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(Hypertension. 2004;43:775.)
© 2004 American Heart Association, Inc.
Scientific Contributions |
From the Department of Medicine (C.J.R., R.R.S., S.H., M.R.D.), Columbia University, New York, NY; Department of Epidemiology (A.V.D.-R.), University of Michigan, Ann Arbor; Neurological Institute (B.B.-A., R.L.S.), Columbia University, New York, NY; Division of Epidemiology (R.L.S.), Columbia University Mailman School of Public Health, New York, NY; Department of Sociomedical Science (B.B.-A.), Columbia University Mailman School of Public Health, New York, NY; and Sergievsky Center (R.L.S.), College of Physicians & Surgeons, Columbia University, New York, NY.
Correspondence to Dr Carlos J. Rodriguez, Division of Cardiology, Columbia University, College of Physicians & Surgeons, 630 West 168th Street, New York, NY 10032. E-mail cjr10{at}columbia.edu
| Abstract |
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Key Words: socioeconomic factors hypertrophy epidemiology race
| Introduction |
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Increased left ventricular mass (LVM) is a predictor of cardiovascular morbidity and mortality independent of arterial hypertension and other traditional risk factors.3,4 Blacks are more likely to have increased LVM than whites.5,6 Increased LVM may account for part of the increased cardiovascular mortality among blacks, and little is known about LVM among Hispanics. Unfortunately, the mechanism by which increased LVM imparts an elevated cardiovascular risk and why its prevalence may be higher among minorities remain largely unknown. An association between lower socioeconomic status (SES) and poorer health, including all-cause mortality and increased cardiovascular morbidity and mortality, has been observed.79 Whether environmental precursors (including underlying socioeconomic factors) contribute to the differential burden of LVM has not been fully investigated. In the United States blacks are known to have considerably lower SES than whites.2,9 Lower SES is associated with increased psychological stress, increased cardiovascular reactivity, and increased incidence of hypertension,1015 all of which may potentially contribute to the development of increased LVM. The aim of the present study is to examine the association between SES, as defined by educational level, and LVM. We will also investigate any influence of raceethnicity on the potential relation between LVM and SES.
| Methods |
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3 months in a household with a telephone. NOMAS subjects with previous myocardial infarction were excluded from this study. Ninety-one percent of those called participated in a telephone interview, and 75% of those who were eligible and invited to participate came to Columbia University Medical Center (CUMC) for an in-person evaluation (overall participation rate 68%). The study was approved by the Institutional Review Board at CUMC. All participants gave consent directly or through a surrogate when appropriate. As part of NOMAS, 3298 participants underwent extensive in-person evaluation, and transthoracic echocardiograms were performed on 2003 eligible subjects. Echocardiograms that were technically adequate for analysis were obtained in 1916 subjects and are included in this study.
Blood pressure was measured with mercury sphygmomanometers and cuffs of appropriate size. Hypertension was defined as a blood pressure recording
140/90 mm Hg (based on the average of 2 blood pressure measurements during one sitting by a trained research assistant), the patients self-report of a history of hypertension, or antihypertensive medication use. Diabetes mellitus was defined by the patients self-report of such a history, use of insulin or hypoglycemic agent, or fasting glucose
126 mg/dL. Physical activity was assessed with a standardized questionnaire that recorded the frequency and duration of 14 different recreational activities during the 2-week period before the interview. These analyses used the total duration of physical activity in hours per week. Height and weight were determined by the use of calibrated scales. Assessments were conducted in English or Spanish, depending on the primary language of the participant. Raceethnicity was based on self-identification through a series of interview questions modeled after the 2000 US census and conformed to the standard definitions outlined by Directive 15.
Transthoracic echocardiography was performed and measurements were taken by standard two-dimensional (2-D) protocols according to the guidelines of the American Society of Echocardiography (ASE). Left ventricular diastolic dimension (LVDD), left ventricular systolic dimension (LVSD), interventricular septal thickness (IVS), and posterior wall thickness (PWT) were measured in all patients. LVM was calculated with the use of the corrected ASE method: 0.8x(1.04x[(IVS+LVDD+PWT)3-LVDD3]+0.6).
LVM was then indexed to body size by dividing raw LVM by height to the allometric power of 2.7 and analyzed as a continuous variable.18,19 Interpretation of echocardiographic studies was performed off-line by researchers blinded to the subjects clinical and demographic characteristics. Four readers over the period of 1993 to 2000 were involved in the analysis of all the echocardiographic studies. For quality-control measures, all readers were trained by senior echocardiographers (S.H. or M.D.T.) and interobserver reliability was periodically assessed among the readers by use of intraclass correlation coefficients for the variables measured, which ranged between 0.59 and 0.74.
Educational level was used as the indicator of SES and classified into 4 categories: "less than high school" included those who never went to high school or had completed only part of high school, "completed high school" included those who had completed high school or other vocational training beyond primary, "some college" included those who had some level of tertiary education, and "college graduate or more" included those who had completed college alone or with a higher degree such as a masters or doctorate.
Means±SD were calculated for continuous variables, and proportions were used for categorical variables. The distribution of sociodemographic and stroke/cardiovascular risk factors was evaluated in the total cohort and among the 3 raceethnic groups. Unadjusted analyses were performed using ANOVA and linear regression models for continuous variables and the
2 test for categorical variables. Unadjusted comparisons of mean LVM across each SES group were performed using ANOVA. Tests for trend were performed using educational levels as ordinal variables. ANCOVA was then used to analyze the association between SES and LVM after adjusting for potential confounding demographic and medical variables, including age, gender, systolic blood pressure, diabetes, physical activity (hours per week), and body mass index (BMI). Least-square adjusted means for the covariates mentioned were computed as an estimate of the marginal means that would be obtained with a balanced population that does not differ with respect to any of the covariates. Statistical significance was determined at the
=0.05 level using two-sided tests. Statistical analyses were conducted using PROC GLM from SAS 8.2 computer software (SAS Institute).
| Results |
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In the total cohort, using unadjusted analysis there was a significant inverse and graded association between SES and mean LVM, ie, subjects of lower SES have higher degrees of LVM. Mean LVM for those in the highest SES category was 10% lower than those in the lowest SES category (P trend<0.0001) (Table 2). On multivariate analysis, this relationship remained independent of age, gender, systolic blood pressure, diabetes, physical activity, and BMI.
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On raceethnic group comparison, using an adjusted multivariate analysis, SES was inversely associated with LVM in blacks (P trend=0.009) but not in whites (P trend=0.86) or Hispanics (P trend=0.47) (Table 2). The difference in adjusted mean LVM between the highest and lowest categories of education was 5.3 g/m2.7 for blacks, 0.0 g/m2.7 for whites, and 1.0 g/m2.7 for Hispanics. This was equivalent to an 11% difference in adjusted mean LVM between the highest and lowest SES categories among blacks.
Table 3 shows the relationship between SES and LVM among the subgroup of 841 subjects without clinical hypertension (SBP was 152±21 mm Hg in hypertensives and 130±14 mm Hg in nonhypertensives; P<0.0001). Differences in unadjusted mean LVM showed that among normotensives, those in the highest SES category had an 11% lower mean LVM than those in the lowest SES category (P trend<0.0001). On raceethnic-specific multivariate analyses, the inverse relationship between LVM and SES again remained significant only among normotensive blacks. Normotensive blacks in the lowest SES category had an 18% higher mean LVM compared with blacks in the highest category (P trend=0.006).
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| Discussion |
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Explanations for the greater LVM differences among blacks remain speculative, but several different factors may play a role. Sympathetic stimulation is one mechanism through which low SES could be associated with greater LVM. Sympathetic stimulation has been shown to vary with environmental exposures, such as job stress and lower SES.12,14 Evidence suggests that lower SES is associated with a disproportionate cumulative burden of stressful life conditions.1215 Chronic intermittent adrenergic stimulation, in the absence of overt hypertension, can cause increased LVM, and the presence of increased LVM may precede the onset of clinically overt hypertension.2022 Lower SES, particularly among blacks, may be associated with increased psychosocial stress and adrenergic stimulation, both of which may produce increased LVM. Being black and of low SES may be a different experience than being white or Hispanic of low SES. Low SES blacks, for example, may be subjected to greater stress producing experiences than low SES whites, resulting in stronger SES patterning of LVM among blacks than among whites. Increased adrenergic receptor sensitivity to norepinephrine infusion has been shown in blacks compared with white hypertensive subjects. These pathways may be either more sensitive (requiring a lower threshold to "trigger") or more responsive (have a higher "gain") or both among blacks leading to increased LVM. Thus, blacks of lower SES may carry a higher sympathetic tone than whites or Hispanics of lower SES, and their LVM may also be more responsive to its effects. Regardless of the mechanisms involved, our observations raise the clinical issue of whether blacks of lower SES require more careful assessment for prehypertension/hypertension and subsequent target organ damage.23
SES may also be related to dietary behaviors. Studies have suggested that blacks retain sodium more avidly than whites,24 and increased sodium retention is associated with increased LVM by way of mechanisms involving circulating volume expansion and possibly activation of the renin-angiotensin system.25 Dietary differences (in the intake of sodium for example) may be greater by SES among blacks than among whites and could also contribute to the greater LVM differences by SES observed in blacks.
Conventional clinical assessment of blood pressure may underestimate the severity and duration of high blood pressure, particularly among subjects of lower SES. Greater LVM among blacks compared with whites is unexplained by differences either in rest or in mean daytime blood pressure.26 However, ambulatory blood pressure that remains elevated at night (non-dipping) rather than exhibiting its normal nocturnal decrease is a strong correlate for the presence of increased LVM and predictive of the LVM differences between blacks and whites.27 Because nondipping status is related to sympathetic tone, and because blacks are more likely to be non-dippers,28,29 ambulatory blood pressure measures and diurnal variation may better explain raceethnic and socioeconomic differences in LVM. Ambulatory blood pressure may also explain the observed LVMSES relation among normotensives. Patients with a normal clinic blood pressure may show abnormalities on ambulatory blood pressure monitoring. This phenomenon of "masked hypertension" may have more clinical relevance because ambulatory blood pressure has a stronger impact than resting blood pressure on end-organ damage and cardiovascular outcomes.6 The development of hypertension may be less well recognized or the severity of hypertension underestimated among blacks of lower SES when compared with their white or Hispanic counterparts in an urban community. These issues warrant further investigation.
Important strengths of this study are its population-based nature and the presence of a tri-ethnic sample. Several limitations exist. First is the lack of variability in educational level observed among Hispanics (nearly 80% were in the lowest educational category), which may have limited our ability to detect SES effects in this group. Income data were not available in our cohort. Educational level was used as a proxy for SES and remains a reasonable but limited measure of SES.7 Educational level may not be a good predictor of SES among an immigrant population, possibly explaining the lack of an association between SES and LVM among our predominantly immigrant Hispanic cohort. Finally, at a given educational level, raceethnic groups may differ in terms of wealth, buying power, living conditions, or access to resources in ways that could not be fully measured.
Perspectives
This study is the first, to our knowledge, to demonstrate that lower SES is an independent predictor of increased LVM among hypertensive and normotensive blacks. Although our results do not establish a causative role for SES in the pathogenesis of increased LVM, they do suggest that a link between SES and LVM exists and open important avenues for further research. It is possible that conditions of lower SES more adversely affect blacks in terms of increased LVM and its sequelae. Thus, this group may warrant more aggressive cardiovascular risk monitoring and intervention.
| Acknowledgments |
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| Footnotes |
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Received October 21, 2003; first decision November 19, 2003; accepted January 12, 2004.
| References |
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