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J Am Coll Cardiol, 2006; 47:1029-1036, doi:10.1016/j.jacc.2005.10.048
(Published online 8 February 2006). © 2006 by the American College of Cardiology Foundation |
Mayo Clinic, Rochester, Minnesota
Manuscript received October 14, 2004; revised manuscript received October 12, 2005, accepted October 18, 2005.
* Reprint requests and correspondence: Dr. Patricia A. Pellikka, Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905 (Email: pellikka.patricia{at}mayo.edu).
| Abstract |
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BACKGROUND: Neither risk scores nor long-term prognostic value of DSE has been described in a large diabetic population.
METHODS: We studied 2,349 patients with diabetes mellitus (1,338 men, 67 ± 11 years of age) during a follow-up of 5.4 ± 2.2 years.
RESULTS: Mortality and morbidity (myocardial infarction and late coronary revascularization) occurred in 1,044 (44%) and 309 (13%) patients, respectively. Addition of stress echocardiographic variables to the clinical and rest echocardiographic model provided incremental prognostic information for predicting mortality (chi-square = 243 to 270, p < 0.0001) and morbidity (chi-square = 38 to 78, p < 0.0001). For each end point, a simple risk score was derived according to the estimated values of beta coefficients of multivariate predictors (insulin therapy, smoking, failure to achieve target heart rate, percentage of ischemic segments, and impaired left ventricular systolic function) and resulted in an assessment of risk among all age groups. The C-statistic values were 0.60 to 0.64, indicating modest discrimination. The estimated five-year event-free survivals of patients in three risk categories were 94%, 86%, and 80% for morbidity (p < 0.00001) and 69%, 60%, and 47% for mortality (p < 0.0001).
CONCLUSIONS: In patients with diabetes mellitus, a simple and practical risk score using clinical variables and results of DSE stratified patients into three risk groups for mortality and cardiovascular morbidity.
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Clinical assessment of cardiac risk may be insufficient in asymptomatic patients with diabetes. Furthermore, inability to exercise and failure to achieve an adequate workload may limit the applicability of exercise stress testing in patients with diabetes mellitus (9). Dobutamine stress echocardiography (DSE) is an accurate and reliable noninvasive technique used for the diagnostic and prognostic assessment of CAD (1014). However, fewer data exist regarding the long-term prognostic role of DSE in a large cohort with diabetes mellitus. The purposes of this study were to determine the prognostic value of DSE for predicting the long-term outcome in a large cohort with diabetes mellitus, regardless of the presence of known or suspected CAD, and to develop a simple model for risk stratification using clinical and stress echocardiographic data.
| Methods |
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140 mg/dl on at least two occasions and/or requirement for insulin or oral hypoglycemic agents (15). Patients who underwent DSE from 1998 to 2000 were diagnosed as having diabetes mellitus according to the criteria by the American Diabetes Association (16), including fasting plasma glucose level
126 mg/dl. Two hundred sixty-nine patients were excluded: 137 had inadequate echocardiographic images, 128 international patients and prisoners were lost to follow-up, and 4 patients refused research authorization. The remaining 2,349 (90%) patients constituted the study population.
Clinical characteristics and results of stress echocardiography were recorded at the time of DSE. Hypertension was defined as systolic blood pressure
140 mm Hg, diastolic blood pressure
90 mm Hg, or the use of antihypertensive medication. Patients were considered to have hyperlipidemia if their total cholesterol was
200 mg/dl or if they were receiving lipid-lowering medication. Age as a cardiovascular risk factor was defined as
45 years for men and
55 years for women.
Dobutamine stress echocardiography.
Dobutamine stress echocardiography was performed according to a previously described protocol using 3-min stages, a peak dose of 40 µg/kg/min and atropine, to a total dose of 2 mg, as needed to augment the heart rate (12). Contrast was used for endocardial border detection when two or more segments could not be adequately visualized. Ejection fraction was evaluated as previously described (17) or by visual estimation. Impaired left ventricular systolic function was defined as ejection fraction <50%. Wall motion was assessed and scored 1 through 5 in each of 16 segments, and left ventricular wall motion score index was calculated at rest and peak stress (12,18). The development of new or worsening wall motion abnormality (WMA), including a deterioration of wall motion after an improvement at low-dose dobutamine, was considered inducible ischemia. A resting WMA unchanged with dobutamine infusion or an akinetic segment that became dyskinetic was defined as fixed (19,20). The percentage of abnormal segments (rest or stress-induced abnormalities) was calculated at rest and stress as the number of abnormal segments divided by the number of visualized segments, multiplied by 100. The percentage of ischemic segments was similarly derived. Normal DSE was defined if there was no WMA at rest or stress. Target heart rate was defined as 85% of age-predicted maximal heart rate (220 age). The dose of dobutamine and heart rate at which the deterioration of wall motion first occurred were recorded. Ischemic threshold was defined as the heart rate at which new or worsening WMA occurred, divided by the age-predicted maximal heart rate, multiplied by 100%. The change in left ventricular end-systolic volume (LVESV) from rest to peak stress was recorded as normal (decrease in LVESV) or abnormal (increase or absence of a decrease). The stress electrocardiogram was positive for ischemia if there were horizontal or downsloping ST-segment depression of
1 mm at 80 ms after the J-point in the absence of baseline ST-segment deviation.
Follow-up. Follow-up information was obtained from medical records, telephone interviews, mailed questionnaires, and the Social Security Death Index. End points were all-cause mortality and cardiovascular morbidity, defined as myocardial infarction (MI) and late (more than three months) coronary revascularization. For the analysis of morbidity, patients who underwent coronary revascularization within three months after DSE were censored at the time of revascularization. For patients who experienced both MI and late coronary revascularization, only the first event was included for the analysis of morbidity. The statistical analyses of all-cause mortality and morbidity were performed separately.
Statistical analysis. Characteristics were summarized as percentages for categorical variables and as mean ± standard deviation for continuous variables. Comparisons between groups were based on Wilcoxon rank-sum test for continuous variables and the Pearson chi-square test for categorical variables. Cumulative probabilities of overall survival or freedom from morbidity were estimated by the Kaplan-Meier method. For the event-free survival analysis, patients were censored at the time of non-cardiac death and at early revascularization. Univariable and multivariable associations of clinical and echocardiographic variables with the end points were assessed using the Cox proportional hazards model. All clinical variables and representative DSE variables were considered in the model, regardless of their univariate significance. The proportional hazards assumption was tested by retaining the model-based risk score and performing a time-dependent proportional hazards analysis in which the risk score was entered along with its interaction both with time and with log (time). Variables were selected in a stepwise forward selection manner with entry and retention set at a significance level of 0.05. Results of these analyses were summarized as hazard ratios with 95% confidence intervals. To determine the incremental value of DSE, models of: 1) clinical variables alone; 2) clinical and rest echocardiographic variables; and 3) clinical, rest, and stress echocardiographic variables were compared via their log likelihood ratio chi-square statistics. For each end point, a simple integer risk score was derived according to the estimated values of beta coefficients of multivariable predictors. The fitted model included age only for the purpose of adjustment; all other models were based on the variables selected in the stepwise algorithm, which were replaced by dichotomous versions to facilitate ease of clinical use. This integer risk score was then divided into three categories, based on choosing the split that maximized the three-group log rank chi-square statistic, as well as the model chi-squared statistic when age and the three risk categories were included in a proportional hazards model. Patients were classified as being at low, intermediate, and high risk according to this categorization. The estimated five-year event survivals of each risk category were derived from the Kaplan-Meier method. In addition, the best model for each end point using the original clinical and echocardiographic variables was summarized by way of its prediction of the one-, three-, five-, or eight-year probability of survival or morbidity-free status, as a function of the respective model variables. This is included in the for users wishing a more accurate prediction model. Finally, the accuracy (calibration) of prediction models was assessed by dividing the patient population into deciles of estimated risk, calculating the average proportional hazards model-based probabilities of survival or morbidity-free status at each time point within these deciles, and comparing them with the corresponding Kaplan-Meier (model-free) estimates of the probabilities within the same deciles of risk. To assess the predictive power of each model, the C-statistic for censored data was calculated and reported for each model. Analyses were carried out using SAS version 8.2 (SAS Institute Inc., Cary, North Carolina).
| Results |
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Outcomes. Follow-up periods for all-cause mortality and cardiovascular morbidity were 5.4 ± 2.2 (maximum, 13.2) years and 3.9 ± 2.7 (maximum, 13.2) years, respectively. Death occurred in 1,044 (44%) patients. Survival probabilities at 1, 3, 5, and 8 years were 89%, 74%, 60%, and 44%, respectively. The cumulative mortality rate was higher in patients with abnormal compared with patients with normal DSE at 1 year (13% vs. 7%), 3 years (30% vs. 19%), 5 years (45% vs. 31%), and 8 years (61% vs. 48%) (p < 0.0001). Patients on insulin had lower survival probabilities compared with patients who were on oral agents or diet alone (87% vs. 90% at 1 year, 71% vs. 76% at 3 years, 57% vs. 62% at 5 years, 42% vs. 46% at 8 years; p = 0.005).
Cardiovascular morbidity occurred in 309 (13%) patients; 193 had MI and 116 had late coronary revascularization. Early coronary revascularization (with no intervening event) was performed in 148 patients. Patients with inducible ischemia were more likely to undergo coronary revascularization than those with nonischemic DSE (19% vs. 9%, p < 0.0001). The DSE data of patients with and without mortality and cardiovascular morbidity are shown in Table 2.
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4, respectively). The coefficient of the risk score for mortality was similar in patients with or without known CAD (hazard ratio, 1.2 and 1.2; p < 0.0001 for each). The C-statistic of this simple risk score model was 0.60. The corresponding mortality rates per person-year of follow-up were 7%, 10%, and 16%, respectively. The estimated five-year survivals of patients in the three risk categories were 69%, 60%, and 47% (p < 0.0001). The Kaplan-Meier survival curves of patients in these three risk categories are shown in Figure 2. The predictive value of this mortality risk score was independent of age (p < 0.0001). Figure 3 shows the estimated five-year mortality in the subsets of patients according to age group and risk category.
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25% (Table 4). The coefficient of the risk score for morbidity was similar in patients both with and without coronary artery disease (hazard ratio, 1.2, p = 0.0002; and 1.3, p < 0.0001; respectively). A risk score of 0, 1 to 2, or
3 permitted classification of patients into three risk categories. This C-statistic of this model was 0.62. The event rates per person-year of follow-up of patients in the three risk categories were 2%, 3%, and 6%. The estimated five-year probability of remaining free of an ischemic event (excluding early revascularization) of patients in the three risk categories were 94%, 86%, and 80% (p < 0.0001). An exact model-based prediction for each end point using the continuous risk models of Tables 3 and 4 is described in the . The C-statistic for each model was 0.64. When these model-based probabilities were averaged within model-based deciles of risk and compared directly with the Kaplan-Meier based probabilities for the same time points and deciles, the 40 discrepancies were within 3% more than 80% of the time for both end points. There was no systematic overestimation or underestimation of risk among the groups.
| Discussion |
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Clinical importance of a high mortality rate in patients with diabetes mellitus. In the present study, the mortality rate of patients with diabetes mellitus was higher than that in a recent study by Sozzi et al. (44% vs. 24%) (21). Several population-based studies of patients with diabetes mellitus, which showed mortality rates of 20% to 48% during follow-up of 5 to 10 years (2224), provide evidence for the adverse impact of diabetes mellitus on mortality. Cardiovascular mortality has been shown to account for the majority of deaths in diabetic patients. Although direct comparisons across these studies are problematic because of differences in baseline characteristics and follow-up duration, the mortality rate is substantial in patients with diabetes mellitus. Therefore, accurate risk stratification is important for optimal patient management.
The present study develops the prognostic information from DSE in patients with diabetes mellitus using a risk score that combines both echocardiographic and clinical variables. This score, which divides patients into three categories, is sufficiently simple for use in clinical practice.
In our study, patients with diabetes mellitus had a high prevalence of CAD risk factors and prior MI, reflecting a high-risk population. Furthermore, inability to exercise in patients with diabetes mellitus, the reason for DSE rather than exercise testing, is a marker not only of a high pretest probability of CAD but also of a poorer prognosis (25,26). Even patients in the lowest risk group had substantial mortality. As a consequence of a long duration of follow-up, the progression of non-obstructive coronary lesions and the development of new obstructive lesions may also contribute to high event rates. The present study also found that survival probabilities were lower in patients on insulin. However, this may be related to the severity of diabetes mellitus rather than to an effect of therapy.
Role of DSE in the diabetic population. Myocardial perfusion scintigraphy has been recommended by the American Diabetes Association and the American College of Cardiology for the evaluation of CAD in patients with diabetes mellitus (27). Because of the paucity of outcome data, the prognostic role of stress echocardiography in patients with diabetes mellitus was not established. Since then, more information regarding the role of stress echocardiography (exercise, dobutamine, dipyridamole, or combination) in patients with diabetes has become available (21,22,2832). Our findings validate the prognostic significance of DSE in predicting long-term outcomes in a large cohort with diabetes mellitus and provide a simple approach for clinical risk stratification.
Study limitations. Data regarding types, duration, complications of diabetes mellitus, degree of glycemic control, and changes in medications after the DSE were not available. Also, criteria for diagnosis of diabetes mellitus evolved during the time period of the study (16). To avoid inaccuracy in defining causes of death, all-cause mortality was selected as an unbiased, objective end point (33). Data on cancer or history of congestive heart failure, important contributors to mortality, were not available in this study. To focus on the prognostic significance of DSE on cardiac outcomes, we regarded MI and coronary revascularization as cardiovascular morbidity, which was analyzed separately. It is possible, especially in this population with diabetes mellitus, that some patients may have had myocardial infarction that was not clinically recognized. Post-test referral bias likely influenced the decision to perform coronary revascularization. Although we excluded early revascularization for this reason, it is possible that test results influenced the decision for late revascularization. Nevertheless, DSE provided similar incremental value in predicting both all-cause mortality and cardiovascular morbidity. The present study provides simple risk scores for risk stratification in patients with diabetes mellitus. However, the C-statistic values of 0.60 to 0.64 for the various models presented indicate modest discrimination. External validity testing with independent samples, ideally from different institutions, would be desirable.
| Conclusions |
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| Footnotes |
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2 Dr. Arruda is currently affiliated with the Heart Institute (InCor), University of São Paulo Medical School, São Paulo, Brazil ![]()
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