Various studies have linked occupational exposures to cardiovascular disease (CVD) incidence and mortality.1
In one estimation, CVD accounts for nearly a third of all mortality attributed to occupation.2
After a few decades of research on occupational characteristics and CVD events,1
several large-scale studies have started to investigate the relationship between job characteristics and intima-media thickness (IMT),3–10
a non-invasive measure of early atherosclerosis. IMT is associated with CVD risk factors11 12
and also predicts clinical CVD events.13 14
Identifying associations between occupation and subclinical disease will facilitate early interventions for CVD.
Most previous studies on occupation and IMT focussed on job characteristics such as job strain (a combination of high job demands and low job control)15
and stress caused by various job demands (eg, complexity of work, physical demands, irregular work schedules). The Atherosclerosis Risk in Communities Study and the Kuopio Ischaemic Heart Disease Risk Factor Study both reported significant associations between job stress and increased IMT, but these associations were attenuated to non-significance after traditional CVD risk factors were included in the analysis.6 7
The Cardiovascular Risk in Young Finns Study, on the other hand, reported that among men, job strain was associated with increased IMT even after adjustments were made for other CVD risk factors.3–5
While these results suggest that certain job characteristics are associated with IMT, at least two issues remain relatively unexplored.
First, these studies did not address occupational category in relation to IMT; therefore, it is not known if an occupational gradient exists regarding IMT. Occupational category is often used as an individual-level indicator of socioeconomic status (SES). Several studies have reported significant associations of IMT with educational attainment or income,10 16 17
but few have examined associations of IMT with occupational category. One exception is the Malmö Diet and Cancer Study, which found that IMT was significantly greater for unskilled manual workers than for high-level non-manual workers.8 10
The Malmö study results suggest an occupational gradient associated with IMT; however, it is not clear if this persists beyond the effects of income and education because these SES indicators were not included in the analyses.
A second point that needs clarification is IMT's segment-specific associations with occupation. The research on IMTas an indicator of early atherosclerosis has identified that the IMTs of different segments have different associations with CVD risk factors.18–20
More specifically, the IMT of the common carotid artery (CCA-IMT) is more strongly associated with blood pressure,19 20
while the IMT of the internal carotid artery (ICA-IMT) is more strongly associated with cholesterol.20
CCA- and ICA-IMT are also reported to have different associations with clinical outcomes.21
Because most previous studies on occupation and IMT either used only CCA-IMT3–6 8
or took the average of IMTs from different carotid artery segments,7
it is not well understood whether occupation has segment-specific associations with IMT. The Malmö study, which did report segment-specific IMT outcomes, found no significant associations of CCA-IMT with occupational status or job strain9 10
; however,the IMT of the carotid bifurcation area was associated with job strain9
and occupational category.10
Different associations of occupational status and/or job characteristics with the IMT of different carotid artery segments would provide detailed information about the role of occupation in early atherosclerosis development; however, such research is still scarce.
The current study examined IMT in relation to both occupational category and job strain. Based on prior work,3–5 10
we hypothesised that (1) IMT would be greater for people in lower status jobs than for those in higher status jobs, (2) IMTwould be positively associated with job demands and negatively associated with job control, and (3) people in high strain jobs (a combination of high demands and low control) would have greater IMT than those in low strain jobs. We investigated these hypotheses with CCA- and ICA-IMT separately in order to explore segment-specific associations.