Purpose: 'Job Strain' (defined as high demands and low control) has been previously associated with increased risk of cardiovascular disease (CVD) and, cross-sectionally, with hypertension and elevated ambulatory blood pressure (AmBP). Our longitudinal cohort study was designed to investigate the hypothesis that exposure to 'job strain' is causally related to increases in mean AmBP.

Methods: The sample consists of 285 healthy male employees, aged 30-60 at initial recruitment, at 8 New York City worksites, 195 of whom were restudied 3 years after (Time 2) their initial participation. Mean systolic (AmSBP) and diastolic (AmDBP) ambulatory blood pressure at work, home and during sleep were computed from 24-hour recordings and diary entries specifying location. The relationship of 'job strain' to AmBP was examined cross-sectionally at each round of data collection. In addition, to take advantage of our information on 'job strain' status at each assessment and to evaluated the impact of changes in exposure, a 'job strain' change variable was constructed with four categories: those defined as having no 'job strain' at either Time 1 or Time 2 (N=138), those reporting 'job strain' at both times (N=15), and two groups which changed 'job strain' status. Multiple regression analysis was used to examine the cross-sectional associations of AmBP with 'job strain' change, controlling for age, body mass, race/ethnicity, smoking status, alcohol consumption, education, sodium and physical exertion level of the job.

Results: Cross-sectional analyses at Time 1 and Time 2 showed consistent significant effects of 'job strain' on AmBP, even though the overlap between the high strain groups at different times was less than 50%. In cross-sectional analyses, subjects with 'job strain' had work AmSBP/AmDBP which were 5-7 / 3-5 mmHg higher than subjects without 'job strain'. Those men facing chronic 'job strain', that is working in high strain jobs at both Time 1 and Time 2 had work AmSBP/AmDBP on average, 10-12/6-8 mmHg higher than those with no 'job strain' at either times. The two crossover groups had intermediate levels of blood pressure. Effect sizes for chronic three-year exposure to 'job strain' are larger than the estimated effect of aging 25 years or gaining 50 pounds in weight. In longitudinal analyses, subjects who changed from exposure to 'job strain' to no exposure three years later had a significant decrease in AmSBP/AmDBP of about 5/3 mmHg.

Conclusions: The previously reported cross-sectional association between 'job strain' and AmBP was replicated at follow-up three years later. Repeated exposure to 'job strain' was associated with the highest levels of AmBP's, but not with any further increases in AmBP. However, changes in 'job strain' status predicted change in AmBP over a 3-year period. 'Job strain' emerges as a consistent and substantial risk factor for AmBP in men. It appears that changes in exposure to work stress over time among working men can obscure associations between stress and outcome and should be taken into account in designing studies to assess the impact of work stress and CVD.