Blood pressure patterns and cardiovascular risk factors in ... - Nature

Journal of Human Hypertension (2000) 14, 489–496  2000 Macmillan Publishers Ltd All rights reserved 0950-9240/00 $15.00 www.nature.com/jhh

ORIGINAL ARTICLE

Blood pressure patterns and cardiovascular risk factors in rural and urban Gambian communities MAB van der Sande1, PJM Milligan1, OA Nyan1, JTF Rowley1, WAS Banya1, SM Ceesay1,2, WMV Dolmans3, Th Thien3, KPWJ McAdam1 and GEL Walraven1 1

Medical Research Council Laboratories, Fajara, The Gambia; 2DUNN Nutrition Unit Cambridge, UK; Department of General Internal Medicine, Nijmegen, the Netherlands

3

Hypertension is emerging as an important public health problem in sub-Saharan Africa. We studied blood pressure (BP) patterns, hypertension and other cardiovascular risk factors in a rural and an urban area of The Gambia. A total of 5389 adults (⭓15 years) were selected by cluster sampling in the capital Banjul and a rural area around Farafenni. A questionnaire was completed, BP, pulse rate, height and weight were recorded. Glucose was measured 2 h after a 75 g glucose load among participants ⭓35 years (n ⴝ 2301); total cholesterol, triglycerides, creatinine and uric acid were measured among a stratified subsample (n ⴝ 1075). A total of 7.1% of the study participants had a BP ⭓160/95 mm Hg; 18.4% of them had a BP ⭓140/90 mm Hg. BP was significantly higher in the urban area. BP increased with age in both sexes in both areas. Increasing age was the major independent risk factor for hypertension. Related cardiovascular risk factors (obesity, diabetes and

hyperlipidaemia) were significantly more prevalent in the urban area and among hypertensives; 17% of measured hypertensives were aware of this, 73% of people who reported to have been diagnosed as hypertensive before had discontinued treatment; 56% of those who reported being on treatment were normotensive. We conclude that hypertension is no longer rare in either urban or rural Gambians. In the urban site hypertension and related cardiovascular risk factors were more prevalent. Compliance with treatment was low. Interventions aimed at modifying risk factors at the population level, and at improving control of diagnosed hypertension are essential to prevent future increases of cardiovascular morbidity and mortality. In view of limited resources and feasibility of intervention in rural Gambia, these could initially be directed towards urbanised populations. Journal of Human Hypertension (2000) 14, 489–496

Keywords: blood pressure; cardiovascular risk factors; sub-Sahara Africa; urban-rural

Introduction Cardiovascular disease (CVD) is the main cause of death in industrialised countries, and is increasingly recognised as a significant cause of morbidity and mortality in sub-Saharan Africa (sSA).1 Hypertension is the most common cardiovascular condition in the world. The risk of CVD associated with hypertension is independent of other risk factors.2 Studies in sSA also show that high blood pressure (BP) is a major independent risk factor for stroke, heart and kidney failure, and for all-cause mortality.3,4 Hypertension has been considered to be rare in traditional rural communities in sSA, and little increase of BP with age was observed.5,6 Associated with increased urbanisation and westernisation of lifestyles, reported prevalences of hypertension have risen. Several studies in sSA report higher prevalences in urban compared to rural areas,7,8 whereas others document no significant difference,9 or even higher rural than urban prevalences.10 A Correspondence: Dr M van der Sande, MRC, PO Box 273, Banjul, The Gambia. Fax: 00.220.495919, E-mail: mvdsande얀mrc.gm Received 2 February 2000; revised and accepted 7 April 2000

study in South Africa suggested that the degree of urbanisation predicted the extent to which hypertension prevalence increased with age.11 A longitudinal study in Kenya documented an almost immediate rise in BP upon migration from a rural to an urban environment.12 The cardiovascular risks associated with hypertension are substantially modified by the presence or absence of other risk factors, such as (central) obesity, diabetes, smoking, physical inactivity and hyperlipidaemia, particularly hypercholesteraemia. This emphasises the importance of a multi-factorial approach to risk assessment and interventions. The main environmental risk factors for hypertension are associated with a shift towards a more westernised and urbanised society; in particular changes in diet (more salt) and changes in lifestyle (less physical activity, more psychosocial stress).13 Similar factors are also associated with an increase in the prevalence of obesity and diabetes, which are independent cardiovascular risk factors and at the same time risk factors for hypertension. The association between hypertension and obesity is reported to be constant across different geographical settings.14 With the progression of a demographic and epide-

BP patterns in urban and rural Gambia MAB van der Sande et al

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miological transition in sSA, the prevalence of hypertension and other cardiovascular risk factors is expected to rise.2,15,16 Treatment needs to be complemented by preventive strategies, which should focus on modifiable risk factors. In a previous nationwide rapid assessment we found that 9.5% of adults above the age of 15 had a BP ⭓160/95 mm Hg.17 The aim of the present study was to assess in more detail BP patterns, and to investigate the prevalence of hypertension and other cardiovascular risk factors in rural and urban settings in The Gambia.

Materials and methods Study area The Gambia is a small West-African country, extending 20–50 km on either side of the River Gambia for about 400 km inland, and situated on the edge of the arid Sahel belt. According to the 1993 census the population was just over 1.03 million, growing at an annual rate of 4.2%. Population density was high (97 km2). Adult literacy rate was low, especially among rural females (18%). The predominant religion (95%) was Islam. Life expectancy was 58 years for men and 60 for women.18 The GNP per capita for the country in 1995 was US$ 325. Between October 1996 and June 1997 we conducted a community-based survey to describe and assess the prevalence of several major non-communicable diseases in an urban community (the capital Banjul) and a rural community (Farafenni area, approx. 150 km inland). Banjul is situated on an island in the River Gambia and has a population of around 40 000. It is the oldest urban settlement in the country. Many of those working in Banjul are involved in small scale industry, trade, crafts, or are employed in the civil service. The vast majority of the population in the villages in the rural Farafenni area, as with most of the rural population in the Gambia, is involved in subsistence farming. The main crops cultivated are millet, groundnuts and rice. The rural Gambian population is poor: in 1993, 45% has an income below US$ 150/year, compared to 7% in the capital Banjul.19 Study design A stratified cluster sample survey was carried out in these two populations among adults aged 15 years and above. The sampling unit was the household in Banjul (selected from a pre-survey enumeration exercise). In the Farafenni area, a demographic surveillance programme had been going on for the previous 15 years, and 20 of the 40 villages taking part in this were randomly selected for participation. In the urban site, subjects were requested to report to a local community centre, whereas mobile clinics were organised in the rural villages. After obtaining informed, signed consent, a general questionnaire was administered by a field worker in the local language, focusing on symptoms of non-communicable diseases, drug use, smoking, alcoholic intake, previous medical history, family history, health seeking behaviour, and socio-economic status. Journal of Human Hypertension

BP and pulse rate were recorded in a standardised way with a validated oscillometric automated digital BP machine (OmronHEM-705CP, Japan20) after a 5min rest, in a seated position with the left arm at heart level. The mean of two readings was used in analysis. Height and weight, waist and hip circumference were measured using standardised equipment. Among participants 35 years and older, an adapted oral glucose tolerance test (OGTT) was performed (n ⫽ 2301). Venous blood glucose was determined with a portable glucose meter (Haemocue-B, AB Haemocue, Sweden). At the same time, among a stratified subsample (one in 10 of participants 15 to 34 years of age; one in five of participants aged 35 and above) (n ⫽ 1075) an additional blood sample was collected. This was analysed for total cholesterol, triglycerides, uric acid and creatinine using a centrifugal biochemical analyser (Cobas Fara, Roche, UK) at the MRC Biochemistry Laboratory in Fajara; daily quality control checks were performed using commercial serum samples. Definitions Following 1993 WHO guidelines, hypertension was defined as a BP ⭓160 mm Hg systolic and/or a BP ⭓95 mm Hg diastolic and borderline hypertension as a BP ⭓140 mm Hg systolic and/or a BP ⭓90 mm Hg diastolic. Since 1998, WHO uses a BP ⭓140/90 mm Hg as cut off for hypertension. Normotensive people who reported to be on antihypertensives, were not included in our definition, but are reported on separately, as we had no means of verifying these data. Mean arterial pressure (MAP) was calculated as the sum of the diastolic BP (DBP) and one-third of the pulse pressure (mm Hg). Obesity was defined as a body mass index (BMI ⫽ weight/height2) ⭓30 kg/m2, and diabetes as a whole blood 2 h glucose after a 75-g glucose load ⭓10.0 mmol/L or currently on antidiabetic medication (and therefore excluded from confirmation by the OGTT). People with a waist/hip ratio in the upper quartile of their sex-specific distribution were defined as centrally obese. Participants who reported that they spent more than half the day on their feet or that they were involved with daily exercise were classified as physically active. Those who spent less than half the day on their feet or led a sedentary life were classed as physically inactive. In line with international high-risk cut-off levels,21 we defined an elevated cholesterol concentration as ⭓5.2 mmol/L (200 mg/dl), elevated triglyceride concentration as ⭓1.8 mmol/L (160 mg/dl), elevated creatinine concentration as ⭓90 ␮mol/L (1.02 mg/dl) for women or ⭓100 ␮mol/L (1.13 mg/ dl) for men, and elevated uric acid concentration as ⭓0.36 mmol/L (6.1 mg/dl) for women or ⭓0.42 mmol/L (7.1 mg/dl) for men. Analysis Data were analysed using the Stata package version 6.0 (Stata Corporation, TX, USA). Linear regression models were used to assess the relationship of BP


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Table 1 General characteristics of the urban (Banjul) and rural (Farafenni area) study populations

No. Mean age (s.d.) % married Ethnic group (%): Mandinka Wollof Fula Serere Other Occupation (%): Manual non-skilled Manual skilled Non-manual Other Education (%): None Islamic only Formal

Urban men

Urban women

Rural men

Rural women

1028 31.7 (14.8) 34.7

1138 33.7 (15.9) 47.9*

1200 38.5 (18.4) 64.3

2023 37.2 (16.4) 77.9*

19.1 29.9 17.5 10.5 22.3

16.7 29.8 10.7* 17.4* 25.4

42.5 30.4 23.3 0.2 3.7

48.0* 30.3 19.1* 0.3 2.3

8.1 44.4 12.6 34.8

43.1* 24.9* 8.5* 23.6*

77.2 9.4 2.9 10.6

94.3* 0.4* 0.3* 5.1*

8.0 26.2 65.9

23.3* 23.1 53.6*

7.3 84.9 7.8

19.8* 77.9* 2.3*

Bold: P ⬍ 0.05 for urban (men and women combined) vs rural (men and women) study area. Occupation = other: includes people not working and students. * P ⬍ 0.05 for men vs women within each area of residence.

with other variables; univariate and multiple logistic regression models for complex survey designs were used to calculate odds ratios. Statistical significance was assigned by a two-sided alpha level of 0.05. The study was approved by the Gambia Government/MRC Ethics Committee.

Results We collected data from 2166/3202 (67.7%) eligible adults in urban Banjul and from 3223/3699 (87.1%) of eligible adults in the rural villages around Farafenni. Overall participation was 78.1%. Mean age was 35.4 years; 58.5% were female. General characteristics of the male and female population of the urban and the rural community are summarised in Table 1. The urban population was younger (mean age 32.2 vs 37.7 rural), with a larger male proportion (47.6% vs 37.3%). There were marked differences between the two study areas in ethnic composition, occupation and education. Figure 1a and 1b show the distribution of SBP and

Figure 1 (a) Mean systolic BP and (b) diastolic blood pressure by area of residence.

DBP among the urban and rural study populations. Mean systolic BP (SBP) and DBP were significantly higher in the urban area compared to the rural area, and among men compared to women (Table 2a). Adjusting for the younger mean age and higher proportion of men in the urban sample, mean BP was significantly higher in the urban area (SBP +5.1 mm Hg (95% CI 4.0–6.3); DBP +4.7 mm Hg (95% CI 4.6–5.8)). Figure 2 shows the combined effect of area and sex on the rise in MAP, SBP, and DBP with age. In all age groups, the MAP was higher for men than for women, and higher in the urban area than in the rural area. Mean DBP in rural women remained low below age 35, and started to increase with age from age 35 onwards; mean SBP started to rise among rural women only after the age of 45. Mean pulse rate was higher in women than in men in all age groups in both areas, except in the urban area among people over 55 years. Figure 3 shows the sharply increasing hypertension prevalence from age 35 among men and women, both urban and rural. Mean BP of people who had immigrated into the area in the last 5 years was similar to that of long term residents of similar age (data not shown). A total of 210 women reported being pregnant; their mean BP was not significantly different from the BP of non-pregnant women of the same age. Hypertension (BP ⭓160/95 mm Hg) was recorded in 380/5372 (7.1%) participants; 988/5372 (18.4%) participants had a BP ⭓140/90 mm Hg. If people with BPs in the normal range but reporting concurrent use of antihypertensive medication were also defined as hypertensive, these percentages were 7.6% and 19.0% respectively. Hypertension prevalence was slightly higher in the urban study site. A BP ⭓160/95 mm Hg was found in 162/2152 (7.5%) urban and 218/3220 (6.8%) rural participants (P ⫽ 0.3; Table 2a). If people reporting concurrent use of antihypertensive drugs were considered hypertensive as well, hypertension was found in 8.9% of urban and 6.9% of rural participants (P ⬍ 0.01). A Journal of Human Hypertension


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Table 2 Cardiovascular data by area and sex (mean (s.d.) and %) (a) Study population

No. Age (years) Systolic BP (mm Hg) Diastolic BP (mm Hg) Pulse rate (beats/min) % BP ⭓140/90 mm Hg % BP ⭓160/95 mm Hg

(b) Hypertensive populations

Urban men

Urban women

Rural men

Rural women

1028 —

1138 —

1200 —

2023 —

126.0 (21.1) 121.0* (23.2) 124.4 (23.3) 117.3* (23.8) 73.4 (12.1) 73.2 (12.5) 71.0 (11.9) 68.9* (12.5) 80.8 (14.4) 82.6* (13.0) 81.5 (14.0) 77.9* (14.2) 22.0 16.9* 20.6 16.0*

Urban men

Urban women

Rural men

Rural women

77 85 91 127 51.8 (17.4) 53.7* (17.0) 62.2 (14.7) 57.2* (14.3) 172.7 (23.4) 174.4 (25.7) 181.1 (22.3) 179.7 (22.8) 98.6 (11.3) 99.7 (15.3) 95.7 (12.7) 95.5 (14.9) 89.3 (15.5) 86.3 (14.6) 79.4 (16.5) 86.1* (17.7)

7.5

7.3

7.6

6.3

BMI (kg/m2) % obese

20.8 (3.4) 1.8

23.9* (5.9) 12.2*

19.8 (2.5) 0.1

20.5* (3.1) 1.1*

23.7 (4.3) 7.8

27.1 (6.5) 33.3*

20.8 (3.3) 1.1

20.6 (4.5) 3.1

Glucosea (mmol/L) % diabetesa

6.5 (2.5) 7.9

7.1 (2.9) 8.7

6.1 (1.6) 2.2

6.0 (1.2) 0.8*

7.6 (2.9) 17.2

8.4 (4.2) 19.4

6.5 (1.5) 4.7

6.6 (1.5) 4.2

Cholesterol (mmol/L)b Triglycerides (mmol/L)b Uric acid (mmol/L)b Creatinine (␮mol/L)b

4.1 0.68 0.32 82.6

(1.1) (0.39) (0.08) (49.8)

4.6 0.68 0.25 66.0

(1.2) (0.44) (0.07) (37.4)

3.6 0.81 0.30 79.8

(0.9) (0.37) (0.06) (18.6)

3.9 0.76 0.21 60.7

(0.9) (0.37) (0.06) (21.4)

4.9 1.13 0.36 95.0

(1.1) (0.43) (0.10) (88.4)

5.0 0.79 0.29 80.5

(1.2) 4.0 (0.8) (0.42) 1.17 (0.33) (0.10) 0.35 (0.07) (73.4) 95.6 (18.4)

4.4 0.83 0.23 71.8

(0.9) (0.33) (0.07) (32.9)

% elevated cholesterolb % elevated triglycerideb % elevated uric acidb % elevated creatinineb

12.5

29.1*

2.3

8.3*

24.5

39.1

4.1

13.7*

4.0

0.6

4.3

2.1

4.1

4.7

1.4

2.9

7.4 15.3

8.5 8.2*

1.4 13.0

1.3 1.4*

18.4 22.2

28.1 10.9

6.8 27.0

6.9 6.9*

% physically active % smokers: current ever

50.8 34.1 46.9

30.2* 1.5* 3.0*

51.9 42.2 57.7

42.0* 5.9* 7.2*

55.8 29.3 45.3

27.4* 4.8* 5.6*

36.3 31.5 56.2

26.0 15.0* 18.9*

BP, blood pressure; BMI, body mass index. *P ⬍ 0.05 between adjacent columns. a Among people aged 35 and above, 2 h after 75 g glucose load. b Means and proportions weighted to reflect stratified subsample.

BP ⭓140/90 mm Hg was diagnosed in 417/2152 (19.4%) of the urban and 571/3320 (17.7%) of the rural population (P ⫽ 0.1). If people who reported concurrent use of antihypertensives were included, prevalences were 20.3% and 17.8% (P ⫽ 0.02) respectively In total, 222 subjects (67% of them were urban residents) reported they had been diagnosed as hypertensive by a doctor. Of the hypertensives, only 66/378 (17%) had been diagnosed as such before, but 37/63 (59%) of them had discontinued treatment. Totals of 58/222 (27%) of ever-diagnosed hypertensives reported current use of antihypertensives; the majority (91%) being urban participants; 26/58 (44%) of the people on treatment were still diagnosed as hypertensive (mean BP 183/101 mm Hg); while the others (56%) on treatment were normotensives. Table 2a shows the prevalence of hypertension and other cardiovascular risk factors by area and sex. Mean pulse was significantly higher in the urban area, just as mean BPs. Obesity, diabetes, hyperlipidaemia and physical inactivity were significantly less prevalent in the rural population than the urban. Overall, smoking was more common in the rural than in the urban study population, and more men than women smoked. Rural smoking frequently Journal of Human Hypertension

meant smoking of tobacco leaves; urban smoking usually implied cigarette smoking. Reported alcohol consumption was negligible in this Islamic population. These general urban-rural differences were comparable to those found among the hypertensive population. Coexistence of other cardiovascular risk factors was more prevalent among the urban than the rural hypertensives. The rural hypertensives had higher mean SBP and lower mean DBP than those in the urban area. (Table 2b). Isolated hypertension, that is hypertension in the absence of other metabolic cardiovascular risk factors examined in this study (diabetes, obesity, hyperlipidaemia), was significantly more prevalent in the rural (132/3171, 4.0%) compared to the urban (40/2107, 1.9%) study population (P ⬍ 0.001). Among the hypertensives only, hypertension was an isolated finding in 132/169 (78.1%) of the rural vs 40/117 (34.2%) of the urban hypertensive participants (P ⬍ 0.001). Obesity and diabetes were significantly more prevalent among hypertensive compared to nonhypertensive participants, both in urban and rural areas (P ⬍ 0.01; Figure 4). Waist circumference, an aggregate measurement of total and abdominal fat accumulation, was positively correlated with BP (P ⬍ 0.001). Central obesity was significantly more


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Figure 4 Obesity and diabetes prevalence by hypertension status, by area. HT, hypertensive (n = 218 rural; n = 162 urban); nonHT, normotensive (n = 3002 rural; n = 1990 urban). *Prevalence of diabetes and of obesity was significantly higher among hypertensives in each area (P ⬍ 0.01).

Figure 2 Mean arterial pressure (MAP), blood pressure (BP) and pulse rate by age group, area and sex.

prevalent among people with hypertension: 52.7% of the hypertensives had a waist/hip ratio in the upper quartile of their sex-specific distribution, vs 22.9% of the non-hypertensives (P ⬍ 0.001). Independent risk factors for hypertension were essentially comparable in the urban and rural com-

Figure 3 Prevalence of hypertension by age, sex and area.

munities (Table 3). Increasing age was the main risk factor. Other independent risk factors associated with hypertension in both study areas, statistically significant after adjustment for age and sex, were (central) obesity, diabetes, and physical inactivity. In the urban area, people who had received formal primary or secondary education were less likely to be hypertensive. Adjusted for age and sex, urban residence was a nearly significant risk factor for hypertension (OR ⫽ 1.8, 95% CI 0.9–3.5). Sex, ethnic group, occupation, family history of hypertension, current or ever smoking history, were not associated with hypertension in either urban or rural communities. Adjusted for age, sex and area of residence, hypertension was independently associated with elevated creatinine concentration (OR 3.6, 95% CI 1.7–7.4). This association was not dependent of hypertension treatment or of obesity. There was no independent association of hypertension with elevated triglyceride, total cholesterol or uric acid concentration. We repeated the analysis to assess independent risk factors associated with a BP ⭓140/90 mm Hg, which is the revised 1999 WHO definition for hypertension.22 Crude and adjusted odds ratios were comparable to results reported for associations with a BP ⭓160/95 mm Hg. Again, increasing age was the main independent risk factor. The association of urban residence with a BP ⭓140/90 mm Hg became statistically significant (OR ⫽ 2.0, 95% CI 1.2–3.2).

Discussion The main finding of our study is that hypertension in no longer rare in either urban or rural Gambia: 19.4% of the urban and 17.4% of the rural population had a BP ⭓140/90 mm Hg. The prevalence of hypertension and the mean pressures we recorded in this study are similar to the data reported for the West Africa study sites in the International Collaborative Study of Hypertension in Blacks,7 but markedly higher than historical data from the Gambia reported in the 1970s.23,24 As in the previous nationwide study we did not observe a significant independent association of urban or rural residence with being hypertensive (BP ⭓160/95 mm Hg), although with the more recent definition (BP ⭓140/90 mm Hg), hypertension was significantly more comJournal of Human Hypertension


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Table 3 Crude and adjusted odds ratios (OR, 95% CI) for hypertension by area Crude OR urban Age vs age 15–24 age 25–34 age 35– 44 age 45–54 age 55+ Female Male

Obese Centrally obese Diabetic Ever smoker Physically active Formal education Non-manual occupation

1 1.5 (0.6–3.4) 9.2 (4.6–18.5) 16.2 (7.9–33.2) 34.2 (17.9–65.8) 1 1.0 (0.7–1.4)

3.1 4.5 3.3 1.0 0.8 0.3 1.3

(2.0– 4.7) (3.3–6.3) (1.9–5.9) (0.7–1.5) (0.6–1.1) (0.2–0.4) (0.8–2.1)

Crude OR rural 1 1.2 (0.4 –3.7) 3.6 (1.5–8.8) 13.5 (6.0–30.4) 39.7 (18.5–85.5) 1 1.2 (0.9–1.6)

3.9 3.2 4.3 1.5 0.6 0.6 1.0

(1.4 –10.6) (2.5– 4.3) (1.8–10.4) (1.1–2.0) (0.4 –0.9) (0.4 –0.8) (0.6–1.7)

Age/sex adjusted ORs urban 1.4 (0.9–2.2) 1.6 (1.1–2.3) 2.9 (1.6–5.2) 0.6 (0.4 –1.0) 0.5 (0.4 –0.7) 0.1 (0.01–0.7) 0.4 (0.1–2.7)

Age/sex adjusted ORs rural 3.5 (1.2–10.7) 1.6 (1.2–2.2) 2.5 (1.0–6.3) 0.9 (0.6–1.3) 0.7 (0.5–0.9) 0.5 (0.1– 4.1) 0.6 (0.1–5.0)

In bold, ORs which are statistically significant.

mon in the urban area. Even in a small country like The Gambia, with good urban-rural communication, different patterns emerge when comparing hypertensives in urban and rural populations. The slightly older rural hypertensive population had a significantly higher systolic pressure but a lower diastolic pressure than the urban population. The rise of BP with age, which could be related to cumulative effects over life span of various environmental exposures,25 and which is considered to be absent in some traditional societies, was more marked in the urban than in the rural community, more in men than in women (Figure 2). The later rise of BP with age we observed among rural women in particular might suggest more traditional patterns are still manifest in this group. Similarly, a significant rise of BP with age was reported in urban Xhosa, and little rise with age in rural Xhosa in South Africa.26 According to the ‘rule of halves’, half of the hypertensive population is diagnosed and known as such, half of the people known to be hypertensive are on treatment, and that half of those who are on treatment actually become normotensive.22 This was not a follow-up study of diagnosed patients on treatment and therefore knowledge of hypertension status and drugs may not always be correct; previous diagnosis may have been incorrect as well. Nevertheless, it is encouraging that we recorded normotensive BPs (⬍160/95 mm Hg) for 56% of the people who reported being on antihypertensive treatment, suggesting that available treatment can be as effective at lowering BP in The Gambia as elsewhere. Unfortunately, compliance with treatment among diagnosed hypertensives was low. This may be related to the quality of advice supplied, the high cost of medication, the investment of time needed or to a limited perceived benefit by asymptomatic people, who may also experience side effects of therapy.27 The association between increased creatinine and high BP regardless of residence suggests that hypertension causes renal impairment even in the rural areas, although not yet manifest as clinical disease. Optimising existing primary care services has Journal of Human Hypertension

been shown to have a marked impact on adherence to treatment and on control of hypertension.28 Decentralising care of hypertensive patients to primary care clinics could therefore have a marked impact on compliance and control. The use of a validated automated electronic BP machine has the potential to improve the reliability and reproducibility of BP measurements in primary care centres, and is gradually becoming common practice in (larger) field studies.7 It is relatively easy to train non-professionals in its use, to check it is used correctly, and observer bias is much reduced. The increased use of objective BP measurements will improve reliability, facilitate hypertension control at primary care level, and enhance comparability between studies. We established in this study the frequent co-existence of other measured cardiovascular risk factors (obesity, diabetes, physical inactivity, hyperlipidaemia) in the urban area in particular, implies a significant threat of impending CVD. Whereas hypertension was associated with similar risk factors both in the urban and the rural site (Table 3), the prevalence of these risk factors was markedly lower in the rural area, except smoking (Table 2b). Our data show that in the capital Banjul, hypertension, obesity, diabetes and hyperlipidaemia frequently occur in the same people; this suggests that the metabolic syndrome may be emerging in this urban population.29 In contrast, nearly 80% of the hypertension in the rural study area was an isolated observation, not associated with obesity, diabetes or hyperlipidaemia. This is important as the cardiovascular risks associated with hypertension are substantially modified by the co-existence of other risk factors. The large proportion, particularly of men, who reported current smoking in our study was alarming. Cigarette companies are shifting their operations to low income countries, where antitobacco measures are less prohibitive than in western countries.30 Health promotion programmes to reduce smoking, together with appropriate legislation to discourage marketing are urgently needed


to prevent the many devastating cardiovascular and other consequences of smoking. Primordial and primary prevention of CVD should focus on prevention of relevant risk factors such as increasing regular physical activity, modification of dietary salt and fat intake, discouragement of smoking, as well as hypertension control. The general effectiveness of antihypertensives on cardiovascular morbidity and mortality through BP reduction has been clearly demonstrated; however compliance with treatment was very low, particularly in the rural area. At a population level, greater cardiovascular benefit might be obtained by reducing the mean population BP even by as little as 2–3 mm Hg, and by modifying other CVD risk factors.31,32 Such a population-wide intervention programme has been successfully implemented in Mauritius.33 People of African origin often display more sodium sensitivity than non-Africans. Therefore, a modest reduction in sodium intake could considerably reduce BP and cardiovascular morbidity.34 Whereas successful health education on these issues might be quite challenging in scattered rural villages, it should be feasible to develop an integrated approach focused on the urban population, where hypertension and other key cardiovascular risk factors are most prevalent. Health education in rural Gambian areas can be integrated in the primary health care system. The lower prevalence of hypertension and other risk factors make it less likely that cardiovascular disease will emerge as a significant health problem among the rural population in the near future.35 Although it is obvious that hypertension is common in rural areas as well the cardiovascular risk of a patient is determined to a large extent by other risk factors, rather than by the level of BP alone.22 In conclusion, this study has documented that hypertension is no longer rare in both urban and rural Gambian populations. Most other cardiovascular risk factors were significantly more prevalent in the urban study site. Innovative strategies must be developed to encourage health care workers and those people who are diagnosed as hypertensive to modify behaviour that reduces other cardiovascular risk factors as well maintain long term follow-up and continue treatment. Interventions and preventive activities could initially be directed towards the urban population, among whom the prevalence of hypertension and of other risk factors is higher and who are thus most at risk of related morbidity.

Acknowledgements This study was funded by the Medical Research Council, UK. We are grateful to the Department of State for Health, in particular the former director of Health Services Dr Alieu Gaye, for their support. We thank all participants for their co-operation, the field workers and MRC support services for their dedication, and Dr Maarten Schim van der Loeff for valuable comments on earlier drafts of this paper. We record our gratitude to Prof Eoin O’Brien for expert advice on BP measurement before we commenced this study.

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References 1 World Bank. World Development Report 1993. Investing in Health. Oxford University Press for the World Bank, 1993, New York. 2 Muna WFT. Cardiovascular disorders in Africa. World Health Stat Q 1993; 46: 125–133. 3 Mensah GA, Barkey NL, Cooper RS. Spectrum of hypertensive target organ damage in Africa: a review of published studies. J Hum Hypertens 1994; 8: 799–808. 4 Kaufman JS et al. The mortality risk associated with hypertension: preliminary results of a prospective study in rural Nigeria. J Hum Hypertens 1996; 10: 461– 464. 5 Shaper AG, Wright DH, Kyobe DH. Blood pressure and body build in three nomadic tribes of northern Kenya. East Afr Med J 1969; 46: 273–281. 6 Truswell AS, Kenelly BM, Hansen JDL, Lee RB. Blood pressure of the !Kung bushmen in Northern Botswana. Am Heart J 1972; 84: 5–12. 7 Cooper R, et al. The prevalence of hypertension in seven populations of West African origin. Am J Public Health 1997; 87: 160–168. 8 Seedat YK, Seedat MA, Hackland DBT. Prevalence of hypertension in the rural and urban Zulu. J Epidemiol Community Health 1982; 36: 256–261. 9 Nair P, Nyamphisi M, Yarnell JW. Lack of difference in blood pressure between the urban and rural population in Lesotho, Africa. Centr Afr J Med 1994; 40: 278–281. 10 M’Buyamba-Kabangu J-R et al. Correlates to blood pressure in rural and urban Zaire. J Hypertens 1987; 5: 371–375. 11 Steyn K et al. Hypertension in the black community of the Cape Peninsula South Africa. East Afr Med J 1996; 73: 758–763. 12 Poulter NR et al. The Kenyan Luo migration study: observations on the initiation of a rise in blood pressure. BMJ 1990; 300: 967–972. 13 Grim CE, Henry JP, Myers H. High blood pressure in blacks: salt, slavery, survival, stress, and racism. In: Larach JH, Brenner BM (eds). Hypertension: Pathophysiology, Diagnosis and Management. Second Edition. Raven Press Ltd: New York, 1995, pp 171–207. 14 Kaufman JS et al. Obesity and hypertension prevalence in populations of African origin. Epidemiology 1996; 7: 398– 405. 15 Beevers DG, Prince JS. Hypertension: an emerging problem in tropical countries. Trans R Soc Trop Med Hyg 1991; 85: 324 –326. 16 Mosley WH, Bobadilla J-L, Jamison DT. The health transition: implications for health policy in developing countries. In: DT Jamison DT et al (eds). Disease Control Priorities in Developing Countries. World Bank: New York, 1993, pp 673–699. 17 Van der Sande MAB et al. Nation-wide prevalence study of hypertension and related non-communicable diseases. Trop Med Int Health 1997; 2: 1039–1048. 18 Central Statistics Department. Population and Housing Census 1993: Mortality Analysis and Evaluation. Central Statistics Department, Banjul, 1998. 19 Ministry of Finance and Economic Affairs. 1993– 4 Household Education and Health Survey Report The Gambia. Central Statistics Department, Banjul, 1995. 20 O’Brien E, Mee F, Atkins N, Thomas M. Evaluation of three devices for self-measurement of blood pressure according to the revised British Hypertension Society Protocol: the Omron HEM-705CP, Philips HP5332, and Nissei DS-175. Blood Press Monit 1996; 1: 55–61. 21 European Atherosclerosis Society. Eur Heart J 1987; 8: 77–88.

Journal of Human Hypertension


496

22 1999 World Health Organization – International Society of Hypertension guidelines for the Management of Hypertension. J Hypertens 1999; 17: 151–183. 23 Wilkins HA. Schistosoma Haematobium in a Gambian community. III The prevalence of bacteriuria and hypertension. Ann Trop Med Parasitol 1977; 71: 179–186. 24 Vaughan JP, Miall WE. A comparison of cardiovascular measurements in the Gambia, Jamaica and the United Republic of Tanzania. Bull WHO 1979; 57: 281–289. 25 Kaufman JS, Owoaje EE, Rotimi CN, Cooper RS. Blood pressure change in Africa: case study from Nigeria. Hum Biol 1999; 71: 641–657. 26 Sever PS, Gordon D, Peart WS, Beighton P. Bloodpressure and its correlates in urban and tribal Africa. Lancet 1980; ii: 60–64. 27 Pobee JOM. Community-based high blood pressure programs in sub-Saharan Africa. Ethnic Dis 1993; 3: S38–S45. 28 Coleman R, Gill G, Wilkinson D. Non-communicable disease management in resource-poor settings: a pri-

Journal of Human Hypertension

29 30 31 32 33

34 35

mary care model from rural south Africa. Bull WHO 1998; 76: 633–640. Laws A, Reaven GM. Insulin resistance and risk factors for coronary heart disease. Baillieres Clin Endocrinol Metab 1993; 7: 1063–1078. Dagli E. Are low income countries targets of the tobacco industry? Int J Tuberc Lung Dis 1999; 3: 113–118. Cooper RS et al. Hypertension treatment and control in sub-Saharan Africa: the epidemiological basis for policy. BMJ 1998; 316: 614 –617. Rose G. Strategy of prevention: lessons from cardiovascular disease. BMJ 1981; 282: 1847–1851. Dowse GK et al. Changes in population cholesterol concentration and other cardiovascular risk factor levels after five years of the non-communicable disease intervention programme in Mauritius. BMJ 1995; 311: 1255–1259. Kaplan NM. Ethnic aspects of hypertension. Lancet 1994; 344: 450– 452. Swai ABM et al. Low prevalence of risk factors for coronary heart disease in rural Tanzania. Int J Epidemiol 1993; 22: 651–659.