Work and Blood Pressure

Thomas G. Pickering, MD, DPhil

Disclosures

The Japanese have a word for it: Karoshi, or death from overwork.[1] For many of us, our jobs are a major source of stress, and there is a perception that those of us who are lucky enough to have jobs are working harder and harder. This raises the issue of the effects of work on health. When 24-hour blood pressure monitoring first came into use nearly 30 years ago, we observed that for most persons blood pressure tends to be highest during hours of work[2] and that blood pressure tends to be higher on a workday than a day away from work.[3,4] We also found that the correlation between left ventricular mass and blood pressure was closer for the blood pressure measured at work than for blood pressure taken at home or during sleep.[5] This led us to look at the effects of occupational stress, but the question was how to measure it.

In general, a situation that is perceived as being stressful is one that poses a threat to well-being over which one has little control. If it can be controlled, either by overcoming the stressor or running away from it, it becomes less of a threat. One of the most widely adopted measures of occupational stress is a model first developed by Robert Karasek and Tores Theorell that is known as the job strain model. This model assesses people's perception of their jobs by two components: job demands and perceived latitude, or job control.[6,7] A highly stressful (high strain) job is described as one that combines high demands with low control. These are not typically executive-type jobs because although these carry high demands, they are also associated with high control. The classic example of a high demands-low control job is assembly line work, where the work is both fast and paced, and the worker has to work at a preset rate. Another example is operating a telephone switchboard. In fact, most classic high-strain jobs are blue collar jobs. However, a number of studies have shown that job title gives only a weak categorization of job strain and much depends on circumstances: a job may potentially be stressful, but a very understanding and sympathetic supervisor can go a long way toward mitigating the level of job strain.

One of the reasons for using the job strain model to evaluate stress was that there were several studies conducted in the United States, the United Kingdom, and Sweden that showed that people in high-strain jobs were at increased risk of developing coronary heart disease.[6,7,8,9] We decided to test the hypothesis that elevated blood pressure might be one way by which the effects of job strain on heart disease are mediated. To test this idea, we initially recruited men employed in a variety of different jobs in New York, NY (to encompass a wide range of job strain), and compared hypertensives with normotensives.[10] We found that the hypertensives were approximately twice as likely to be working in high-strain jobs as the normotensives, and when we analyzed blood pressure according to the level of job strain, it was clear that the pressure was only raised if the person's job was characterized by both high demands and low control. Others have reported similar results.[11,12] One implication of this is that there are some people whose resting clinic blood pressures may be normal, but during the working day their blood pressures enter the hypertensive range. We identified a substantial number of these persons in our work site study, and originally referred to the phenomenon as "occult workplace hyperten-sion"[13] and subsequently as "masked hyperten-sion."[14] Interestingly, we found that while men in high-strain jobs showed increased blood pressure, there was absolutely no effect of job strain on blood pressure in women.[15] This was a surprise, because women are in general more likely to be in high-strain jobs than men, as in fact was the case in our sample.

Two other studies found that job strain was not correlated with ambulatory blood pressure in nurses,[16,17] although one reported that those who reported high job demands showed a bigger difference between work and home pressures than those with less demanding jobs.[16] Why there should be a gender difference in the effects of job strain on blood pressure is not clear, but two other studies have helped clarify this. A study of a biracial sample of working men and women by Light et al.[18] found that the combination of high-effort coping and a high-status job was related to higher blood pressure at work in women but not in men. The former was defined by giving positive answers to questions in the John Henryism active coping scale, such as "I don't let my personal feelings get in the way of doing a job," and "hard work has really helped me get ahead in life." This combination did not affect blood pressure in white men, but was associated with higher work pressures in African Americans. In the Light study[18] most of the participants were relatively highly educated, which limits the generalizability of the results, but it is of interest that it too found that it was a combination of factors (high demands and high status) that resulted in higher work pressures, in the same way that we found that the combination of high demands and low control was associated with increased blood pressure in men.

Another study[19] (from Canada) found that job strain on its own was not related to higher blood pressure in women unless they also had "family responsibilities," defined as a combination of having children at home and doing domestic chores. The effect of the combination of a high-strain job and family responsibilities was most pronounced in women who were also highly educated, which is consistent with the findings of Light et al.[18] Thus, women are not immune from the effects of work-related stress. In another study of working women,[20] we found that women who reported their jobs as being more stressful than their home environment had higher blood pressures than women for whom the home environment was more stressful, both while at work and during sleep, while in those for whom the home environment was more stressful there was a correlation between perceived level of home stress and sleep blood pressure.

These studies were all cross-sectional, which raises a question about the causal relationship between job stress and blood pressure. People do not choose their jobs at random, but it seems improbable that persons with hypertension would be any more likely to select a high-strain job than normotensives. Over a 3-year follow-up we found that the relationship between job strain and blood pressure was still present, but about half of the subjects had changed their job-strain status.[21] Furthermore, men who remained in high-strain jobs over the 3-year period had substantially higher ambulatory systolic blood pressures than those who were in low-strain jobs, and those whose job strain status changed showed small changes in the expected directions. These findings are consistent with the idea that job strain contributes to the development of hypertension.

Another relevant factor is personality. A good illustration of this is a study conducted by Brondolo et al.[22] A group of healthy subjects was studied with ambulatory monitoring. They were asked to record all social interactions in a diary and to grade the interactions as being pleasant or unpleasant. Subjects were also scored for hostility. The researchers found that hostility was not related to blood pressure on its own, except during a negative social interaction. In other words, if you are a hostile person your blood pressure is not any higher when you are on your own, but when you get irritated by other people your blood pressure goes up. The same group found that positive social interactions influence blood pressure at work. In a study of New York City traffic wardens, it was found that having a supportive supervisor was associated with a lower work blood pressure in women, and having supportive coworkers had the same effect in men.[23]

One of the classic studies of the effects of work-related factors on cardiovascular disease is the Whitehall II study[24] conducted in 10,000 British civil servants. The main finding of this study was that those in the lowest grades of the administration are at the highest risk for cardiovascular and other diseases. There was a small gradient in blood pressure in men but not in women when it was measured in the conventional way, with higher readings occurring in the lower grades. In a subgroup that wore an ambulatory monitor there were differences in systolic pressure during the morning hours (128.9 mm Hg in the lowest grade, 122.6 mm Hg in the middle grade, and 123.3 mm Hg in the highest grade).[25] During the rest of the day and in the evening, however, there were no differences.

Overtime may also affect blood pressure. A Japanese study classified employees according to the amount of overtime they worked and found that while there were no significant differences in the conventionally measured blood pressures, persons who worked an average of 60 hours or more per month of overtime had slightly higher 24-hour blood pressure levels.[1] Both 24-hour blood pressure and heart rates were higher during periods when the workers were accumulating more overtime.

One of the interesting issues raised by these studies is the extent to which transient stress-induced increases of blood pressure may contribute to the onset of sustained hypertension, which is characterized by a resetting of the entire 24-hour blood pressure profile to a higher level. Some studies of occupational stress have found that blood pressure is increased only during working hours, whereas we have found that job strain raises it to the same extent at home and during sleep as at work.[15] Thus, if environmental stress is a causal factor in the evolution of hypertension -- as some of us like to think -- job strain appears to be a promising candidate for study.

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