© 1999 American Heart Association, Inc.
Original Contributions |
Socioeconomic Status and Determinants of Hemostatic Function in Healthy Women
From the Department of Public Health Sciences, Division of Preventive Medicine, Karolinska Institute, Stockholm, Sweden (S.P.W., M.H., K.O.-G.); the Department of Epidemiology, Harvard School of Public Health, Boston, Mass (M.A.M.); and the Department of Cardiology (M.E., K.S.-G.), and the Atherosclerosis Research Unit, King Gustaf V Research Institute (A.H., A.S.), Karolinska Hospital, Stockholm, Sweden.
Correspondence to Sarah P. Wamala, MSc, Karolinska Institutet, Department of Public Health Sciences, Division of Preventive Medicine, Novum Plan 7, Post fack 30, S-141 57 Huddinge, Sweden. E-mail Sarah.Wamala{at}phs.ki.se
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Abstract |
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Abstract—Hemostatic factors are reported to be associated with coronary heart disease (CHD). Socioeconomic status (SES) is 1 of the determinants of the hemostatic profile, but the factors underlying this association are not well known. Our aim was to examine determinants of the socioeconomic differences in hemostatic profile. Between 1991 and 1994, we studied 300 healthy women, aged 30 to 65 years, who were representative of women living in the greater Stockholm area. Fibrinogen, factor VII mass concentration (FVII:Ag), activated factor VII (FVIIa), von Willebrand factor (vWF), and plasminogen activator inhibitor-1 (PAI-1) were measured. Educational attainment was used as a measure of SES. Low educational level and an unfavorable hemostatic profile were both associated with older age, unhealthful life style, psychosocial stress, atherogenic biochemical factors, and hypertension. Levels of hemostatic factors increased with lower educational attainment. Independently of age, the differences between the lowest (mandatory) and highest (college/university) education in FVII:Ag levels were 41 µg/L (95% confidence interval [CI], 15 to 66 µg/L, P=0.001), 0.26 g/L (95% CI, 0.10 to 0.42 g/L, P=0.001) in fibrinogen levels, and 0.11 U/mL (95% CI, 0.09 to 0.12 U/mL, P=0.03) in levels of vWF. The corresponding differences in FVIIa and PAI-1 were not statistically significant. With further adjustment for menopausal status, family history of CHD, marital status, psychosocial stress, lifestyle patterns, biochemical factors, and hypertension, statistically significant differences between mandatory and college/university education were observed in FVII:Ag (difference=34 µg/L; 95% CI, 2 to 65 µg/L, P=0.05) but not in fibrinogen (difference=0.03 g/L; 95% CI, -0.13 to 0.19 g/L, P=0.92) or in vWF (difference=0.06 U/mL; 95% CI, -0.10 to 0.22 U/mL, P=0.45). An educational gradient was most consistent and statistically significant for FVII:Ag, fibrinogen, and vWF. Age, psychosocial stress, unhealthful life style, atherogenic biochemical factors, and hypertension mediated the association of low educational level with elevated levels of fibrinogen and vWF. Psychosocial stress and unhealthful life style were the most important contributing factors. There was an independent association between education and FVII:Ag, which could not be explained by any of these factors.
Key Words: hemostatic function • life style • psychosocial stress • socioeconomic status • women
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Introduction |
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Hemostatic factors have been reported to be associated with coronary heart disease (CHD) morbidity1 2 3 4 5 6 7 8 9 10 and mortality11 12 in both men and women. Fibrinogen, factor VII, and von Willebrand (vWF) are coagulation factors that may increase the likelihood of atherothrombotic diseases, whereas plasminogen activator inhibitor-1 (PAI-1) is the rapid inhibitor of the endogenous fibrinolytic enzyme system. Fibrinogen may contribute to CHD by influencing the progression of coronary atherosclerosis2 13 14 as well as by precipitating the formation of occlusive thrombus.15 16 vWF and PAI-1 have been suggested to be associated with carotid atherothrombotic diseases17 and to be related to recurrent myocardial infarction (MI).7 In particular, vWF has been related to both recurrent MI and death in patients who survived a first MI.8 9 11 Activated factor VII (FVIIa) has been reported to be a useful index of thrombin production during coagulation.18
A social gradient in CHD has been found in both men and women, the risk of heart attack being higher in low socioeconomic strata. Educational attainment is the most consistent and reliable socioeconomic status (SES) measure that is associated with CHD.19 Among the hemostatic factors, fibrinogen has received greatest attention, particularly in men. Inverse relations with education have been reported.5 20 21 22 In the Atherosclerosis Risk in Communities (ARIC) Study, a univariate association between low education and elevated levels of coagulant factor FVII (FVII:C) was observed in both men and women.23 In contrast, an educational gradient in vWF, FVIIa, and PAI-1 has not been well documented. Although levels of hemostatic factors have been reported to be more elevated in women than in men,1 5 6 21 23 24 factors contributing to their SES differentials among women are not well known. Understanding the determinants of hemostatic profile in relation to SES may be useful in designing preventive strategies geared to reducing socioeconomic differences in atherosclerosis and thrombosis or premature CHD.
Some studies have reported strong associations of hemostatic factors with major cardiovascular risk factors, such as genetic factors,25 26 27 smoking,3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 19 23 24 28 29 a sedentary life style,23 30 obesity,4 5 17 23 30 31 low alcohol consumption,23 24 31 poor lipid profile,1 3 4 5 20 23 30 hypertension,3 4 23 30 social isolation,29 and psychosocial job stress.32 33 34 The main objective of this study was to investigate the association between SES and hemostatic profile and to assess the contribution of other cardiovascular risk factors to this association. Educational attainment was used as a measure of SES. Our hypothesis was that low education is associated with psychosocial stress, which may lead to an unhealthful life style, thereby resulting in an unfavorable hemostatic profile. Cardiovascular risk factors included background factors (age, family history of CHD, menopausal status, and marital status), psychosocial factors (social isolation and work stress), lifestyle patterns (smoking, physical activity, alcohol consumption, dietary habits, and obesity), biochemical factors (glucose, C-reactive protein [CRP], triglycerides, total cholesterol, HDL, and LDL), and hypertension. To the best of our knowledge, this is the first study to examine the association between SES and hemostatic profile in relation to a wide range of cardiovascular risk factors in women.
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Methods |
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The study group comprised 300 healthy women aged 30 to 65 years. They constitute the control subjects of the Stockholm Female Coronary Risk Study, which is a population-based case-control study.35 The study was approved by the Karolinska Hospital Ethics Committee (No. 91;119), Stockholm, Sweden, and the study subjects gave informed consent to participate. The healthy women were chosen from the population register of the greater Stockholm area, which includes each person's identification numbers (based on birth date and sex). "Healthy" was defined as being free from symptoms of heart disease and without hospitalization for any illness during the prior 5-year period. The healthy women were compared with a random sample of 2500 women of the same age range from the general population of Stockholm,36 and no differences were found in the distribution of educational attainment or lifestyle patterns (smoking and body mass index). Seventeen percent of the healthy women declined to participate, mainly due to difficulties in arranging time off from work. There were no significant differences in the mean age between participants and nonparticipants. More detailed information about recruitment of study subjects has been given elsewhere.35
Data Collection
A questionnaire on lifestyle and psychosocial factors was mailed
to the subjects before their visit to the research clinic.
Questionnaires were completed at home and brought to the research
clinic, where the research nurse reviewed them together with the
subject to complete missing answers. Internal nonresponse rate was
<10%. Anthropometric measures, gynecological interview, blood
pressure, and fasting blood samples were all collected and assessed at
the research clinic.
Blood Samples
Venous blood samples were drawn from the right arm of each
subject in the resting position by antecubital vein puncture with a
1.4-mm Wasserman needle. The blood sampling was performed between 8 and
9 AM after 12 hours of fasting. After drawing the first 2
mL, blood was allowed to run freely into the tubes. Samples for the
hemostatic factors were drawn first.
For the analyses of hemostatic factors, venous blood was drawn into 10-mL plastic tubes containing 0.13 mol/L trisodium citrate, 9 parts of blood to 1 part of citrate solution. The blood samples were immediately mixed and centrifuged at 2000g for 15 minutes at room temperature. The supernatant plasma was snap-frozen and stored at -70°C.
For the analyses of lipids and lipoproteins, venous blood was drawn into a 10-mL, precooled, sterile tube containing 0.12 mL of 0.34 mol/L tripotassium EDTA and kept on ice until they were centrifuged at 3000g. Plasma (4 mL) was obtained, immediately frozen to -70°C, and sent in batches to the processing laboratory once per month for further analyses.
Hemostatic Factors (Main Outcome)
Fibrinogen determination was performed with a polymerization
rate method.37 vWF was determined immunochemically by
means of ELISA (Asserachrom vWF, STAGO).38
Plasma levels of FVIIa were determined by a clotting assay using soluble, recombinant, truncated tissue factor (a kind gift from Prof James H. Morrissey, Oklahoma Medical Research Foundation, Oklahoma City),39 40 as previously described.41 Coagulation times were converted to FVIIa concentration (µg/L) by comparison with a standard curve constructed from varying concentrations of purified recombinant FVIIa (a kind gift from Dr Mirella Ezban, Novo Nordisk A/S, Gentofte, Denmark). Data were collected on a Compaq Presario 425 microcomputer (Compaq Computer Corp) and analyzed using Windows Research Software supplied with the ACL-300 coagulometer (Instrumentation Laboratories), as described.41 Intra-assay and interassay coefficients of variation were 3.9% and 9.1%, respectively. FVII mass concentration was determined as FVII antigen (FVII:Ag) by using an enzyme immunoassay kit (Novoclone FVII EIA kit, Dako A/S; a kind gift from Dr Mirella Ezban). PAI-1 was determined by a functional spectrophotometric method (Biopool AB).42
Measure of SES
The highest education attained was reported by subjects in a
self-administered questionnaire, which was later examined by a research
nurse. Educational attainment was categorized into 3 levels, as
previously done in studies of hemostatic profile23 : (1)
low education (mandatory or less than high school), corresponding to 9
years of schooling or less; (2) medium (high school), corresponding to
10 to 13 years of schooling; and (3) high (college or university),
corresponding to 14 years of schooling or more.
Background Factors
Age at examination was obtained from the date of birth given in
the census register. Family history of CHD was defined as having a
first-degree relative with a history of CHD at any age. Menopausal
status was categorized as premenopausal or postmenopausal (having had
no menses for at least 6 months before the examination) with or without
hormone replacement therapy (HRT). Women >50 years of age who were
started on HRT before menopause or who had undergone bilateral
oophorectomy were classified as postmenopausal. Marital status was
categorized as single, widowed, divorced, or cohabiting (married or
living with a male companion).
Psychosocial Stress
Social isolation was derived as an index of 3 instruments:
social support,43 leisure social activities (such as going
with others to the movies, concerts, theaters, museums, church,
restaurants, or clubs, or inviting guests into one's home), and
household size (total number of people living in one's household).
Women were defined as socially isolated when their total scores were
75th percentile. Psychosocial work stress was described as lacking
control or having low decision latitude at work, based on the Swedish
version of the Karasek demand/control questionnaire.44
Work stress was considered present when the total scores were
25th percentile.
Lifestyle-Related Factors
Cigarette smoking and physical activity were assessed according
to the World Health Organization criteria. Diet was assessed using an
88–food item frequency questionnaire with relative portion
sizes.45 Dietary variables included total energy
intake, total fat, carbohydrate, total fiber, protein intake, and
alcohol consumption. Nutrient calculation of dietary factors has been
documented elsewhere.46 The total average amount of
alcohol (100% ethanol) consumed was calculated in grams per day, after
taking into account the frequency, amount, and alcohol content in
specific beverages.47 Low alcohol consumption was defined
as not drinking or consumption 25th percentile. Obesity was defined
as body mass index >29 kg/m2 or a waist-to-hip
circumference ratio >0.85.
Biochemical Factors
Total cholesterol was determined with CHOD-PAP and
triglycerides with GPD-PAP enzymatic methods with reagents
from Boehringer Mannheim. HDLs were determined on the basis of
the isolation of LDL and VLDL from serum by precipitation. The
cholesterol content of the supernatant was measured
enzymatically.48 CRP was analyzed by
immunoturbidimetric determination (Orion
Diagnostica).49 Fasting serum glucose was
analyzed by the GOD-PAP method.50 All measurements
were carried out in the same laboratory (CALAB), with an automated
multichannel analyzer.51 Cutoff points of
biochemical factors were adapted from the treatment guidelines of the
European Atherosclerosis Society.52
Hypertension
Hypertension was defined as systolic blood pressure
>140 mm Hg, diastolic blood pressure >90
mm Hg, or a history of hypertension (according to the physician's
diagnosis). Systolic and diastolic blood pressures
were measured with subjects in the supine position after a 5-minute
rest; phases I and V of the Korotkoff sound were used.
Statistical Methods
To attain normality, hemostatic factors were logarithmically
transformed in all analyses. Geometric means are
presented. The SEs of geometric means were calculated using the
delta method. To retain power, PAI-1 levels at zero were assigned a
value of 0.001 before logarithmic transformation. ANOVA and ANCOVA
(using the standard least-squares method) were performed for
univariate and multivariate
analyses, respectively. In the multivariate
analyses, 6 models were generated. Risk factors were grouped
into sets as follows: lifestyle patterns (smoking, physical activity,
obesity, and alcohol consumption); psychosocial stress (social
isolation and work stress); biochemical factors (glucose, CRP,
triglycerides, and LDL and HDL cholesterol);
and hypertension. Least-squares means of hemostatic factors were
estimated for each level of education. The linear trend for the effect
of education was assessed by computing P values for trend.
JMP Statistics and Stata for Macintosh were used to run the
analyses.
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Results |
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The distribution of hemostatic and other cardiovascular risk factor characteristics are shown in Table 1
.
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Univariate/Bivariate Analyses
Cardiovascular Risk Factors and Hemostatic
Profile
Elevated levels of fibrinogen were observed among women who were
divorced (mean=3.42 g/L), widowed (mean=3.31 g/L), and single
(mean=3.22 g/L) compared with women who were married (mean=3.11 g/L,
P=0.01). The association with marital status was not
statistically significant for other hemostatic variables. Elevated
levels of FVII:Ag (r=0.12, P=0.05) and of PAI-1
(r=0.14, P=0.02) were associated with high total
fat intake and also with a high total energy intake (r=0.15,
P=0.02 for FVII:Ag and r=0.13, P=0.03
for PAI-1). The associations of carbohydrate, protein, and total fiber
intake with hemostatic profile were not statistically significant. An
unfavorable hemostatic profile was associated with smoking, a sedentary
life style, obesity, psychosocial stress, atherogenic biochemical
factors, and hypertension, as shown in Table 2.
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Educational Attainment and Cardiovascular Risk
Factors
Comparing women with mandatory with those with a
college/university education revealed that a larger proportion were
older (mean=58 versus 55 years, P=0.001), were hypertensive
(26% versus 11%, P=0.02), were obese (34% versus 16%,
P=0.01), were smokers (38% versus 20%, P=0.01),
were socially isolated (34% versus 11%, P=0.0003), and
reported work stress (56% versus 22%, P<0.0001). Compared
with college/university, mandatory education was also associated with
lower levels of HDL cholesterol (mean=1.7 versus 1.9
mmol/L, P=0.05) and elevated levels of total
cholesterol (mean=6.2 versus 5.8 mmol/L,
P=0.004), LDL cholesterol (mean=4.0 versus
3.6 mmol/L, P=0.006), and triglycerides
(mean=1.1 versus 0.9 mmol/L, P=0.02). Although there
was a tendency for women with low education to have unhealthful dietary
habits, this association did not show statistical significance.
Educational Attainment and Hemostatic Profile
Women with the lowest (mandatory) education had elevated levels of
hemostatic factors compared with women with the highest
(college/university). After comparing women of mandatory with those of
a college/university education level, statistically significant
differences were found in fibrinogen(difference=0.32 g/L; 95% CI, 0.16
to 0.48 g/L), vWF (difference=0.15 U/mL; 95% CI, 0.05 to 0.25 U/mL),
and FVII:Ag (difference=46 µg/L; 95% CI, 21 to 73 µg/L). The
corresponding figures for FVIIa (difference=0.3µg/L; 95% CI, -0.1
to 0.7 µg/L) and PAI-1 (difference=1.8 IU/mL; 95% CI, -4.0 to 7.6
IU/mL) did not reach statistical significance (Table 3).
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After adjustment for age, statistically significant differences between
the lowest and highest education were observed for levels of fibrinogen
(0.26 g/L; 95% CI, 0.10 to 0.42 g/L), vWF (0.11 U/mL; 95% CI, 0.10 to
0.12 U/mL), and FVII:Ag (41 µg/L; 95% CI, 15 to 66 µg/L; Table 3
).
Multivariate Analyses
We further examined the contribution of life style, psychosocial
stress, biochemical factors, and hypertension to the association of
education with fibrinogen, vWF, and FVII:Ag (Table 4).
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Fibrinogen and vWF
For both fibrinogen and vWF, statistically significant
associations with education persisted after adjustment for biochemical
factors and hypertension in models 4 and 5 (P<0.05). These
associations, however, weakened after adjustment for psychosocial
stress and lifestyle patterns (P>0.05, Table 4).
After simultaneous adjustment for all risk factors in model
6, there was no statistically significant difference in fibrinogen
concentration between the 2 extreme levels of education
(difference=0.03 g/L; 95% CI, -0.13 to 0.19 g/L, P=0.92).
Independently of other risk factors, elevated levels of fibrinogen were
observed in women who were hypertensive (3.39 versus 3.06 g/L,
P=0.002), smokers (3.19 versus 2.97 g/L,
P=0.004), and obese (3.29 versus 3.10 g/L,
P=0.03) and in those who had elevated levels of CRP
(ß=1.12, P=0.04). After adjustment for family history of
CHD, marital status and menopausal status did not alter these results
(Table 4).
After adjustment for all risk factors, the difference in levels of vWF
when women of mandatory education were compared with those with a
college/university education was 0.06 U/mL (95% CI, -0.10 to 0.22
U/mL), and this difference was not statistically significant
(P=0.45, Table 4). None of the other risk factors had
a statistically significant independent effect on vWF.
Factor VII Antigen
A statistically significant difference in the concentration of
FVII:Ag between mandatory and college/university education persisted
after individual adjustment for psychosocial stress, lifestyle
patterns, biochemical factors, and hypertension in models 2 through 5
(difference >30 µg/L; P=0.01, Table 4). This
difference persisted after simultaneous adjustment for all
risk factors in model 6 (difference=34 µg/L; 95% CI, 2 to 65 µg/L,
P=0.05). Another independent association of FVII:Ag was
observed with triglycerides (ß=1.5, P=0.03).
Further adjustment for total fat and total energy intake did not
substantially alter these results. The associations of FVII:Ag with
total fat intake and total energy were at borderline statistical
significance (P=0.09). Further adjustment for family history
of CHD, marital status, and menopausal status did not alter the
associations observed above.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Low education and unfavorable hemostatic profile were both associated with older age, unhealthful life style, psychosocial stress, atherogenic biochemical factors, and hypertension. Levels of hemostatic factors increased with lower educational attainment, and this association was strongest and most consistent for FVII:Ag, fibrinogen, and vWF. The associations of education with FVIIa and PAI-1 were not statistically significant. Further adjustment for age, lifestyle patterns, psychosocial stress, biochemical factors, and hypertension explained a substantial part of the educational gradient in fibrinogen and vWF, but the gradient for FVII:Ag could not be fully explained by these factors. In the multivariate analyses, statistically significant independent effects on fibrinogen were found with smoking, obesity, CRP, and hypertension, whereas independent effects on FVII:Ag were found only with triglycerides.
Adjustment for psychosocial stress and lifestyle factors substantially weakened the associations of education with fibrinogen and vWF, whereas adjustment for biochemical factors and hypertension had a much smaller impact on these associations. This observation provides some support for the hypothesis that low education is associated with psychosocial stress, which may lead to an unhealthful lifestyle and result in an unfavorable hemostatic profile.
To the best of our knowledge, this is the first study to show the strong associations of low education with elevated levels of vWF and also an independent association between low education and elevated levels of FVII:Ag, after adjustment for a wide range of cardiovascular risk factors. The observed associations of low education with elevated levels of fibrinogen are consistent with earlier studies.5 20 21 22 In Finnish men and women (45 to 64 years old), the association between education and FVII:Ag was absent (P>0.80),22 whereas in the ARIC Study, low education was strongly associated with elevated levels of FVII:C.23 The absence of an association between educational attainment and PAI-1 has been previously reported.29 The univariate associations of hemostatic factors with major cardiovascular risk factors in the present study are consistent with results from other studies.1 3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 23 24 25 29 30 31
Although the biological pathways between psychosocial stress and hemostatic profile are not well known, it is suggested that a combination of low SES, social isolation, and work stress may gradually increase the susceptibility to stress. Stressful situations may influence endocrine responses, including circulating cortisol levels,53 which may in turn increase insulin resistance, resulting in increased circulating fatty acids, raising triglycerides and lowering HDL levels. For example, plasma triglyceride has been shown by the present study and others23 to be an independent determinant of FVII. In addition, work stress has previously been associated with low HDL levels, after adjustment for age and other cardiovascular risk factors.54 Furthermore, work stress has previously been associated with obesity and was reported to contribute to some of the socioeconomic gradient in obesity among women.46 These studies44 54 suggest possible mechanisms by which psychosocial stress may influence biological risk factors known to disturb the hemostatic profile.5 23 31
Hemostatic Function and CHD
How hemostatic proteins may damage the circulatory system is not
fully known. Hemostatic risk factors have been suggested to play a
causative role in the development of
atherosclerosis.5 6 23 24 55 56 57 58 59 In
addition, they may be markers of inflammation and
endothelial dysfunction.60 High plasma
concentrations of fibrinogen, vWF, and FVII may also precipitate
coronary events by enhancing procoagulant activity and reducing
fibrinolytic defenses in the presence of a disrupted
intracoronary plaque.61
Limitations and Strengths of the Study
The design of this study does not account for time trends of risk
factor levels (eg, quitting smoking or exercising more) that may
influence the strength of the association between SES and the
hemostatic profile. In addition, measurement of some hemostatic
factors, such as FVII, is biochemically difficult, because 2 forms of
FVII molecules are present in plasma. The major proportion of FVII
circulates in the zymogen single-chain form. However, low levels of the
activated double-chain form (
1% of the FVII mass) are also
present and appear to serve a priming function for triggering the
clotting cascade. It is difficult to compare results obtained in
different studies because of large variations in laboratory techniques
and analyses. Of note, we had the opportunity to assess the
association of educational attainment and other
cardiovascular risk factors with both FVII mass and
FVIIa, free of FVII zymogen, by using sensitive and specific methods.
Furthermore, the measured levels of hemostatic factors are influenced
by gene polymorphisms27 and gene-environment
interactions that were not investigated in this study. It should also
be emphasized that the study sample is community based and
representative of Swedish women living in a
metropolitan area. The response rate was extremely high (83%).
Importantly, the report is based on healthy women whose hemostatic
levels were not influenced by MI or any other acute-phase reactions, as
has been earlier reported.62
Conclusions
The educational differentials in hemostatic profile are suggested
to be strongly mediated by lifestyle patterns and psychosocial stress
and in part by biochemical factors and hypertension. Unfavorable
cardiovascular risk factor profiles in persons with low
educational attainment appear to be associated with an increased risk
of atherosclerosis and thrombosis, which is reflected
in the hemostatic profile. Smoking, a sedentary life style, obesity,
hypertension, hyperlipidemia, and psychosocial stress
are modifiable risk factors that should be considered in preventive
strategies aimed at reducing both atherosclerosis and
thrombosis, particularly among persons with low SES.
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Acknowledgments |
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This work was supported by grant No. HL45785 from the National Institutes of Health, Bethesda, Md (to K.O.-G., K.S.G.); grants No. B93-19X-10407 and K98-19X-8691 from the Swedish Medical Research Council (to K.O.-G.); a grant from the Swedish Heart-Lung Foundation (to K.O.-G.); and a grant from the Swedish Labor Market Insurance Company (to K.O.-G.).
Received December 29, 1997; accepted July 21, 1998.
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