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(Hypertension. 1996;28:256-264.)
© 1996 American Heart Association, Inc.
Articles |
the Medlantic Research Institute, Washington, DC (B.V.H.); Center for Epidemiologic Research, University of Oklahoma Health Sciences Center, Oklahoma City (E.T.L., J.L.Y., O.G.); National Heart, Lung, and Blood Institute, Bethesda, Md (R.R.F.); Cornell Medical Center, New York, NY (R.B.D.); and Aberdeen Area Indian Health Service, Rapid City, SD (T.K.W.).
| Abstract |
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Key Words: Indians, North American cardiovascular diseases diabetes mellitus ethnic groups
| Introduction |
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In other groups, hypertension has not been as well studied. Although CVD is the leading cause of death among American Indians,8 9 a population in which prevalence rates of diabetes and obesity are high, relatively little is known about hypertension and its correlates in this population. The Strong Heart Study was initiated in 1988 to determine, using standardized methodology, the prevalence of CVD and its risk factors in 13 American Indian communities in three geographic regions. This report presents data from the phase I examination on the prevalence of hypertension, its awareness and control, and relationships of BP to other CVD risk factors and physiological variables.
| Methods |
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Study Communities
The study population included people between 45 and 74 years of age who were members of the following tribes: (1) Pima, Maricopa, and Tohono O'odham (Papago) of Central Arizona (residing in the Gila River, Salt River, and Ak Chin communities), (2) the seven tribes of southwestern Oklahoma (Apache, Caddo, Comanche, Delaware, Fort Sill Apache, Kiowa, and Wichita), and (3) the Oglala and Cheyenne River Sioux in South Dakota and the Devils Lake Sioux in the Fort Totten area of North Dakota (Fig 1
).
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The Arizona center includes three Indian communities: (1) the Gila River Pima/Maricopa Indian community, located approximately 30 miles (48 km) south of Phoenix; (2) the Salt River Pima/Maricopa Indian community, located on the eastern border of the greater Phoenix metropolitan area; and (3) the Ak Chin Papago/Pima Indian community, located approximately 25 miles (40 km) southwest of Phoenix. All three of these communities comprise primarily Pima Indians but include some of the very closely related Maricopa and Papago (Tohono O'odham) tribes. Anthropological evidence indicates that the Pima/Maricopa/Papago Indians are descendants of the Paleo Indians, who originated in northwest Asia between 14 000 and 38 000 BCE, and that they have been relatively stable communities, with little intermarriage with other Indian tribes or with whites.
The Oklahoma center comprises the Seven Tribes of southwestern Oklahoma: Comanche, Kiowa, Apache, Fort Sill Apache, Wichita, Caddo, and Delaware. The first five were originally classified as "Plains" tribes. The Fort Sill Apache were organized by descendants of Geronimo, who, along with his group of followers, were relocated to Fort Sill, Oklahoma Territory. The Caddo tribe was a "Woodland" tribe. Although they were agriculturists, the Caddo took on much of the culture of the Plains tribes; these two tribes now live together in southwestern Oklahoma. The Delaware originated in the northeastern United States; two groups of Delaware now live in Oklahoma, one with the Cherokee Nation and one in close association with the Plains tribes. Members of the Seven Tribes in Oklahoma live among the general community; there are no reservations in Oklahoma.
The Dakota center comprises three separate Sioux tribes: the Oglala and Cheyenne River Sioux of South Dakota and the Devils Lake Sioux of North Dakota. The Sioux originated from the Black Hills area of South Dakota. They led a primarily hunter/gatherer lifestyle, traveling as far as the east and west coasts at times. There are three main Sioux groups designated by language dialects. The Oglala and Cheyenne River Sioux are from the Lakota subgroup, and the Devils Lake Sioux originated from the Dakota subgroup. The Oglala Sioux tribe now resides on the Pine Ridge Indian Reservation in southwestern South Dakota. Pine Ridge is the second largest (about the size of Connecticut) reservation and tribe in the United States. The Cheyenne River Sioux reside in the Eagle Butte area. The Devils Lake Sioux are located in eastern North Dakota. These three Sioux tribes have more admixture with non-Indians than do the Arizona and Oklahoma tribes.
Clinical Examination
The first (phase I) clinical examination was conducted between July 1989 and January 1992. Participation rates were 72% in the Arizona center, 62% in the Oklahoma center, and 55% in the Dakota center.13 Nonparticipants did not differ significantly from participants in age or self-reported frequency of diabetes. A higher proportion of participants were female (59%) than were nonparticipants (42%), and participants had a higher self-reported frequency of hypertension than nonparticipants (42% versus 30%). Data on all participants who attended the clinical examination were used for determination of prevalence rates of CVD and its risk factors.
The clinical examination consisted of a personal interview and physical examination. After participants gave informed consent, fasting blood samples were obtained for measurements of lipids and lipoproteins14 (total cholesterol and triglycerides; very-low-density, low-density, and high-density lipoprotein cholesterol; and very-low-density lipoprotein triglycerides), insulin,15 plasma fibrinogen,16 and glycated hemoglobin.17 A 75-g oral glucose tolerance test was performed on all participants (except for diabetic people being treated with insulin or oral hypoglycemic agents and those with a fasting glucose level
12.5 mmol/L [
225 mg/dL] as determined by an Accu-Check II [Baxter Healthcare Corp]). A random urine sample was obtained for measurement of albumin18 and creatinine.19 Anthropometric measurements included weight, height, and waist-hip circumferences. Percent body fat was estimated with an impedance meter (model B1A101, RJL Equipment Co) and an equation based on total body water (M Singer, RJL Equipment Co, personal communication, 1992).
Brachial artery BP (first and fifth Korotkoff sounds) was measured three consecutive times on seated participants after they had rested 5 minutes with the use of a mercury sphygmomanometer (WA Baum Co). An appropriately sized cuff was placed on the right arm, pulse occlusion pressure was determined, and the cuff was inflated to 20 mm Hg above that pressure. The mean of the last two of these measurements was used for estimation of BP. Stages of hypertension were defined by criteria listed in JNC-V (1993).1 Hypertension awareness was defined if the participant had been told by a physician or healthcare provider that he or she had hypertension. Hypertension control was defined as those individuals taking BP medication who had SBP less than 140 mm Hg and DBP less than 90 mm Hg.
Questions administered during the interview assessed demographic information, family health history, lifestyle, and medical history, including the Rose Questionnaire for angina pectoris.20 Alcohol consumption was assessed by asking participants how many drinks they consumed per day; one drink was defined as 360 mL (12 oz) beer, 120 mL (4 oz) wine, or 30 mL (1 oz) hard liquor. Binge drinking was defined as consumption of more than five drinks on any 1 day within the past year. Leisure-time exercise and occupation-related physical activities were measured with the instrument developed by Kriska et al,21 which was modified for use with American Indians.
Definitions
Individuals were classified as diabetic according to World Health Organization criteria.22 In some analyses, a nondiabetic group was defined as those individuals with either NGT (fasting and 2-hour glucose <7.8 mmol/L) or IGT (fasting glucose <7.8 mmol/L and 2-hour glucose from 7.8 to 11.0 mmol/L [140 to 199 mg/dL]).
Obesity was defined according to NHANES criteria as BMI greater than or equal to 31.1 kg/m2 for men and greater than or equal to 32.3 kg/m2 for women.23 Urinary albumin excretion was estimated by the ratio of albumin (milligrams) to creatinine (grams)24 : Microalbuminuria was defined as 30 to 299 mg/g and macroalbuminuria as 300 mg/g or higher. Diabetes and hypertension therapy were assessed by personal interview; before the interview, individuals were asked to bring all of their medications to the examination site.
Statistical Analysis
Associations of BP with physiological and lifestyle variables were assessed by multivariate analysis; a stepwise multiple regression procedure was used separately for men and women, with BP as the dependent variable and center, age, percent body fat, glucose, urinary albumin, insulin, smoking, education, alcohol consumption, and antihypertensive therapy as independent variables. Analyses were conducted on all participants and also on those not receiving antihypertensive medications. Removing individuals from the analysis who were using antihypertensive medication did not affect the conclusions, and therefore the analyses are presented for the population as a whole.
| Results |
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The prevalence of hypertension was similar in the Arizona and Oklahoma sites and lower in the Dakota site (P<.001) (Table 2
). In Oklahoma, rates were higher in men than in women (P<.0001), but no significant sex differences were found in the other two sites. Means for both SBP and DBP were lower in the Dakotas than in the other centers (P<.001) (Table 2
). The prevalence of hypertension increased with age in both sexes (Table 3
), more steeply in women than in men. Rates increased to approximately 49% in 65- to 74-year-old men and to approximately 57% in 65- to 74-year-old women. Mean levels of SBP also increased with age (Table 3
). The proportion of hypertensive participants who were aware of their disorder ranged from 69% to 82% (Fig 2
) and was higher by a modest margin in women younger than 65 years of age. In women, more than 60% of hypertensive participants in the two younger decades were receiving treatment compared with 50% of those in the 65 to 74 age range. In men, 51% to 55% were receiving treatment in all three decades. The proportion of men and women with hypertension whose BP was controlled averaged approximately 30% and decreased with increasing age in both sexes, except in 65- to 74-year-old men.
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Individuals were divided into categories according to JNC-V criteria.1 The majority of those with hypertension were classified as stage I, representing more than two thirds of all those with hypertension, except in 65- to 74-year-old women (Fig 3
). Fig 3
also shows that a high proportion of the population had optimal BP, defined as SBP less than 120 mm Hg and DBP less than 80 mm Hg. This proportion, which averaged almost 30% in 45- to 54-year-old men and more than 40% in 45- to 54-year-old women, declined with increasing age.
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Prevalence rates of hypertension were greatest in individuals with diabetes in both sexes (Fig 4
). Prevalence rates in women and 45- to 54-year-old men with IGT were higher than in those with NGT but lower than in those with diabetes. In 55- to 64-year-old men, prevalence rates in those with IGT were similar to rates in those with NGT; in 65- to 74-year-old men, prevalence rates in those with IGT were slightly higher than in those with diabetes.
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Univariate and multivariate analyses of possible physiological correlates of BP in this population are shown in Table 4
. Analyses were performed by diabetic status because of the high prevalence of diabetes in this population. Age was significantly and positively related to SBP and negatively related to DBP in both men and women. These relationships were significant even after adjustment for center, insulin, glucose, and obesity and were seen in both nondiabetic and diabetic individuals.
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In women, SBP and DBP were significantly but inconsistently related to obesity (Table 4
), as measured by percent body fat; this relationship was significant after adjustment for center, insulin, glucose, use of antihypertensive medication, and age, except for DBP in diabetic women. DBP was related to obesity in nondiabetic men and SBP was negatively related to obesity in diabetic men; both of these relations remained significant after adjustment for center, insulin, glucose, and age. The differences in SBP with obesity in women were small. For example, nondiabetic women in the first decile of body fat (mean BMI 21 kg/m2) had a mean SBP only 8 mm Hg lower than those in the 10th decile of body fat (mean BMI 43 kg/m2). The corresponding differences in men were even smaller. When the relationship between BP and obesity was evaluated with BMI as the criterion for obesity, results were similar (data not shown); there were no significant relations between BP and waist-hip ratio.
Correlations between BP and plasma insulin concentrations were very weak and were not statistically significant after adjustment for center, glucose, age, and obesity, except for a negative relationship between DBP and insulin in diabetic men (Table 4
). The relationship of insulin in the one group showing significant positive correlation before adjustment (SBP in nondiabetic women) was small; those women in the first decile of insulin (mean, 28.7 pmol/L [4 µU/mL]) had a mean SBP only 9 mm Hg lower than those in the 10th decile (mean fasting insulin, 294.18 pmol/L [41 µU/mL]). Insulin was not significantly related to BP if analyses were conducted without individuals taking antihypertensive medication.
Albuminuria was significantly and positively related to SBP and DBP in both diabetic and nondiabetic men and women. These relationships remained significant after adjustment for center, glucose, age, obesity, and insulin (Table 4
).
Lifestyle correlates of BP are shown in Table 5
. In both men and women, current drinking or binge drinking was not significantly related to SBP or DBP. Physical activity, measured as a combination of occupational and leisure activities, showed a trend toward association with lower SBP, although none of the trends with SBP reached statistical significance after adjustment for center, age, obesity, insulin, and glucose, and there was a positive relation between physical activity and SBP in diabetic men (Table 5
). Education was negatively related to SBP in both nondiabetic and diabetic women and in nondiabetic men (Table 5
), although relationships lacked significance after adjustment for center, age, obesity, insulin, and glucose, except with SBP in nondiabetic women.
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The various correlates of BP were evaluated with a forward stepwise multiple regression procedure (Table 6
). In women, albuminuria was the most significant positive correlate of SBP; the next most significant was age. Use of antihypertensive medication, center, obesity, and alcohol consumption (current) also were significant positive correlates, whereas glucose and smoking were independent negative correlates. In men, albuminuria was also the most significant correlate of SBP (Table 6
); the second variable to enter the model was age. Use of antihypertensive medication, center, alcohol consumption, and physical activity were positive correlates, and smoking was a significant negative correlate in men.
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For DBP in women, use of antihypertensive medication was the most significant correlate, with age second and albuminuria third; obesity, physical activity, alcohol consumption, and center also entered the model as positive correlates and glucose and smoking as negative correlates (Table 6
). In men, albuminuria was the most significant correlate of DBP; age entered the model second. Use of antihypertensive medication, alcohol consumption, center, and obesity also were significant positive correlates, and smoking and educational attainment were negative correlates. Insulin was not a significant independent correlate of either SBP or DBP in men or women.
| Discussion |
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Compared with analogous data available from NHANES III for whites and blacks in the United States, prevalence rates for hypertension in American Indians in Arizona and Oklahoma were similar to the rates for whites, but rates were lower in Indians in the Dakotas.25 26 The degree of hypertension appeared mild, with most American Indians in the first and second categories of hypertension as defined by JNC-V,1 and the proportions treated and controlled were higher than in NHANES III.26 In addition, many nonhypertensive participants had BPs in the optimal range. One possible explanation for the lower rates in the Dakota center compared with the Arizona and Oklahoma communities is that smoking rates are high in the Sioux25 and other Plains Indians27 and smoking is a significant negative correlate of SBP and DBP. The center difference, however, persisted after adjustment for several confounding variables, including smoking, and thus other factors must be explored to explain the center difference.
It is difficult to compare the present data with other data available in American Indians because of differences in methodology and population sampling strategies. Although most BP measurements or previous studies were taken from seated participants, methods did not fully adhere to JNC-V guidelines. Nevertheless, most previous studies reached the conclusions that hypertension prevalence is lower and that hypertension may be milder in these populations. Gillum et al27 reported average BP values of approximately 126/77 mm Hg for men and 119/72 mm Hg for women among American Indians in Minnesota, and DePrez et al28 reported a 23% hypertension prevalence in Penobscot Indians. A survey of 640 Navajo Indians showed lower BPs and a lower hypertension prevalence than in whites and blacks, and BP levels did not show as great an increase with age.29 Mean BPs for Papago Indians in Gila Bend, Arizona, were slightly lower than those of NHANES II whites, and no associations were found between any measures of sodium intake and BP.30 Among 817 diabetic Navajo Indians, only 47.5% had diagnosed hypertension, as determined by medical record review.31 This value is slightly lower than the rate observed in diabetic Indians in the present study, although the age range for the Navajo study was younger. A survey of 25- to 65-year-old Pascua Yaqui Indians showed mean BPs similar to those of the present study.32 The latter was a population-based sample of this community but involved only 230 subjects.32 BP data from the ongoing diabetes study among Pima Indians in the Gila River Indian Community showed lower average values than those in the present study, but the measurements were taken with the participant in the supine position and with the use of different procedures.33 Finally, regional rates based on IHS outpatient records were lower than rates published for NHANES II whites and blacks.34 35 Variation in rates was observed among IHS service areas but in a different pattern compared with the present study, with the highest rates in the northwestern communities and lowest rates in the Southwest.
It is of interest that hypertension prevalence rates in the present, more recent survey, compared with data from NHANES III,36 are similar to the overall prevalence rate among US whites, except for the Dakota center. NHANES III data suggest that the prevalence of hypertension is declining among the general US population, although the data available to date on American Indians may show the opposite trend, with earlier studies showing differences in comparisons with other US groups that are not now apparent. Longitudinal analysis of data from our second examination may shed more light on this relationship.
The present study indicates that diabetes is associated with significantly higher BP among American Indians; prevalence rates were approximately twofold higher in those with diabetes than in those with NGT, and they were also higher in those with IGT. This finding has been observed in many other studies of both Indians and non-Indians. An ongoing longitudinal study of diabetes and its complications among Pima Indians has shown a consistently higher prevalence of high BP in individuals with diabetes in all age groups,33 and BP has been a significant predictor of diabetic retinopathy3 and renal disease.37 Thus, the occurrence of hypertension in American Indians deserves rigorous attention because of the high prevalence of diabetes and the high morbidity and mortality attributable to retinopathy and end-stage renal disease.
As with other populations, BP increases with age in American Indians. Conversely, obesity appears to have only a modest effect on BP, with the association being more significant in women than in men. This is in contrast to reports in whites, blacks, and Hispanics, in whom BP is considerably higher in obese individuals.38 39 40
In contrast to recent reports in other populations, BP had no independent relationship to plasma insulin in the American Indians in the present study. This is consistent with a previous study of the relationship between insulin resistance and BP in Pima Indians, in which no significant relationships were found in Pima men or women between insulin resistance, measured by a euglycemic clamp, and BP, although significant positive relationships were found in whites.41 Interestingly, in Mexican Americans, who have significant American Indian admixture, BP and insulin are correlated, but only weakly.42 It has been postulated that insulin and insulin resistance lead to hypertension43 either by enhancing renal sodium retention44 or through sympathetic overactivity.45 It is likely that these mechanisms are inoperative in this ethnic group.
In our study, we also were interested in the relationship between hypertension and lifestyle variables. When all covariates were included in a multivariate analysis, current alcohol consumption (measured in drinks per day) was a significant independent correlate of SBP and DBP in women and men. Many studies have demonstrated that alcohol consumption is associated with increased BP, and moderation of alcohol intake is almost universally suggested as a strategy for control of hypertension.46 47 48 49 Thus, lowering alcohol consumption should be promoted in these communities as a strategy for prevention and control of hypertension. Physical activity showed some trends with SBP in univariate analysis, but the relationships were inconsistent and did not achieve statistical significance, perhaps because activity level in this 45- to 74-year-old cohort was low in almost all participants. Increasing activity levels in this population might also affect BP control.
Univariate analysis indicated a negative relation between education level and SBP, although this negative relation was significant in the full multivariate model only for SBP in women. A negative association between education and hypertension prevalence has been observed in other studies.4 Smoking was inversely related to SBP and DBP in men and women. This relationship has also been observed in previous studies; although smoking is associated with an acute rise in BP,50 population studies show an overall lower BP in smokers.51 Despite its negative relationship with BP, smoking, as measured by pack-years, was associated with coronary heart disease in the Strong Heart Study participants52 and likely contributes to high rates of mortality from CVD and cancer in American Indian populations in whom smoking rates are high.53
In summary, these data from the Strong Heart Study indicate that despite high rates of diabetes and obesity, the prevalence of hypertension among American Indians in Arizona and Oklahoma is similar to national rates for whites, with lower levels and rates in Sioux Indians in South and North Dakota. Hypertension prevalence appears to be somewhat less strongly influenced by physiological variables such as obesity and insulin resistance than in other populations. On the other hand, BP and the prevalence rate of hypertension are significantly higher in individuals with diabetes and are strongly correlated with albuminuria. The IHS has an aggressive program for care of hypertension, and this is reflected in the observations of a relatively high percentage of hypertensive individuals taking medication and a higher proportion who are controlled than in NHANES III. Because of the strong predictive value of hypertension for the development of diabetes-associated complications such as retinopathy and renal disease and because of the strong association between hypertension and CVD in these communities,52 attention must be increasingly focused on the treatment and control of high BP in American Indian populations. Community-based programs to educate people about the dangers of hypertension, the importance of BP screening and medication use, and the possible effects of increased physical activity and reduced alcohol consumption should prove effective.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received November 16, 1995; first decision December 4, 1995; accepted April 10, 1996.
| References |
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