1 Faculty of Medicine, Ramathibodi Hospital, Mahidol University, 270 Rama VI Road, Rajdevi, Bangkok 10400, Thailand.
2 Institute for International Health, University of Sydney, PO Box 576, Newtown, NSW 2042, Australia.
3 Medical and Health Office, Electricity Generating Authority of Thailand, 53 Jaransanitwong, Bangkruay, Nonthaburi 11130, Thailand.
4 Faculty of Medicine, Prince of Songkhla University, Hat Yai, Sonkhla 90110, Thailand.
Correspondence:
Piyamitr Sritara, Department of Medicine, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, 270 Rama VI Road, Rajdevi, Bangkok 10400, Thailand. E-mail:
rapst{at}mahidol.ac.th
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Abstract |
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Methods In 1985, male and female employees of the Electricity Generating Authority of Thailand took part in a cardiovascular risk factor survey. In 1997, a follow-up survey was conducted and causes of death were determined for those subjects known to have died. Changes in levels of vascular risk factors over 12 years, and the associations of baseline risk factors with vascular mortality, were calculated.
Results The 1985 survey recruited 3499 volunteers (average age 43 years) of whom 23% were female. In 1997, vital status was determined for 3318 (95%) and 2967 (85%) of the study participants were resurveyed. Mean levels of systolic blood pressure (SBP) and diastolic blood pressure (DBP), body mass index, total cholesterol and high density lipoprotein (HDL) cholesterol all increased over the 12-year follow-up period. Over the same time, the prevalence of diabetes also rose but the proportion of current smokers decreased. Vascular diseases were the most frequent cause of death during follow-up (n = 46), were positively associated with baseline age, SBP, DBP, smoking, diabetes, male sex, and total cholesterol, and were negatively associated with HDL cholesterol.
Conclusions Levels of most vascular risk factors worsened over the 12-year period between 1985 and 1997. The associations between baseline risk factor levels and vascular mortality were consistent with those observed in other populations. Interventions that control vascular risk factors have the potential to avert much premature vascular disease in Thailand.
Accepted 23 December 2002
Worldwide, vascular diseases were the leading causes of death in 1999,1 when stroke and coronary heart disease were responsible for about 5.1 and 7.1 million deaths, respectively.1 While there are trends toward improvement in the rates of premature stroke and coronary heart disease in many industrialized countries,2,3 the converse is true of most developing countries.4 Since much of the worlds population lives in developing countries, the global burden of vascular disease is projected to rise considerably over the next two decades.4 In large part, this growth is likely to be explained by sociodemographic changes and associated deterioration in the levels of vascular risk factors in developing countries.5,6
In Thailand, vascular diseases have been the leading cause of death since 1987. In 1998 the five leading causes of death, in order, were heart disease, malignant neoplasms, accidents or poisonings, suicide or homicide or other injury, and hypertension or cerebrovascular disease. In 1998 there were over 54 000 deaths due to vascular causes7 and between 1985 and 1997the prevalence of heart disease in Thailand tripled, from 56 per 100 000 population to 168 per 100 000 population.8 Cross-sectional studies have provided some limited information about levels of vascular risk factors in different population groups within Thailand,915 but changes in risk factor levels with age and the associations of established vascular risk factors with mortality are not well documented. The Electricity Generating Authority of Thailand (EGAT) study was originally designed in 1985 as a cross-sectional study of vascular risk factor levels among employees. In 1997, the same individuals were resurveyed or else information about the cause of death was sought for those known to have died in the interim. The primary aim of these analyses was to describe 12-year changes in vascular risk factors in this cohort of subjects. A secondary aim was to determine the associations between baseline risk factors levels and the risk of vascular death.
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Methods |
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Blood samples were obtained after a 12-hour overnight fast. Serum total cholesterol, high density lipoprotein (HDL) cholesterol and triglycerides were measured using enzymatic-calorimetric assays (Boehringer Mannheim, Mannheim, Germany). High total cholesterol was defined as a total cholesterol level of 6.2 mmol/l or current use of cholesterol lowering therapy.17 Blood glucose levels were measured using a glucose oxidase method on capillary blood samples in 1985 (Reflocheck, Boehringer Mannheim, Mannheim, Germany) and on plasma samples in 1997 (Peridochrome, Boehringer Mannheim, Mannheim, Germany). On each occasion, oral glucose tolerance tests were performed by measuring blood glucose levels on fasting samples and on samples drawn 2 hours after the ingestion of a 75-g glucose load. Diabetes was defined on the basis of any of the following (1) a prior clinical diagnosis, (2) a fasting capillary glucose
6.1 mmol/l, (3) a fasting plasma glucose
7.0 mmol/l or (4) a 2-hour capillary or plasma glucose
11.1 mmol/l).18 Overweight or obesity was defined as body mass index (BMI)
25 kg/m2.19
Causes of death
An independent adjudication committee, of two cardiologists, one neurologist, one gastroenterologist and one internist, determined causes of death based upon all available evidence and using international criteria. Information about the likely cause of death was sought through review of information obtained from interviews with relatives and colleagues, medical notes, medical insurance reimbursement claims, records of hospital admissions, and death certificates. Deaths were classified as being due to one of the following eight broad causes: coronary heart disease (fatal myocardial infarction or sudden unexplained death), stroke (including subarachnoid haemorrhage), other vascular death (e.g. heart failure, valvular heart disease or peripheral arterial disease), respiratory disease (excluding malignancy), gastrointestinal or hepato-biliary disease (excluding malignancy), malignancy (of any kind), injuries (including suicide), or other.
Statistical methods
For analyses of changes in vascular risk factors over time, only those subjects for whom both baseline and follow-up data were available were included. For each survey (baseline and follow-up), sex-specific mean values of risk factors or proportions of the study population with each risk factor were calculated for continuous and categorical measures of exposure, respectively. Differences in the level or prevalence of each risk factor between the two surveys were tested using paired t-tests or McNemar tests.20 On each occasion, differences between risk factor levels in men and women were examined using t-tests for continuous data and 2 tests for categorical data. All P-values were based on two-tailed tests of significance. Secondary analyses compared those people aged 4754 years at the first survey with those in the same age group in the second survey. This age group was chosen since it represented the only age range that was common to the two surveys. Analyses were similar to the above, except that two-sample tests for independent samples were used.
Associations between risk factors and vascular death (coronary heart disease, stroke or other vascular death) were estimated using data on all subjects for whom data on vital status were available in 1997. Analyses were performed using Cox survival models that included age, sex, smoking, BMI, diabetes, total cholesterol, HDL cholesterol, and SBP or DBP. The estimates for SBP and DBP and total cholesterol were corrected for measurement error (regression dilution bias) using attenuation factors derived from the Asia Pacific Cohort Studies Collaboration.21
All statistical analyses were performed using SPSS software (SPSS Version 10, SPSS Inc., Chicago, IL, USA).
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Results |
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Changes in vascular risk factors levels between 1985 and 1997
The mean age of the study population was 42.5 years at baseline (1985) and 54.5 years at follow-up (1997). Mean levels of risk factors and proportions of individuals with abnormal risk factors levels are shown for men and women in Table 1. Over the 12-year study period, for both sexes, mean SBP, mean DBP, mean serum total cholesterol levels, mean serum HDL cholesterol levels, mean triglyceride levels and mean BMI all increased (all P < 0.001). Similarly, the proportion of the study population satisfying the criteria for hypertension, high total cholesterol levels, diabetes and obesity each rose markedly, for both men and women (all P < 0.001). In contrast, the proportion of current smokers decreased by approximately half in both sexes (both P < 0.001).
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Table 2 shows the same comparisons as Table 1
but for a common age group, 4754 years. The changes from 1985 to 1997 are similar to those seen for the cohort comparison in Table 1
, except that there is now no evidence of an increase in levels of triglycerides and the magnitude of differences is generally less, particularly where percentages of hypertension and diabetes are concerned.
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Discussion |
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The average rise in blood pressure (16 mmHg [SBP]/7 mmHg [DBP]) and the increase in the proportion of the population with hypertension (from 18% to 48%) were greater than would be expected for ageing alone.26,27 While minor methodological differences between the procedures used to measure blood pressure at the baseline and follow-up survey may have contributed to the observed differences, these are unlikely to account for all the changes observed. Studies that have compared supine and seated blood pressure measurements in the same subjects demonstrate that SBP is unaffected by posture while DBP is approximately 5 mmHg higher when measured with subjects in the seated position.28,29 Thus, while the observed increase in DBP might be explained by the altered posture during measurement, this difference in technique is unlikely to have accounted for the rise in SBP. Deteriorating dietary and behavioural practices of the study population most likely also contributed importantly to the rises in blood pressure observed.23 Similar levels of changes were observed for undiagnosed and treated hypertension.
There were also methodological differences in the techniques used for assay of glucose levels in 1985 (capillary blood) and 1997 (venous blood) and it is therefore not possible to reliably compare the mean glucose levels recorded on the two occasions. However, recent World Health Organization criteria for the diagnosis of diabetes provide definitions based on both capillary and plasma samples,18 and it is therefore possible to compare the prevalence of diabetes on the two occasions. The observed, almost threefold, rise in the prevalence of diabetes over the 12-year follow-up period is greater than the differences in the prevalence of diabetes observed in cross-sectional studies in other populations,30,31 or amongst the 4754 year olds at the two time points in this study. Again, therefore, the changes observed in the EGAT participants most likely comprise both the effects of the increased mean age of the cohort and other changes such as a greatly increased prevalence of overweight and obesity. Changes in sex-specific diabetes prevalence were almost completely due to diagnosed diabetes; undiagnosed diabetes hardly altered. This is partially explained by the effect of being in the study, i.e. as a result of diagnosis in 1985.
The proportion of smokers among the EGAT participants declined sharply during the 12 years of follow-up. Similar trends in smoking habits have been observed in other cross-sectional surveys conducted in Thailand, although the magnitude of the decrease observed in EGAT is greater than that observed in national surveys.3234 This may reflect a particularly good uptake of a national anti-smoking campaign by this well educated and affluent cohort.34 The observed decrease in smoking is all the more remarkable given recent evidence from Europe that, despite extensive public health campaigns, the prevalence of smoking has declined little over the last few years, even among those with established coronary heart disease.35
The risk factor levels and the changes recorded among the participants of the EGAT study are likely to be precise and reliable. However, the employees of EGAT are largely middle-class, well educated, urban-dwelling individuals receiving an above average level of income and medical care and as such, are not representative of the overall Thai population.36 Further, classic sources of bias in epidemiological studies are sure to have played a role: the healthy worker effect20 (employment of relatively healthy people); non-response (the response rate in the first survey was only 45%women had higher response rates than men and the older had higher response rates than the younger); and loss to follow-up (which probably led to underestimation of the effects of smoking and diabetes). Accordingly, the absolute levels of risk factors recorded in these surveys, and the absolute changes in risk factor levels over time, are unlikely to be generalizable to the Thai nation as a whole. It is likely, nonetheless, that the observed changes do give an indication of the most likely trends in risk factor levels in Thailand and that these data might usefully contribute to the development of national strategies for cardiovascular disease prevention.
In addition to clear data about risk factor levels, this study also provides the first ever information about the effects of vascular risk factors on the occurrence of major vascular events in a Thai population. The total number of fatal events recorded in the EGAT participants during the 12-year follow-up period was small and, consequently, evidence about the determinants of vascular death is rather imprecise. However, the associations observed are consistent with those seen in other much larger studies conducted in Eastern and Western populations,24,25 and confirm the likely importance of established vascular risk factors for the Thai population.
In summary, the changes in the levels of vascular risk factors observed in the EGAT study are consistent with the increase in mortality from vascular disease observed in Thailand over the last few decades.8 While trends in smoking were favourable, the deterioration in most other major vascular risk factors was substantial, and for blood pressure in particular, the increases appear to exceed those expected from ageing of the population alone. The implementation of interventions that improve levels of vascular risk factors have great potential to prevent much premature vascular disease in Thailand over the next few decades.
KEY MESSAGES
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Acknowledgments |
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References |
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