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J Am Coll Cardiol, 2005; 45:1787-1793, doi:10.1016/j.jacc.2005.02.072
© 2005 by the American College of Cardiology Foundation
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CLINICAL RESEARCH: CORONARY ARTERY DISEASE

C-Reactive Protein and Electrocardiographic ST-Segment Depression Additively Predict Mortality

The Strong Heart Study

Peter M. Okin, MD, FACC*,*, Mary J. Roman, MD, FACC*, Lyle G. Best, MD{dagger}, Elisa T. Lee, PhD{ddagger}, James M. Galloway, MD, FACC§, Barbara V. Howard, PhD|| and Richard B. Devereux, MD, FACC*

* Division of Cardiology, Department of Medicine, Weill Medical College of Cornell University, New York, New York
{dagger} Missouri Breaks Industries Research Inc., Timber Lake, South Dakota
{ddagger} College of Public Health, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma
§ University of Arizona, Tucson, Arizona
|| MedStar Research Institute, Washington, DC

Manuscript received December 8, 2004; revised manuscript received February 15, 2005, accepted February 22, 2005.

* Reprint requests and correspondence: Dr. Peter M. Okin, Weill Medical College of Cornell University, 525 East 68th Street, New York, New York 10021. (Email: pokin{at}med.cornell.edu).


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
OBJECTIVES: This study was designed to examine whether high-sensitivity C-reactive protein (CRP) and electrocardiographic (ECG) ST-segment depression (STD) have additive utility for predicting cardiovascular disease (CVD) death and all-cause death (ACD).

BACKGROUND: C-reactive protein, a marker of systemic inflammation, and ECG STD, an index of myocardial ischemia and hypertrophy, independently predict mortality.

METHODS: Electrocardiograms and CRP levels were examined in 2,155 American Indian participants in the second Strong Heart Study examination. ST-segment depression ≥50 µV (n = 127) and CRP >7.0 mg/l (defining the upper quartile of CRP levels, n = 540) were considered abnormal.

RESULTS: After 5.2 ± 1.2 years follow-up there were 95 CVD deaths and 310 ACD. In univariate Cox analyses, the combination of CRP and ECG STD improved risk stratification compared to either alone, with the presence of both CRP >7.0 and ECG STD associated with a 7.7-fold increased risk of CVD death (95% confidence interval [CI] 3.3 to 18.2) and a 6.5-fold increased risk of ACD (95% CI 4.1 to 10.3). After adjustment for age, gender, and relevant risk factors, the combination of high CRP and STD remained predictive of CVD death and ACD, with the presence of both abnormal CRP and STD associated with the highest risks of CVD death (hazard ratio [HR] 3.2, 95% CI 1.1 to 10.5) and ACD (HR 3.9, 95% CI 2.1 to 7.2) and the presence of either high CRP or abnormal STD associated with intermediate risks of CVD death (HR 2.2, 95% CI 1.4 to 3.4) and ACD (HR 1.5, 95% CI 1.2 to 2.0).

CONCLUSIONS: The combination of ECG STD and CRP increases the risk of mortality, demonstrating the additive impacts of active inflammation and preclinical CVD on prognosis.

Abbreviations and Acronyms
  ACD = all-cause death
  CI = confidence interval
  CRP = C-reactive protein
  CVD = cardiovascular disease
  ECG = electrocardiogram/electrocardiographic
  HDL = high-density lipoprotein
  HR = hazard ratio
  LDL = low-density lipoprotein
  STD = ST-segment depression

The resting electrocardiogram (ECG) is an inexpensive and widely used noninvasive tool for assessing risk in population-based studies and clinical practice. ST-segment depression (STD) on the ECG, a sign of ventricular repolarization abnormality, is a well-established marker of risk in the general population (1–10) that is also strongly associated with underlying cardiovascular disease (CVD), including left ventricular hypertrophy (11,12). Indeed, even minimal degrees of computer-measured STD are associated with higher left ventricular mass and greater prevalence of anatomic ventricular hypertrophy (11) and provide additional prognostic information beyond that afforded by echocardiographic hypertrophy and traditional risk factors (10).

C-reactive protein (CRP), a marker of systemic inflammation, has been demonstrated to predict incident CVD, hypertension, systemic atherosclerosis, sudden death, and CVD and all-cause mortality (13–26). The additive prognostic value of combining CRP with other markers of risk, such as low-density lipoprotein (LDL) or total cholesterol (19), glycated hemoglobin (20), metabolic syndrome (21,22), systemic hypertension (23), and Framingham risk scores (24,25) to improve risk stratification has been well documented. However, there are few data regarding the relationship between STD and CRP (27,28), and whether these two readily available noninvasive risk factors provide additive prognostic value has not been examined. Thus, the present study examined the relationship between high-sensitivity CRP and ECG STD and whether CRP and STD have additive utility for predicting CVD and all-cause mortality, controlling for clinical and demographic variables that could confound these relationships.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Study population.   The Strong Heart Study is a population-based study of CVD and its risk factors in American Indians from 13 communities in Arizona, Oklahoma, and North and South Dakota. Information about the population, methods, and enrollment procedures for the study has previously been reported in detail (8,9,11,29,30). The current study examined a subset of 2,155 of the 3,638 original participants in the second Strong Heart Study exam (64% women, mean age 59 ± 8 years) who had digital ECG records showing sinus rhythm with no bundle branch block and with CRP levels.

Electrocardiography.   Standard 12-lead ECGs were performed with MAC-PC or MAC-12 digital ECG systems (GE Medical Systems, Milwaukee, Wisconsin) as previously described (8,9,11). Absolute ST-segment deviation was measured by computer with 5-µV precision at the midpoint of the ST-segment. ST-segment depression ≥50 µV in any lead (excluding aVR) was considered abnormal, a value that corresponded to the 95th percentile of ECG STD and that stratified mortality risk in participants in the first Strong Heart Study examination (8).

CRP determination.   C-reactive protein was measured using an in-house developed enzyme-linked immunosorbent assay using purified CRP and anti-CRP antibodies from CalBioChem (La Jolla, California) (31). This assay has been used extensively in epidemiologic studies and in the validation of the commercially available assay for high-sensitivity CRP (32). The coefficient of variation is approximately 8%. C-reactive protein was considered abnormal if >7.0 mg/l, defining the upper quartile of CRP levels in the study population.

Determination of end points.   Deaths were identified and verified and were classified as due to CVD if caused by myocardial infarction, stroke, sudden death from coronary heart disease, or congestive heart failure as determined by standardized review by the mortality committee (8,29).

Data and statistical analyses.   Data were analyzed with SPSS, release 12.0 (SPSS Inc., Chicago, Illinois). Data are presented as mean ± SD for continuous variables and as proportions for categorical variables. Mean values were compared between groups using two-way analysis of variance to adjust for possible gender differences. Proportions were compared by chi-square tests. Mortality rates were calculated and plotted by the Kaplan-Meier product-limit method; death rates were compared between groups with the log-rank test. Mortality analyses were performed by fitting Cox proportional hazards models to the data with stratification by center. The estimated hazard ratio of death associated with positive test outcomes was computed as the antilog of the estimated coefficient for dichotomous variables. The 95% CI of each relative risk was calculated from the estimated coefficients and their standard errors, and Wald chi-square statistics and probability values were calculated. To test the independence of STD and CRP as predictors of mortality, both variables were entered together into multivariate Cox models with a forward selection procedure. The models also included covariates that were significant predictors of CVD or all-cause mortality found in the univariate Cox models. To test the complementary information provided by CRP and STD criteria, a combined test criterion was derived that incorporated both measures into three categories: both STD and CRP negative, either STD or CRP positive, and both STD and CRP positive. For all tests, a two-tailed p value <0.05 was considered significant.


   Results
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 Abstract
 Methods
 Results
Discussion
 References
 
Patient characteristics.   After a mean follow-up of 5.2 ± 1.2 years there were 310 deaths from all causes, including 95 CVD deaths. Clinical characteristics of participants grouped according to the level of CRP and degree of STD are examined in Table 1. Compared to those with lower CRP levels, the 540 participants with CRP >7.0 mg/l were slightly younger, more likely to be women, had higher body mass indexes, lower LDL cholesterol, higher fasting glucose and fibrinogen levels, greater albuminuria, higher prevalences of coronary heart disease and diabetes, and were slightly less likely to be current smokers, but did not differ with respect to systolic or diastolic pressure, high-density lipoprotein (HDL) cholesterol or triglyceride levels, or degree of STD. The 127 participants with STD ≥50 µV had higher systolic pressures, higher triglyceride, fasting glucose, fibrinogen, and CRP levels; had greater albuminuria and prevalences of coronary heart disease and diabetes; and were less likely to be current smokers, but did not differ with respect to age, gender, body mass index, diastolic pressure, and HDL or LDL cholesterol levels compared to participants with less STD. C-reactive protein and STD were only minimally correlated with each other (r = –0.06, p = 0.006).


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  		Table 1. Clinical Characteristics According to Level of C-Reactive Protein and Degree of ST-Segment Depression at Baseline
 
Prediction of cardiovascular and all-cause mortality.   In Cox analyses stratified for study center, abnormal STD and CRP were each significant predictors of CVD and all-cause mortality (Table 2, Figs. 1 and 2). ST-segment depression ≥50 µV was associated with a >4-fold increased risk of CVD death and with a >2.5-fold increased risk of all-cause mortality, with CVD mortality of 15.0% (19 of 127) and all-cause mortality of 32.3% (41 of 127), compared with 3.7% (76 of 2,028) and 13.3% (269 of 2,028), respectively, in those with STD <50 µV. C-reactive protein >7.0 mg/l was associated with a two-fold increased risk of CVD death and with a nearly two-fold higher risk of death from any cause, with CVD mortality of 6.7% (35 of 540) and total mortality of 20.0% (128 of 540), compared with 3.7% (59 of 1,615) and 12.5% (202 of 1,615), respectively, in those with lower CRP levels.


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  		Table 2. Cox Proportional Hazards Models of High-Sensitivity C-Reactive Protein and ST-Segment Depression Criteria for Prediction of All-Cause and Cardiovascular Mortality
 


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  		Figure 1 Kaplan-Meier plots of cumulative cardiovascular mortality (A) and all-cause mortality (B) according to the magnitude of ST-segment depression (STD) partitioned at 50 µV.

 


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  		Figure 2 Kaplan-Meier plots of cumulative cardiovascular mortality (A) and all-cause mortality (B) according to high-sensitivity C-reactive protein (CRP) partitioned at 7.0 mg/l.

 
After adjustment for the possible predictive values of age, gender, body mass index, diastolic and systolic blood pressures, HDL and LDL cholesterol, triglyceride, fasting glucose and fibrinogen levels, albuminuria, alcohol use, diabetes, and history of smoking or prevalent coronary heart disease, both STD ≥50 µV and an elevated CRP remained significant predictors of CVD and all-cause mortality (Table 2). Predictive values of STD and CRP for CVD and all-cause mortality were not dependent on the partition values selected for test positivity: considering STD and CRP as continuous variables preserved their independent predictive value for both CVD (HR = 1.25, 95% CI 1.03 to 1.52 per 25 µV of STD and HR = 1.015, 95% CI 1.000 to 1.039 per 1 mg/l of CRP) and all-cause mortality (HR = 1.13, 95% CI 1.03 to 1.25 and HR = 1.014, 95% CI 1.005 to 1.023) in multivariate analyses.

Combined high-sensitivity CRP and ECG STD criteria.   Because CRP and STD criteria provided independent prognostic information, the ability of the combination of these variables to improve prediction of mortality was assessed (Table 3, Fig. 3). In Cox analyses stratified by study center, the combined CRP and ECG STD variable improved risk stratification compared to either CRP or STD alone for both CVD and all-cause mortality, with the presence of both ECG STD and an elevated CRP associated with the greatest risks. Cardiovascular disease mortality was 16.7% (6 of 36) in participants with both STD ≥50 µV and CRP >7.0 mg/l, 7.2% (43 of 595) in those with either ECG STD or an abnormal CRP, and only 3.0% (46 of 1,524) in those in whom both variables were negative (p < 0.0001). All-cause mortality was 55.6% (20 of 36) in the presence of both ECG STD and an elevated CRP, 18.3% (109 of 595) when either variable was abnormal, and 11.9% (181 of 1,524) when both tests were negative (p < 0.0001). Multivariate Cox analyses (Table 3) demonstrated that after adjustment for other potential predictors of mortality, the combination of ECG STD and an elevated CRP level remained a significant predictor of CVD and all-cause mortality, with the presence of both ECG STD and increased CRP associated with a 3.2-fold increased risk of CVD death and a 3.9-fold increased risk of all-cause mortality after adjusting for covariates. Predictive value of the combined criterion was not dependent on use of the upper quartile of CRP to define abnormality: the combined predictive value of CRP and STD remained significant if CRP was partitioned using the population median (3.8 mg/l) or 90th percentile (15.0 mg/l) values.


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  		Table 3. Cox Proportional Hazards Models of Combined High-Sensitivity C-Reactive Protein and ST-Segment Depression Criteria for Prediction of All-Cause and Cardiovascular Mortality
 


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  		Figure 3 Kaplan-Meier plots of cumulative cardiovascular mortality (A) and all-cause mortality (B) according to combined ST-segment depression (STD) and high-sensitivity C-reactive protein (CRP) criteria. (STD–/CRP– represents both negative; STD+ or CRP+ represents either positive; STD+/CRP+ represents both positive).

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
This study demonstrates that the combination of STD on the ECG and elevated high-sensitivity CRP is associated with an increased risk of CVD and all-cause mortality. Electrocardiographic STD and increased CRP provide additive prognostic information, independent of each other and of other risk factors known to predict mortality. The additive prognostic value of increased CRP and STD persists across the full range of CRP and STD values and is independent of factors previously demonstrated to provide additional prognostic information in the presence of an elevated CRP (19–26). The absence of both an elevated CRP and ECG STD identifies a large group at relatively low five-year risk of death, whereas the presence of both abnormalities identifies a subgroup with a markedly increased five-year mortality. These findings support the ability of combining simple computerized ECG STD and CRP criteria to improve risk stratification.

Relationship of CRP and STD to outcome.   The separate predictive values of CRP and ECG STD have been well documented in a range of populations, with convincing evidence for continuous relationships between increasing values of both STD and CRP and event rates (8,9,11,13–26). Quantitative measures of STD using computerized ECG have been associated with an increased risk of anatomic hypertrophy (11) and with both CVD and all-cause mortality (8,9), with the combination of ECG STD and echocardiographic left ventricular hypertrophy providing complementary prognostic information for these outcomes (9). Similarly, the prognostic value of CRP has been extensively documented in a variety of prospective epidemiologic studies, with evolving evidence that CRP provides additive prognostic information beyond that afforded by assessment of cholesterol levels (19) and the Framingham risk score (24,25), further supporting the additive value of CRP as a risk factor for vascular disease and outcomes. However, only limited data exist on the relationship between CRP and ECG findings (27,28), and the value of combining CRP and ECG STD for risk prediction has previously not been examined.

In a population-based study of 8,076 subjects, Asselbergs et al. (27) demonstrated that although ST-segment and T-wave abnormalities by Minnesota code were modest univariate correlates of an increased CRP, only Q-wave myocardial infarction by Minnesota code remained associated with increased CRP levels after adjusting for standard cardiovascular risk factors. The current study demonstrates only a weak correlation between CRP and ECG STD as continuous variables and that each provides significant independent prognostic information for both CVD and ACD (Table 2), providing the impetus for combining the variables to enhance risk stratification. Importantly, the present study further demonstrates that the additive predictive value of STD and CRP is independent of the prognostic value of cholesterol and other risk factors that constitute the Framingham risk score, and of serum fibrinogen levels and albuminuria, risk factors previously demonstrated to predict outcome in this population (9,30). In light of the recent report demonstrating the additive value of ECG STD and echocardiographic hypertrophy for predicting mortality in this population (9), it is important to note that inclusion of echocardiographic hypertrophy in alternative multivariate Cox models (data not shown) did not affect the predictive value of the combined ECG STD and CRP variable.

These findings suggest that active inflammation (reflected by increased CRP) and preclinical CVD (as denoted by even minor degrees of STD on the ECG) provide complementary and additive stimuli in the development of mortality due to atherosclerosis. Accumulating evidence suggests that CRP may play a direct role in atherogenesis at the level of the endothelial cell and vascular smooth muscle (33–36), whereas STD on the ECG has been directly linked with CVD, including left ventricular hypertrophy (1–12). The interaction of inflammation with ventricular and vascular hypertrophy to produce atherosclerosis provides an attractive hypothesis for the additive impact of these two risk markers in predicting outcome.

Several aspects of the study population need to be considered with respect to these findings. First, it is unclear to what degree these findings in American Indians can be extrapolated to other ethnic populations. However, the demonstrated predictive value of CRP and minor degrees of STD in other populations when examined separately from each other (1–7,13–26) suggests that the combination of CRP and ECG STD will stratify risk in other populations as well. Second, the current population has a high prevalence of diabetes and metabolic syndrome and is predominantly women, with the attendant issues of gender differences in CRP levels and possible effects of estrogen on CRP. However, CRP performed similarly in men and women and there were no significant interactions between gender and CRP in Cox analyses of the entire population. Additionally, estrogen use was limited in the population (n = 137, 6.4%), and neither including estrogen use as an additional variable nor excluding women using estrogen altered the results.

A number of other potential limitations should also be taken into account. First, the absence of serial CRP and ECG STD determinations precludes analysis of the impact of changes in CRP and/or STD on risk. Second, the values of CRP in this population of American Indians are significantly higher than those found in most prior population-based studies of the prognostic value of CRP (13–26). However, the predictive value of both STD and CRP in the current population persisted when CRP was examined using both lower (3.8 mg/l) and higher (15.0 mg/l) partition values and when CRP was considered as a continuous variable. Previous analyses have also documented the predictive value of very high (≥10 mg/l) levels of CRP (25). In addition, exclusion of participants with CRP levels in the highest decile (≥15 mg/l) did not substantively alter the results of the current analyses. Lastly, it should be noted that use of computer-measured STD ≥50 µV for prediction of risk has been validated in American Indian participants only in the Strong Heart Study (8,9,11). However, utility of this threshold of STD, using both manual and computerized ST-segment measurements, has been extensively examined and confirmed in other populations using Minnesota codes 4-2 and 4-3 (37,38), which are defined by the presence of STD ≥50 µV and <100 µV.

The major implication of this study is that both high-sensitivity CRP and ECG STD aid in routine clinical identification of patients at high risk. The recent interest in development of novel biomarkers that provide additional information beyond that available from standard risk factors and are inexpensive and readily available to practitioners (39) provides further impetus for the use of CRP levels and determination of the magnitude of STD from the widely available and inexpensive digital ECG. Although data are emerging that demonstrate clear associations between improved clinical outcomes and treatment-related reductions in CRP (40,41) and ECG markers of risk (42), whether therapies targeted specifically at reducing these biomarkers will improve prognosis requires further evaluation and will prove crucial in further delineating the role of these biomarkers in the serial assessment of risk.


   Acknowledgments
 
The authors wish to thank the Strong Heart Study participants, staff, and coordinators.


   Footnotes
 
This work was supported by grants HL-41642, HL-41652, HL-41654, and HL-65521 from the National Heart, Lung, and Blood Institute, Bethesda, Maryland; and by grant M10RR0047-34 (GCRC) from the National Institutes of Health, Bethesda, Maryland. The views expressed in this paper are those of the authors and do not necessarily reflect those of the Indian Health Service.


   References
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 Abstract
 Methods
 Results
 Discussion
 References
 

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S. Cicala, G. de Simone, M. J. Roman, L. G. Best, E. T. Lee, W. Wang, T. K. Welty, J. M. Galloway, B. V. Howard, and R. B. Devereux
Prevalence and Prognostic Significance of Wall-Motion Abnormalities in Adults Without Clinically Recognized Cardiovascular Disease: The Strong Heart Study
Circulation, July 10, 2007; 116(2): 143 - 150.
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P. M. Rautaharju, C. Kooperberg, J. C. Larson, and A. LaCroix
Electrocardiographic Predictors of Incident Congestive Heart Failure and All-Cause Mortality in Postmenopausal Women: The Women's Health Initiative
Circulation, January 31, 2006; 113(4): 481 - 489.
[Abstract] [Full Text] [PDF]

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