Stress, social support and cardiovascular activity over the working day
Introduction
There is a substantial literature relating stress and social support with cardiovascular disease, psychological well-being and other health outcome (Coyne and Downey, 1991, Czajkowski and Shumaker, 1994, Uchino et al., 1996). Social support and social networks are generally found to be protective, although social interactions may also have negative consequences for health and well-being (Burg and Seeman, 1994). The extent to which social support has direct effects on health outcomes or buffers the impact of life stress remains controversial, since data to endorse both viewpoints continues to emerge (Lepore, 1992, Cohen et al., 1997, Krumholz et al., 1998).
Laboratory studies have been used to evaluate the mechanisms linking social support with physiological function. A number of studies have evaluated cardiovascular and neuroendocrine reactions to mental stress tests in relation to the presence of supportive others (for reviews, see Uchino et al., 1996, Lepore, 1998). In general, reactions to stressors are attenuated by the presence of social supports, but results vary in relation to the familiarity of the supportive other, gender, and other factors. A limited amount of work has linked acute physiological stress reactivity with questionnaire measures of social support. Boyce and Chesterman (1990) found that social support was unrelated to cardiovascular stress reactivity in a sample of adolescent boys. Recently, we found that in young working men, high social support was associated with greater cardiovascular stress reactivity but also with more prompt post-task recovery, suggesting enhanced efficiency of counter-regulatory mechanisms (Roy et al., 1998).
The relevance of acute psychophysiological responsivity to everyday life experience has been questioned by a number of authorities (Pickering and Gerin, 1990, Parati et al., 1991). Ambulatory monitoring studies provide complementary information, since they allow associations between social support and cardiovascular function to be investigated under naturalistic conditions. Ambulatory blood pressure monitoring typically involves measurements every 15–30 min using automated apparatus for several hours, while participants go about their normal activities. Studies evaluating blood pressure during social interactions have shown mixed results to date. Crowther et al. (1987) found that cardiovascular activity was greater when participants reported social involvement than at other times, while blood pressure was lower in another study when measures were taken in the presence of family than when strangers or friends were present (Spitzer et al., 1992). More recently, a study involving sophisticated concurrent assessments of activities at the time of blood pressure registration showed no effects of social conflict on blood pressure or heart rate levels (Kamarck et al., 1998). However, these findings may depend on the nature of the group under investigation, since an investigation of people engaged in more socially confrontive work (traffic agents or wardens) has shown a clear effect of social interaction on ambulatory cardiovascular activity (Brondolo et al., 1999).
Few studies have explored the influence of perceived social support on ambulatory cardiovascular activity. Linden et al. (1993) studied 129 students and found that systolic blood pressure over the day was inversely associated with social support, but only among women and not men. A 24-h ECG monitoring study of a working population showed that heart rate was lower during sleep, work and leisure time in participants reporting high social support (Unden et al., 1991).
Ambulatory monitoring methods can also be used to evaluate the possible interactions between stress and support. By asking participants to make ratings of stress and other factors at the time of each blood pressure reading, it is possible to compare levels of cardiovascular activity during periods of high and low stress. We have previously shown that blood pressures are elevated at times when people report stress or anger (Steptoe et al., 1996). As noted by Carels et al. (1998), ambulatory blood pressure research would benefit from more specific testing of hypotheses related to psychosocial factors. In the present analysis, comparisons were therefore made of the cardiovascular responses to naturally occurring periods of stress in participants reporting high or low social support. It was predicted that if social support has a buffering effect, then high and low support groups should differ in cardiovascular activity during periods of high but not low stress, with low support individuals being more responsive to stressful episodes. If on the other hand, social support has direct effects, then blood pressure and heart rate should be less in the high than low support groups under conditions of both high and low stress.
One problem in the investigation of psychological influences on ambulatory cardiovascular activity is that momentary fluctuations in blood pressure and heart rate are strongly influenced by recent physical activity and by body posture (Gellman et al., 1990, Johnston et al., 1990, Kario et al., 1999). It is possible therefore that blood pressure and heart rate might be greater during stressful episodes because people are more active, or more likely to be standing than sitting under these circumstances. In order to address this possibility, concurrent energy expenditure was estimated from three-dimensional accelerometers throughout the ambulatory monitoring period. Differences in energy expenditure between high and low stress episodes were analysed, and concurrent energy expenditure was used as a covariate in the analyses of cardiovascular function.
Accelerometers have been developed to assess physical activity, and have been validated on that basis (Sherman et al., 1998). However, as noted above, body posture also influences blood pressure and heart rate, with higher heart rate and diastolic pressure in the upright compared with the supine and seated positions (Lundin et al., 1986). The sensitivity of accelerometers to postural variations has not been well established. Consequently, a secondary aim in these analyses was to ensure that estimates of energy expenditure were greater in the standing than seated position. Accordingly, blood pressure, heart rate and accelerometer readings were grouped for sitting and standing, and comparisons were made within-subjects between the two postures. These analyses were conducted on data from a larger study described elsewhere (Steptoe et al., 1999a, Steptoe et al., 1999b).
Section snippets
Participants
Full-time school teachers were selected from a larger survey of teachers in south London (for details, see Steptoe et al., 1999a). After 12 months, participants were re-contacted and 137 (84.6%) carried out ambulatory blood pressure monitoring over a work day. Ten of the 25 who did not take part in the follow up had resigned or retired from teaching, six were pregnant, one was severely ill, one filled in questionnaires but did not complete blood pressure monitoring, and the remaining seven did
Results
One hundred and thirty-three participants in this study carried out ambulatory cardiovascular monitoring. However, accelerometers were not available at the beginning of the study, so adequate measures of social support, ambulatory cardiovascular data, diary ratings and energy expenditure for the assessment of stress effects were obtained from 104 individuals (37 men, 67 women). Comparison between these people and the remaining 29 revealed no significant differences in gender distribution, age,
Discussion
This study involved a detailed analysis of blood pressure and heart rate measurements associated with episodes of naturally occurring perceived stress during the working day. Previous studies have shown that blood pressure is higher during episodes of subjective stress or negative emotion over the day (Steptoe et al., 1996), particularly in individuals who report more variable emotional states during ambulatory monitoring (Carels et al., 2000). The results replicated and extended previous
Acknowledgements
This research was supported by the Medical Research Council, UK. I am grateful to Dr Mark Cropley and Dr Jayne Griffith for their involvement in data collection and processing.
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