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(Stroke. 2000;31:19.)
© 2000 American Heart Association, Inc.
Original Contributions |
Geographic Variation in Stroke Risk in the United States
Region, Urbanization, and Hypertension in the Third National Health and Nutrition Examination Survey
Presented in part at the 38th Annual Conference on Cardiovascular Disease Epidemiology and Prevention, Santa Fe, NM, March 18–21, 1998.
From Howard University College of Medicine, Washington, DC (T.O.O.), and Centers for Disease Control and Prevention, Hyattsville, Md (C.M.V., R.F.G.).
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Abstract |
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Background—In the United States, stroke mortality is higher in the south than in other regions. Hypertension is the main risk factor for stroke among older adults; however, few studies have examined group-specific regional and urbanization differences in hypertension prevalence.
Methods—Data from the Third National Health and Nutritional Examination Survey (NHANES III), 1988 to 1994, were analyzed to calculate the prevalence of hypertension (systolic >140 mm Hg and/or diastolic >90 mm Hg and/or taking antihypertensive medication) by region and urbanization for age (40 to 59 and 60 to 79 years), sex, and ethnic subgroups. Logistic regression models were fitted to estimate the association of hypertension with region and urbanization.
Results—With age and urbanization kept constant, southern residence was associated with hypertension among middle-aged non-Hispanic white men (odds ratio [OR], 1.49; 95% confidence interval [CI], 1.12 to 1.90; P<0.006), non-Hispanic black men (OR, 1.36; 95% CI, 1.05 to 1.66; P=0.019), and non-Hispanic black women (OR, 1.23; 95% CI, 1.01 to 1.45; P=0.034). Among older non-Hispanic white men, a significant interaction was noted between region and urbanization (P=0.01), with a higher prevalence in the south only for nonmetropolitan residents (OR, 1.32; 95% CI, 1.06 to 1.56; P<0.013). A similar but not statistically significant trend was also confirmed among non-Hispanic black men in logistic regression analysis (OR, 1.38; 95% CI, 0.97 to 1.68; P=0.061). No statistically significant association was observed for urbanization or region in the other subgroups.
Conclusions—Southern residence was associated with increased hypertension prevalence among middle-aged non-Hispanic white men, non-Hispanic black men and women, and older non-Hispanic white men.
Key Words: aged • blacks • cross-sectional studies • geography • hypertension
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Introduction |
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Stroke is a leading cause of death and disability among the elderly in the United States, and hypertension is a major risk factor for stroke, coronary artery disease, congestive heart failure, renal insufficiency, and peripheral vascular disease.1 2 3 4 5 Similarities in the geographical distribution of stroke mortality and several major stroke risk factors, including hypertension, have been reported.5 Therefore, epidemiological studies targeted at improved understanding of the role of stroke risk factors in producing geographical variation in stroke mortality and incidence are of high public health interest.
The positive association between systolic and diastolic blood pressures and stroke risk has long been recognized among persons with and without coronary artery disease.6 7 Several interventions targeting lifestyle risk factors for hypertension and treatment modalities currently exist.1 However, for these interventions to have optimal benefit and to maximize their relative cost-effectiveness, it may be desirable to target high-risk populations. The southeastern part of the United States has long been identified as the stroke belt region, thus making it a target for hypertension intervention.5 8 9 However, few studies have examined group-specific regional and urbanization differences in the prevalence of hypertension and consequently stroke risk in the US population. Such studies are even rarer in the US elderly population.
In this study, we examined regional and urbanization-based differences in the prevalence of hypertension, a prime risk factor for stroke, in 8 sex, ethnic, and age subgroups. Our hypotheses were that hypertension prevalence is higher in the south than in other regions and that hypertension prevalence differs between metropolitan and nonmetropolitan areas after adjustment for regional differences.
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Subjects and Methods |
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To investigate regional variations in the prevalence of hypertension in the United States, we analyzed data from the Third National Health and Nutrition Examination Survey (NHANES III), 1988 to 1994. NHANES III was conducted by the National Center for Health Statistics of the Centers for Disease Control and Prevention between 1988 and 1994 through a national multistage probability sample of the civilian noninstitutionalized population of the United States.10 The initial NHANES III sample consisted of 33 994 individuals 2 months to 99 years of age; the survey design ensured oversampling of non-Hispanic blacks, Mexican-Americans, persons 2 months to 5 years of age, and those
60 years of age. Data were
collected from each participant through face-to-face interview,
physical examination, and laboratory analyses. The overall
survey design and provisions for informed consent have been previously
detailed.10
The age limits for inclusion in the analysis were set along
sampling stratum. First, we set the lower age limit at 40 years because
of the low rates of hypertension in this age group. Second, we set the
upper age limit at 79 years because a significantly higher percent of
persons 80 years of age who suffered disproportionately higher rates
of hypertension and other comorbid conditions were institutionalized
and excluded from the NHANES III survey. Overall, this analysis
was restricted to the 6278 noninstitutionalized non-Hispanic whites and
non-Hispanic blacks 40 to 79 years of age for whom complete data on
hypertension were available.
Data on systolic or diastolic blood pressures were missing for 30 participants. Even though NHANES III provides information on Mexican-Americans, we chose not to include them in this study because of their geographical concentration outside the historic stroke belt. NHANES III includes Texas, with its high percentage of Mexican-American population, in the southern region. The inclusion of Texas may therefore redefine the stroke belt and make the overlap of the south and the stroke belt less exact. Persons of other ethnic origins were excluded because of small sample size.
Region and Urbanization Classification
Urbanization classification was based on the USDA rural-urban
continuum codes11 12 that describe metropolitan
communities by degree of urbanization and nearness to metropolitan
areas (Tables 1
and 2). The USDA codes were recoded
into 2 categories, metro and nonmetro, to prevent identification of
counties that were sampled in the survey. Region of residence describes
4 broad geographic regions as defined by the Bureau of the Census.
Table 3 shows the states included
in the southern region. In the analysis presented here,
the northeast, midwest, and west were combined as the nonsouthern
region.
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Blood Pressure Measurements
Blood pressure was measured on 2 occasions. Three measurements
for participants 17 years of age were taken during the home interview
by a trained interviewer, and 3 more measurements were taken from all
participants 5 years at the Mobile Examination Center by the
examining physician. All 6 measurements were taken with the participant
in the sitting position after 5 minutes of rest. Blood pressure was
measured with a standard mercury sphygmomanometer (Baumanometer, WA
Baum Co, Inc) and 1 of 5 available cuffs (infant, regular, adult,
large, and thigh) selected on the basis of the circumference of the
participant’s arm. Before the survey started and periodically
thereafter, all blood pressure observers, both interviewers and
physicians, received training in the use of a standardized protocol for
measurement of blood pressure.10 Maximum inflation levels
were predetermined before the first blood pressure
measurement.13 The first and fifth Korotkoff sounds
defined systolic and diastolic blood pressures in
participants 20 years of age. The average of all available readings
is reported here. A positive response to the question "Are you taking
prescribed medication?" indicated the use of antihypertensive
medication. This question was asked of the participants who reported
having been told by a doctor that they had hypertension and also
reported having received a prescription for antihypertensive
medication.10 In this study, we define hypertension as an
average systolic blood pressure 140 mm Hg and/or
diastolic blood pressure 90 mm Hg and/or currently
taking antihypertensive medication.1 For descriptive
analyses, persons reporting a medical history of hypertension
were categorized separately as "hypertension aware" group.
Individuals currently taking antihypertensive medications were
classified as treated; of these subjects, those having systolic
blood pressure <140 mm Hg and/or diastolic blood
pressure <90 mm Hg were considered as persons with controlled
hypertension.
Statistical Analysis
Age-adjusted hypertension prevalence and percent of hypertension
awareness, treatment, and control were calculated by the direct method
with the 1990 US population as standard. Because of the well-known
difference in mean blood pressure by sex, race-ethnic, and age
groups,14 analyses of the distribution of
hypertension by southern and nonsouthern and metro and nonmetro
residence were conducted for each of the 8 demographic subgroups formed
by the combination of sex, race-ethnicity, and age groups.
Analyses were performed separately for persons 40 to 59 and 60
to 79 years of age. The 
2 test statistics and
95% confidence intervals (CIs) were used to determine statistical
significance. We performed exploratory logistic regression
analysis of hypertension prevalence15 using the
Statistical Analysis System (SAS).16 Initial
logistic regression analyses including all participants showed
that there was a significant interaction between age and hypertension
prevalence. To address this interaction, the study population was
divided into 2 age groups for further analyses: those 40 to 59
and those 60 to 69 years of age. Separate logistic regression models
were constructed for each of the 8 demographic subgroups. Both regions
(south versus nonsouth) and urbanization (metro versus nonmetro) were
examined in a multivariate model. To assess for
possible interaction between region and urbanization, separate
additional logistic regression models testing the association between
region and hypertension were fitted, including the interaction terms
"region*urbanization," "region*age," and
"urbanization*age." Significant "region*urbanization"
interaction was noted only in the older age group. For this
reason, separate models were constructed for metro and nonmetro areas
including region and age in the group 60 to 79 years of age for
non-Hispanic white men and non-Hispanic black men. Final models for
women 60 to 79 years of age and for the entire group 40 to 59 years of
age included region, urbanization, and single years of age. Because of
the multistage complex survey design of NHANES III, all final
analyses including the interaction models were performed in
SUDAAN, software that accounts for the design effect in computing
variance estimates17 Appropriate sampling weights were
used to account for oversampling and nonresponse.10 The
adjusted odds ratios were corrected by use of the method described by
Zhang et al18 to avoid overestimation of risk because the
prevalence of hypertension in the study population was >30%.
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Results |
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In Table 4
, we present
age-adjusted prevalence of hypertension, mean systolic blood
pressure, and percent aware of, treated for, and control of
hypertension in region and urbanization categories among participants
40 to 59 years of age. In this age group, non-Hispanic black men and
women had the highest mean systolic pressure, whereas a
relatively low percent of non-Hispanic black men achieved control of
their high blood pressure. Statistically significant regional
differences in mean systolic blood pressure were noted in
southern non-Hispanic white men (127.3 versus 123.6 mm Hg,
P=0.002) and non-Hispanic black men (133.0 versus 129.7
mm Hg, P=0.033) for south and nonsouth, respectively. Only
non-Hispanic black women residing in the south showed a higher rate of
blood pressure control while receiving medication treatment compared
with non-Hispanic black women residing in nonsouthern regions (38.3%
versus 25.8%, P=0.047, respectively). Overall, non-Hispanic
white women had the lowest prevalence of hypertension of all the
demographic subgroups without significant regional differences. Also,
33.1% of southern non-Hispanic white men (95% CI, 27.3 to 38.9)
compared with 23.0% of nonsouthern non-Hispanic white men (95% CI,
18.4 to 27.6) had hypertension.
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Table 5 shows the results of a logistic
regression analysis examining regional variations in the
prevalence of hypertension while controlling for age and urbanization
for those 40 to 59 years of age. Statistically significant associations
between residence in the south and hypertension were noted in
non-Hispanic black women (OR, 1.23; 95% CI, 1.01 to 1.45;
P=0.034), non-Hispanic black men (OR, 1.36; 95% CI, 1.05 to
1.66; P=0.019), and non-Hispanic white men (OR, 1.49; 95%
CI, 1.12 to 1.90; P<0.006) but not among non-Hispanic white
women (OR, 1.05; 95% CI, 0.71 to 1.49; P=0.801). However,
no statistically significant urbanization differences were observed in
any of the demographic subgroups 40 to 59 years of age.
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Overall, in the United States, age-adjusted prevalence of hypertension
was higher among 60- to 79-year-old southern residents compared with
nonsouthern residents of the United States (59% versus 54%;
P=0.04; Table 6). A higher
percent of older southern non-Hispanic white women and non-Hispanic
white men had hypertension compared with similar ethnic-sex groups in
the nonsouthern region. Table 6 shows age-adjusted hypertension
prevalence and mean systolic blood pressure in persons 60 to 79
years of age by region and urbanization. Older southern non-Hispanic
white men (54.9% versus 47.4%, P=0.012) and non-Hispanic
black men (66.6% versus 62.3%, P=0.050) exhibited higher
rates of hypertension than nonsouthern residents. Interestingly, older
non-Hispanic black men in the south demonstrated higher awareness
(78.3% versus 67.3%, P=0.043) and treatment (65.7% versus
53.1%, P=0.035) rates but a lower rate of blood pressure
control with medication (34.5% versus 45.4%, P=0.041)
compared with their nonsouthern counterparts. Mean systolic
blood pressure showed differences only within urbanization groups, with
levels higher among southern nonmetro non-Hispanic white men (139.9
versus 133.8 mm Hg, P=0.001) and non-Hispanic black
men (143.7 versus 133.4 mm Hg, P=0.003) compared with
nonmetro nonsouthern residents of similar ethnicity. Non-Hispanic white
women showed no regional differences but higher rates of awareness
(P=0.010) and treatment (P=0.036) among southern
nonmetro residents compared with their nonsouthern counterparts.
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A significant interaction was noted between region and
urbanization in older non-Hispanic white men (P=0.006), with
a higher prevalence in the south only for nonmetro residents.
Similarly, statistically significant region and urbanization
interaction was noted in older non-Hispanic black men
(P=0.031), with hypertension prevalence also being higher
for nonmetro residents in the south. However, in age-adjusted logistic
regression analysis for the group 60 to 79 years of age, in an
examination of regional differences in the prevalence of hypertension
among the different sex-ethnic groups, only nonmetro southern
non-Hispanic white men (OR, 1.32; 95% CI, 1.06 to 1.56;
P=0.013) showed a higher risk of having hypertension
compared with their nonsouthern counterparts (Table 7). A similar but not statistically
significant trend was also confirmed among non-Hispanic black men in
logistic regression analysis (OR, 1.38; 95% CI, 0.97 to 1.68;
P=0.061). No statistically significant association was
observed for urbanization in the other demographic subgroups.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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A statistically significantly higher prevalence of hypertension was noted among younger southern non-Hispanic white men and non-Hispanic black men and women compared with those residing in other regions (Table 4
). Geographical variation in the prevalence of
hypertension was not apparent among non-Hispanic white women. In those
60 to 79 years of age, a higher age-adjusted rate of hypertension was
noted among southern non-Hispanic white men compared with nonsouthern
residents of the United States only for nonmetropolitan residents
(Table 6). The difference was not statistically significant for
non-Hispanic black men and women after controlling for urbanization.
Other significant differences by urbanization were not noted.
The higher rates of stroke incidence and mortality, as well as
hypertension prevalence and systolic blood pressure, in the
southeastern part of the United States make it a target for stroke
intervention.1 2 3 4 5 6 7 8 9 19 Regional variation in the
distribution of stroke, for which hypertension is the leading cause,
has been observed in the United States and in other parts of the
world.1 2 3 4 5 6 7 8 9 19 20 21 22 23 Recently, the south-central region has
shown similar high rates because of a more rapid decline in stroke
mortality among non-Hispanic whites in the southeastern
area.5 8 9 Although most recent studies suggest an overall
decline in the rate of hypertension,24 if more vigorous
hypertension control efforts such as education and treatment had been
mounted in the southeast in the 1980s, the result may have been a more
rapid decline in hypertension prevalence and stroke mortality. A
slowing down of the decline in overall stroke mortality rate in the
United States has also been reported,24 25 making the need
for more effective, targeted interventions more urgent. Explanations
for geographical differences in stroke risk have been elusive largely
because of complex interactions of multiple causes, some of which may
have differential effects on atherosclerosis and
hypertension.26 Furthermore, hypertension is a
multifactorial disorder that emanates from a genetic and environment
interaction.27 28 These causes may have geographical
distribution and therefore produce geographical variations in the
prevalence of hypertension.29 30 Consequently, profiles
that delineate risk in 1 ethnic group may not necessarily
represent risk factors in other groups.31
Recently, Kiefe et al32 also showed persistent regional
differences in the rates of hypertension after adjusting for body mass
index, weight gain, physical activity, dietary intake, alcohol,
tobacco, education, contraceptive use, and family history of
hypertension. Although environmental toxic elements such as lead and
cadmium33 were implicated in geographic variation in some
studies, others have reported high salt intake, low dietary potassium,
and magnesium and protein intake, particularly among southern
blacks.34 35 36 Also, the relationship between hypertension
and psychological stress and socioeconomic status as measured by
education, occupation, and area of residence may contribute to the
geographical distribution.37 Moreover, the prevalence of
counseling that results in increased physical activity level and
improved blood pressure control was lowest in the south and highest in
the midwest.38 Non-Hispanic blacks were substantially less
likely than non-Hispanic whites and Hispanics to be counseled about
physical activity. Similarly, a greater proportion of persons 50 to 64
years of age than those in other age groups was more likely to be
counseled.38 Collectively, these correlates of
hypertension appear to parallel those reported for stroke with similar
geographic, ethnic, and age distribution in the United States. One
study suggested that the overall differential rates of stroke and
hypertension may reflect a markedly higher rate of severe hypertension
among southern blacks.23 This notion is supported in the
present study by the relatively lower rate of blood pressure
control among older southern hypertensive black men treated with
medication (Table 6). Furthermore, the interaction of region and
urbanization among older white men is similar to the interaction
reported previously for ischemic heart disease and stroke
mortality.19 Previously, explaining the higher rate of
stroke among non-Hispanic white men in the south has been difficult.
However, the findings of higher rates of hypertension, a statistically
significant association between hypertension and residence in the
south, and a significant interaction between region and urbanization
among non-Hispanic white men help explain increased stroke risk in the
region (Table 7).
The higher rates of hypertension among black men in the south are consistent with stroke mortality patterns or stroke incidence in the United States.5 19 Similarly, previously discussed higher prevalence rates of self-reported hypertension among southeastern US residents is consistent with the findings of the present study.39 The lack of an overall association between region and hypertension among elderly black men and women may be due to selective mortality among hypertensives23 and exclusion of the institutionalized population. Early onset of hypertension, the "ceiling effect," in which the rates were already high, and bias or confounding by factors not included in this analysis are alternative explanations for the lack of regional differences in the older age group.39 Reduced access to healthcare services and consequent underutilization of preventive services is 1 possible reason for higher rate of stroke among the lower socioeconomic groups in the rural south.40 41 Although a relatively higher awareness and treatment rate exists among older black men in the south compared with nonsouth, this treatment does not appear to be sustained because blood pressure control by both region and urbanization was poorer in this group, thus underscoring the contributions of socioeconomic and education factors to higher rates of sustained hypertension and stroke risk among older blacks in the south. It is also possible that black men in particular respond very poorly to treatment compared with persons of other ethnic origins. Additionally, lower rates of preventive services use reported in the southeast region and metropolitan areas in previous national surveys in 1973 and 198241 42 support the hypothesis of inadequately treated hypertension as a possible explanation for increased stroke risk in this region. The high rates of treatment that are not complemented by high rates of blood pressure control for those on medication treatment in the present study warrant increased focus on sustained blood pressure treatment and control, in addition to ongoing awareness education particularly among older blacks in the southern United States.
The strength of the this study lies in the fact that NHANES III provides the best available data for examination of group-specific regional differences in the prevalence, awareness, and treatment of hypertension in the US noninstitutionalized population. In addition to the large sample size, the accuracy of blood pressure was ensured by taking measurements on 2 occasions and then averaging them to get a more precise estimate. This avoids some inherent bias posed by pseudohypertension, particularly white-coat hypertension. The major limitation of the study is the lack of power for subgroup analysis of differences in awareness, treatment, and control of hypertension among subgroups 40 to 59 years of age. Other limitations include possible bias resulting from survey nonresponse and possible confounding by variables not controlled for in the analysis. The presence of higher rates of hypertension, stroke prevalence, and mortality among the institutionalized population may further bias the data in older age group.
In conclusion, analysis of the data from NHANES III showed regional differences in the prevalence of hypertension. Specifically, southern non-Hispanic white men and non-Hispanic black men and women 40 to 59 years of age had higher rates of hypertension compared with persons in other regions. One challenge for the next decade is the reduction of ethnic, socioeconomic, and regional variations in hypertension.42 Group- and region-targeted hypertension and multiple risk factor intervention could be a useful and cost-effective measure to reduce the prevalence of hypertension and associated mortality. Despite numerous questions that remain unanswered regarding the cause of geographical variations in stroke incidence and mortality in the United States, currently available evidence supports an increased focus on hypertension education and intervention program in the southern United States. Improved access to health care and optimization of hypertension treatment strategies may reduce stroke rates in high-risk areas.
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Acknowledgments |
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This work was support in part by Health Care Finance Administration grant No. 20-C-90721/3. We acknowledge the technical assistance provided by Thomas C. Socey of NOVA Research Co.
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Footnotes |
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Reprint requests to Thomas O. Obisesan, MD, MPH, Section of Geriatrics, Department of Medicine, Howard University Hospital, 2041 Georgia Ave NW, Washington, DC 20060.
Received May 21, 1999; revision received October 15, 1999; accepted October 15, 1999.
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