, 2.22 (1.124.41); P = 0.023].
Consistent with the results from the total study cohort, the MetS remained significantly predictive of vascular events after adjustment for HOMA insulin resistance in both the subgroup of patients with type 2 diabetes [adjusted HR, 4.05 (1.1414.42); P = 0.031] and the subgroup of nondiabetic patients [adjusted HR, 2.31 (1.154.65); P = 0.019]. As for the total study cohort, HOMA insulin resistance conversely added incremental prognostic value to the clinical diagnosis of the MetS among patients with type 2 diabetes, with a standardized adjusted HR of 1.55 (1.162.08; P = 0.003). Among nondiabetic patients, HOMA insulin resistance was not significantly predictive of the incidence of vascular events after adjustment for the MetS [HR, 1.21 (0.751.94); P = 0.442]. Again, the impact of HOMA insulin resistance on the incidence of vascular events was not significantly different between patients with type 2 diabetes and nondiabetic patients (for interaction of HOMA insulin resistance x diabetes status, P = 0.299).
| Discussion |
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The MetS, as defined by ATP III criteria, significantly increased cardiovascular risk among our patients with type 2 diabetes. Because diabetes is associated with a 2- to 3-fold increase in cardiovascular risk (27), cardiovascular risk factors in patients with diabetes deserve particular attention. However, there is a paucity of prospective data on the impact of the MetS on the incidence of vascular events in patients with diabetes. Recently, Bonora et al. (28) reported an increased risk of cardiovascular events in diabetic patients with the MetS defined according to WHO criteria, which include insulin resistance per se, whereas Bruno et al. (29) found similar HRs for cardiovascular mortality in diabetic patients with and without the WHO-defined MetS. However, on the basis of ATP III criteria, before our investigation no data had been published for diabetic patients on the prospective impact of the MetS on the incidence of vascular events. Also, we provide the first report that the MetS is significantly predictive for future vascular events among men with established CAD, thereby extending a previous observation on the impact of the MetS on women with CAD (20) to the larger population of male coronary patients.
Insulin resistance is regarded as a key player in the pathophysiology of the MetS (12, 30). In our investigation, insulin resistance increased gradually from patients with none through those with five markers of the MetS. This increasing severity of insulin resistance with an increasing number of markers of the MetS reflects and confirms the pathophysiological rationale behind the ATP III criteria for the diagnosis of the MetS.
In Cox regression models controlling for non-MetS risk factors, both the MetS and insulin resistance were significant predictors of vascular events in our cohort of coronary patients. Importantly, the MetS remained significantly predictive for the incidence of vascular events even after adjustment for HOMA insulin resistance. This result suggests that insulin resistance alone does not explain the full amount of future vascular risk that is conferred by the MetS stigmata; the MetS, as defined by ATP III criteria, conferred vascular risk beyond insulin resistance. Conversely, insulin resistance was significantly predictive for vascular events even after adjustment for the MetS. This suggests that insulin resistance exerts additional vascular risk beyond current ATP III MetS criteria. These prospective results from our investigation are well in line with a recent cross-sectional study reporting independent associations with coronary artery calcification for both the MetS and insulin resistance after mutual adjustment (31).
HOMA insulin resistance was significantly higher in the subgroup of patients with diabetes than in the subgroup of nondiabetic individuals. In subgroup analyses, the MetS strongly affected cardiovascular prognosis in both patients with type 2 diabetes and nondiabetic individuals; the increased risk associated with insulin resistance, in contrast, was carried primarily by the patients with diabetes. However, there was no statistically significant difference in the impact of insulin resistance on the incidence of vascular events between patients with diabetes and nondiabetic subjects, and from the results of our subgroup analyses, we cannot exclude that a significant association between insulin resistance and vascular events could be demonstrable in nondiabetic coronary patients with the greater statistical power of a larger study or a longer period of follow-up. Recently, Stern et al. (32) reported data from a population with a high proportion of Mexican-Americans, in whom, among subjects with a history of cardiovascular disease, the ability of the MetS to predict cardiovascular mortality was entirely due to the inclusion of diabetic patients among those with the MetS. In that population, no additional effect of the MetS on cardiovascular mortality was found after adjustment for diabetes mellitus. However, in our investigation, adjustment for diabetes status in regression analyses confirmed that both the MetS and insulin resistance were strong and significant predictors of vascular events in the total study population, independent of diabetes status.
Our study shows that the MetS identifies coronary patients at a high cardiovascular risk. However, our data also indicate that cardiovascular risk increases gradually with the number of MetS stigmata. Patients with four and, even more so, those with five of these markers are at a particularly high cardiovascular risk. Therefore, our results suggest that considering the number of risk factors that cluster in an individual enhances the prognostic strength of MetS, at least in an unselected angiography population.
Our investigation is characterized by the typical limitations and strengths of studies enrolling patients who undergo coronary angiography for the evaluation of CAD. This is a selected group of patients; our results therefore are not necessarily applicable to the general population. However, these patients undergoing coronary angiography are at a high absolute risk for future cardiovascular events, and prospective data on cardiovascular risk factors in this patient cohort are of major clinical importance.
We conclude that both the clinical diagnosis of the MetS and biomarkers of insulin resistance should be considered in cardiovascular risk estimation. Whether pharmacological improvement of the MetS and insulin sensitivity, as with thiazolidinedione treatment, translates into improved cardiovascular outcome is a question of major clinical importance. Indeed, this hypothesis has been investigated in a large-scale clinical trial with pioglitazone (33), the results of which are awaited soon.
| Acknowledgments |
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| Footnotes |
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First Published Online August 9, 2005
Abbreviations: CAD, Coronary artery disease; HDL, high-density lipoprotein; HOMA, homeostasis model assessment; HR, hazard ratio; LDL, low-density lipoprotein; MetS, metabolic syndrome.
Received April 12, 2005.
Accepted July 29, 2005.
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