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Stress, social support and cardiovascular activity over the working day
International Journal of Psychophysiology 37 Ž2000. 299᎐308
Stress, social support and cardiovascular activity over the working day Andrew SteptoeU Department of Psychology, St. George’s Hospital Medical School, Cranmer Terrace, London SW17 ORE, UK Received 8 December 1999; received in revised form 20 March 2000; accepted 30 March 2000
Abstract The influence of stress on ambulatory blood pressure monitored over the working day, and the potential buffering effect of social support, was assessed in 104 school teachers Ž37 men and 67 women.. Blood pressure and heart rate were measured every 20 min and energy expenditure was assessed using accelerometers. Participants rated the degree of stress they were experiencing at the time of each measurement on a seven-point scale. Episodes of both high and low stress during the working day were reported by 62 participants. They were divided by median split into high and low social support groups on the Interpersonal Support Evaluation List. After controlling for body mass and concomitant energy expenditure, high stress was associated with increased systolic blood pressure, diastolic blood pressure and heart rate. However, the impact of stress was buffered by social support, with no significant increase in blood pressure or heart rate with stress in the high support group. The accelerometers were also shown effectively to discriminate between blood pressure readings taken in the seated and standing positions in terms of energy expenditure. The results corroborate laboratory studies, in showing that social support buffers the impact of episodic stress on cardiovascular activity under naturalistic conditions during the working day. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Blood pressure; Heart rate; Stress; Social support; Posture; Ambulatory monitoring
1. Introduction There is a substantial literature relating stress and social support with cardiovascular disease, psychological well-being and other health outU
Tel.: q-44-208-725-5603; fax: q44-208-767-2741. E-mail address: [email protected] ŽA. Steptoe..
come ŽCoyne 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 ŽBurg 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
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viewpoints continues to emerge ŽLepore, 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 Žfor 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 Ž1990. 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 ŽRoy et al., 1998.. The relevance of acute psychophysiological responsivity to everyday life experience has been questioned by a number of authorities ŽPickering 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. Ž1987. 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 ŽSpitzer 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 ŽKamarck 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 Žtraffic agents or wardens. has shown a clear effect of social interaction on ambulatory cardiovascular activity ŽBrondolo et al., 1999.. Few studies have explored the influence of perceived social support on ambulatory cardiovascular activity. Linden et al. Ž1993. 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 ŽUnden 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 ŽSteptoe et al., 1996.. As noted by Carels et al. Ž1998., 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 ŽGell-
A. Steptoe r International Journal of Psychophysiology 37 (2000) 299᎐308
man 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 ŽSherman 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 ŽLundin 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 withinsubjects between the two postures. These analyses were conducted on data from a larger study described elsewhere ŽSteptoe et al., 1999a,b..
2. Methods 2.1. Participants Full-time school teachers were selected from a larger survey of teachers in south London Žfor details, see Steptoe et al., 1999a.. After 12 months, participants were re-contacted and 137 Ž84.6%. 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
301
seven did not reply. Ambulatory blood pressure data were satisfactory in 133 participants. 2.2. Social support measure Social support was assessed using the Interpersonal Support Evaluation List ŽISEL, Cohen et al., 1985.. This questionnaire consists of 40 items designed to assess perceived availability of four separate types of support: tangible support or the perceived availability of practical aid, appraisal support or the availability of people to talk to and provide advice, self-esteem support measuring self-confidence and positive social comparison, and belonging support or the perceived availability of social contacts with whom one can do things. Responses to the 40 items Žscored yesrno. were summed to generate a total support score, with higher values reflecting greater perceived support. The internal consistency ŽCronbach ␣ . for the scale in this sample was 0.89. 2.3. Ambulatory monitoring procedures Ambulatory blood pressure monitoring was carried out using the SpaceLabs 90217 monitor ŽRedmond, WA, USA., an instrument that satisfies international instrumentation standards ŽBaumgart and Kamp, 1998.. The monitor was fitted between 08.00 and 09.00 h on a working day at each participating school. After confirmation of accurate functioning, the participant wore the monitor for the day and evening. Blood pressure was measured at 20-min intervals between 09.00 and 17.40 h, and subsequently at 30-min intervals. Participants were instructed to keep their arms still while the cuff inflated and deflated. Each reading was accompanied by an entry in a diary in which the participant recorded location, posture and activity. In addition, participants rated the extent to which they felt stressed at that moment, assessing the level of subjective pressure they were experiencing from 1 s extreme pressure to 7 s ¨ ery little pressure. The blood pressure readings were subsequently reviewed and outliers were excluded according to the methods described by Berardi et al. Ž1992.. At the end of the moni-
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toring day, participants were asked whether the day had been more stressful than average, average, or less stressful than average. Energy expenditure was recorded in 1-min epochs using a TriTrac-R3D research ergometer ŽReining International, Madison, WI, USA.. This solid state monitor contains accelerometers that assess movements in three planes ᎏ mediolateral, antero-posterior and vertical ᎏ and uses the integrated vector magnitude together with information concerning gender, age, weight and height to calculate estimated energy expenditure in kilocalories. Validation of the TriTrac and related triaxial accelerometers indicate that the instruments provide reliable and acceptable assessments of energy expenditure ŽBouten et al., 1994; Sherman et al., 1998.. Height, body weight, and body mass index were assessed using standard procedures. Smoking status was also recorded. So as to provide a measure of work status, participants were classified as class teachers, or teachers in supervisory grades who had additional administrative duties. 2.4. Data analysis Blood pressure and heart rate data recorded over the working day Ž9.00᎐17.40. were analysed in this report. The evening values were excluded since the physical environment and psychological pressures on individuals were very different from those experienced over the day. The total energy expenditure in kilocalories for the 5 min preceding each cuff inflation was computed. Average levels of blood pressure, heart rate and energy expenditure over the working day were calculated. For the analysis of cardiovascular activity and stress, the blood pressure, heart rate and energy expenditure values associated with each of the seven levels of subjective pressure rating were averaged. As not all participants had blood pressures associated with each of the stress ratings, data were subsequently grouped into low stress Žratings 6 and 7. and high stress Žratings 1᎐4. periods. Data were analysed using analysis of variance, with social support Žhighrlow. as the between-subject factor, and stress level Žhighrlow.
as the within-subject factor. Gender was included as a factor in preliminary analyses, but gender effects are not described unless they interacted with social support or stress level. In the analysis of posture, blood pressure, heart rate and energy expenditure were averaged for all readings when the participant was seated, and all readings during standing. These data were subsequently analysed with posture Žseatedrstanding. as the within-subject factor. Data are presented as means " S.D.
3. 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 Ž37 men, 67 women.. Comparison between these people and the remaining 29 revealed no significant differences in gender distribution, age, body mass index, smoking status, employment grade, social support, or in blood pressure and heart rate recorded during the day. Out of a maximum of 26 possible recordings, the mean number successfully obtained was 23.4" 6.2. Sixty-two individuals reported periods of both high and low stress associated with blood pressure readings over the working day. The remaining 42 did not have any high stress ratings over the day, so comparisons between high and low stress periods were not possible. Further comparison was made between the 62 participants in the final analysis, and the remaining 42 subjects. There were no significant differences in any of the background characteristics listed above, or in cardiovascular activity. However, only 10.3% of the 42 excluded individuals rated the work day as more stressful than average, compared with 27.3% of the 62 included in the main analyses Ž 2 s 4.10, P- 0.05.. In the 62 participants analysed in this study, high and low stress ratings were distributed evenly through the working day.
A. Steptoe r International Journal of Psychophysiology 37 (2000) 299᎐308 Table 1 Characteristics of the high and low social support groups
a
High social support Ž n s 30. Menrwomen Age Žyears. % Smokers % Supervisory grade Social support ŽISEL. Stressfulness of day Above average Average or less than average Body mass index Systolic blood pressure ŽmmHg. Diastolic blood pressure ŽmmHg. Heart rate Žb.p.m.. Energy expenditure Žkcalr5 min. a
303
Low social support Ž n s 32.
11r19 40.2 Ž8.7. 13.3% 53.3% 37.5 Žl.5.
11r21 37.3 Ž8.7. 18.8% 50.0% 28.7 Ž6.3.
22.2% 77.8% 26.7 Ž3.9. 124.6 Ž3.9. 81.0 Ž7.3. 79.9 Ž9.2. 8.60 Žl.9.
32.1% 67.9% 24.3 Ž2.9. 126.0 Ž2.9. 81.4 Ž2.9. 78.7 Ž9.0. 8.53 Ž2.3.
Means ŽS.D.. and percentages.
The 62 participants were divided by median split into high and low social support groups. The median criteria Ž33 for men, 35 for women. were based on gender, since scores on social support were on average higher in women Ž34.3" 5.9. than men Ž31.5" 6.9, F1,103 s 4.63, P- 0.05.. The characteristics of the high and low support groups are summarised in Table 1. The two groups did not differ in gender distribution, age, smoking status, or employment grade. Since the groups were defined in terms of social support, there was a substantial difference in social support scores Ž F1,60 s 55.6, P- 0.0001.. There was also a significant difference in body mass index, with the high support group being significantly heavier than the low support group Ž F1,60 s 7.34, P- 0.01.. As body mass is known to influence blood pressure, it was therefore included as a covariate in all cardiovascular analyses. As can be seen in Table 1, high and low support groups did not differ in average systolic blood pressure, diastolic blood pressure, heart rate or energy expenditure over the working day. The high and low support groups did not differ in the extent to which they rated the day as more stressful than average. 3.1. Stress, social support and blood pressure An average of 53.5" 24.3% of blood pressure
and heart rate readings were taken at low stress levels, and 25.3" 19.9 at high stress levels. This difference did not vary with social support group. The analyses of energy expenditure in the 5 min preceding high and low stress periods revealed a near significant main effect of stress level Ž F1,60 s 3.80, Ps 0.056.. Energy expenditure averaged 9.21" 4.2 kcalr5 min during the high stress, and 8.32" 1.9 kcalr5 min during low stress periods. The analyses of blood pressure and heart rate during high and low stress periods were therefore carried out with concomitant energy expenditure and body mass index as covariates. The main effect for stress level was significant in the analysis of systolic blood pressure Ž F1,59 s 3.89, P- 0.05., together with the support by stress level interaction Ž F1,59 s 6.91, P- 0.01.. These data are summarised in Fig. 1. It is evident that systolic blood pressure was raised during high compared with low stress periods, but only in the low social support group. Post hoc analyses of the social support groups separately indicated that the main effect for stress level was significant in the low support group Ž F1,30 s 9.55, P- 0.005., but not in the high stress group. In the analysis of diastolic blood pressure, the main effect of stress level was again significant Ž F1,59 s 4.48, P- 0.05., while the interaction of social support and stress level was not reliable Ž F1,59 s 2.29, Ps 0.14.. The
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Fig. 1. Mean levels of systolic blood pressure in mmHg Žadjusted for concurrent energy expenditure and body mass index. during periods of low and high stress over the working day, in the low and high social support groups. Error bars are S.E.M.
adjusted means in Fig. 2 suggest a similar but not so marked pattern as that found for systolic blood pressure. In separate post hoc analyses of the two groups, the main effect for stress level was significant in the low stress Ž F1,30 s 7.52, P- 0.01., but not the high stress group. The analyses of heart rate during the day again revealed a main effect of stress level Ž F1,59 s 7.67, P- 0.01., but no interaction of social support and stress Žsee Fig. 2.. The increase in heart rate between low and high stress levels was significant in the low support group Žaveraging 76.6" 10.5 vs. 82.1" 9.7 b.p.m., F1,30 s 8.97, P- 0.005., but not in the high support group Ž78.9" 9.6 vs. 81.1 " 11.3 b.p.m... The cardiovascular data are consistent with the notion that social support buffers the impact of perceived stress on ambulatory blood pressure and heart rate under naturalistic conditions. 3.2. Posture, energy expenditure and cardio¨ ascular acti¨ ity Data were available for 112 participants in the comparison of cardiovascular activity and energy expenditure for readings taken in the seated and standing positions. The results are summarised in Table 2. It can be seen that, as predicted, energy expenditure was significantly greater in the 5 min preceding blood pressure readings taken in the
standing than seated positions Ž F1,111 s 87.7, P0.0001.. Heart rate was also higher when participants were standing, with a mean increase from the seated position of 4.67 b.p.m. Ž F1,110 s 92.4, P- 0.0001.. There was no significant difference in systolic pressure between the two body positions, but diastolic blood pressure was higher when participants were standing Ž F1,111 s 18.5, P - 0.0001.. Despite the absence of differences between systolic blood pressures on sitting and standing, it should be noted that between-subjects, there was a significant positive correlation between systolic blood pressure and energy expenditure for readings taken when seated Ž r s 0.35, P- 0.001., and when standing Ž r s 0.28, P- 0.005.. 4. 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 ŽSteptoe et al., 1996., particularly in individuals who report more variable emotional states during ambulatory monitoring ŽCarels et al., 2000.. The results replicated and extended previous findings on the effects of stress on blood
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305
Fig. 2. Mean levels of diastolic blood pressure in mmHg Župper panel. and heart rate in b.p.m. Žlower panel., adjusted for concurrent energy expenditure and body mass index, during periods of low and high stress over the working day, in the low and high social support groups. Error bars are S.E.M.
pressure, in showing that pressor responses were confined to participants who reported low social support. These findings are consistent with perceived social support having a buffering effect, reducing the impact of naturally occurring stress on cardiovascular activity. The absolute effects are small, but this is consistent with other data on
psychosocial and mood correlates of ambulatory data ŽSchwartz et al., 1994.. The study was carried out with a sample of school teachers. This profession was selected because teaching is reported to be a stressful occupation, with multiple demands from pupils, parents, and administrators, coupled with limited fi-
Table 2 Effects of body position on cardiovascular activity and energy expenditure a Seated Energy expenditure Žkcalr5 min. Heart rate Žb.p.m..
7.47 Ž1.6.
Standing 8.71 Ž2.1.
75.6 Ž9.1.
80.3 Ž9.5.
Systolic blood pressure ŽmmHg.
123.4 Ž12.0.
123.7 Ž11.9.
Diastolic blood pressure ŽmmHg.
79.6 Ž6.9.
81.1 Ž7.1.
a
Means ŽS.D.. and statistical significance.
P 0.0001 0.0001 0.62 0.0001
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nancial rewards ŽTravers and Cooper, 1996.. In other analyses of the sample from which these data were derived, we have reported high levels of stress and increased risk of emotional disturbance ŽCropley et al., 1999.. Nevertheless, the proportion of participants who reported any episodes of stress during the day was only 60.4% Ž64r104. of the total population. Even among those individuals, the majority of blood pressure readings were obtained during periods of low stress, and only 25.3% at high stress levels. Little is known about how questionnaire ratings of work stress translate into moment-to-moment experiences, and it is possible that teachers might rate their jobs as generally stressful, even if they did not experience episodes of acute stress on a typical day. However, another possibility is that relatively few stressful episodes occurred over the day because participants limited their activities. Blanchard et al. Ž1990. compared self-reported activity ratings on a day of ambulatory blood pressure monitoring and a control day without monitoring. They showed that people restricted their activity levels during the ambulatory monitoring day, spending more time at home and less in miscellaneous settings than they did on the control day. We have recently reported in a subsample of the present cohort that energy expenditure was lower on the day of ambulatory monitoring than on an adjacent day ŽCosta et al., 1999.. It is possible therefore that teachers made choices about their activities during the day of blood pressure monitoring that reduced the probability of becoming stressed. There is some evidence to support this notion in the near significant difference between participants included and excluded from the high vs. low stress period analysis. Individuals who were excluded from the main analysis by virtue of the fact that none of their blood pressure readings were associated with significant stress were also more likely than others to rate the day average or less stressful than usual. The problem of controlling for concurrent physical activity during ambulatory blood pressure monitoring was addressed by having participants wear accelerometers. A number of measures of activity have been utilised in recent am-
bulatory psychophysiological studies, including actigraphs or accelerometers worn at the wrist or waist ŽFahrenberg, 1996; Shapiro and Goldstein, 1998; Kario et al., 1999., and muscle tension from the leg ŽJohnston et al., 1990.. The Tritrac accelerometer has been extensively employed in studies of sport, rehabilitation and ageing as an index of energy expenditure ŽBouten et al., 1994; Sherman et al., 1998; Welk et al., 1998.. There is no agreement about the period of activity measurement that should be taken into account when assessing ambulatory blood pressure and heart rate. In this study, the 5 min preceding cuff inflation were included, while Kario et al. Ž1999. used the 6 min before each measurement. The role of body position is important since it not only influences blood pressure and heart rate during ambulatory monitoring, but also affects stress responsivity ŽTurner and Sherwood, 1991.. As can be seen in Table 2, the Tritrac effectively discriminated between blood pressure readings obtained in the seated and standing positions, suggesting that it might be used to index body posture as well as concurrent activity. Across participants, we also found that systolic blood pressure was greater in those who were more physically active at the time. Interestingly, the main effect of stress level on energy expenditure approached statistical significance, suggesting a tendency towards participants being more physically active during periods of high than low stress. The finding confirms the importance of estimating energy expenditure in the interpretation of psychophysiological ambulatory monitoring studies. Nevertheless, the stressbuffering effects of social support were independent of concurrent energy expenditure. This implies that the differences in blood pressure and heart rate between low and high stress periods were stimulated by central nervous system activation through autonomic or neuroendocrine pathways. The limitations of this study should be noted. The investigation was carried out within a single working group, and it is not known whether findings were generalised to other populations. The haemodynamic mechanisms underlying the influence of stress on cardiovascular activity could not
A. Steptoe r International Journal of Psychophysiology 37 (2000) 299᎐308
be evaluated with the measures available. It is also not known what environmental events stimulated the reports of high stress over the working day. Nevertheless, this analysis corroborates the results of both laboratory and epidemiological studies in finding that stress provokes cardiovascular activation. High levels of perceived social support were protective, not by altering the appraisal of stressful events, but reducing the impact of these events on blood pressure in everyday life.
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Lundin, S., Jern, S., .Hansson, L., 1986. Testing neural control of the circulation in patients: hemodynamic responses to exercise and stressful stimuli. In: Zanchetti, A., Tarazi, R.C. ŽEds.., Handbook of Hypertension, vol. 8: Pathophysiology of Hypertension ᎏ Regulatory Mechanisms. Elsevier Science, Amsterdam, pp. 237᎐249. Parati, G., Trazzi, S., Ravogli, A., Casadei, R., Omboni, S., Mancia, G., 1991. Methodological problems in evaluation of cardiovascular effects of stress in humans. Hypertension 17 ŽSuppl.., 11150᎐11155. Pickering, T.G., Gerin, W., 1990. Cardiovascular reactivity in the laboratory and the role of behavioral factors in hypertension: a critical review. Ann. Behav. Med. 12, 3᎐16. Roy, M.P., Steptoe, A., Kirschbaum, C., 1998. Life events and social support as moderators of individual differences in cardiovascular and cortisol reactivity. J. Pers. Soc. Psychol. 75, 1273᎐1281. Schwartz, J.E., Warren, K., Pickering, T.G., 1994. Mood, location and physical position as predictors of ambulatory blood pressure and heart rate: application of a multi-level random effects model. Ann. Behav. Med. 16, 210᎐220. Shapiro, D., Goldstein, I.B., 1998. Wrist actigraph measures of physical activity level and ambulatory blood pressure in healthy elderly persons. Psychophysiology 35, 305᎐315. Sherman, W.M., Morris, D.M., Kirby, T.E. et al., 1998. Evaluation of a commercial accelerometer ŽTritrac-R3 D. to measure energy expenditure during ambulation. Int. J. Sports Med. 19, 43᎐47. Spitzer, S.B., Llabre, M.M., Ironson, G.H., Gellman, M.D., Schneiderman, N., 1992. The influence of social situations on ambulatory blood pressure. Psychosom. Med. 54, 79᎐86.
Steptoe, A., Roy, M.P., Evans, O., 1996. Psychosocial influences on ambulatory blood pressure over working and non-working days. J. Psychophysiol. 10, 218᎐227. Steptoe, A., Cropley, M., Joekes, K., 1999a. Job strain, blood pressure, and responsivity to uncontrollable stress. J. Hypertension 17, 193᎐200. Steptoe, A., Cropley, M., Griffith, J., Joekes, K., 1999b. The influence of abdominal obesity and chronic work stress on ambulatory blood pressure in men and women. Int. J. Obesity 23, 1184᎐1191. Travers, C.J., Cooper, C.L., 1996. Teachers Under Pressure. Routledge, London. Turner, J.R., Sherwood, A., 1991. Postural effects on blood pressure reactivity: implications for studies of laboratoryfield generalization. J. Psychosom. Res. 35, 289᎐295. Uchino, B.N., Cacioppo, J.T., Kiecolt-Glaser, J.K., 1996. The relationship between social support and physiological processes: a review with emphasis on underlying mechanisms and implications for health. Psychol. Bull. 119, 488᎐531. Unden, A.L., Orth-Gomer, ´ K., Elofsson, S., 1991. Cardiovascular effects of social support in the work place: twenty-fourhour ECG monitoring of men and women. Psychosom. Med. 53, 50᎐60. Welk, G.J., Corbin, C.B., Kampert, J.B., 1998. The validity of the Tritrac-R3D activity monitor for the assessment of physical activity: II Temporal relationships among objective assessments. Res. Q. Exerc. Sport. 69, 395᎐399.
Stress, social support and cardiovascular activity over the working day Andrew SteptoeU Department of Psychology, St. George’s Hospital Medical School, Cranmer Terrace, London SW17 ORE, UK Received 8 December 1999; received in revised form 20 March 2000; accepted 30 March 2000
Abstract The influence of stress on ambulatory blood pressure monitored over the working day, and the potential buffering effect of social support, was assessed in 104 school teachers Ž37 men and 67 women.. Blood pressure and heart rate were measured every 20 min and energy expenditure was assessed using accelerometers. Participants rated the degree of stress they were experiencing at the time of each measurement on a seven-point scale. Episodes of both high and low stress during the working day were reported by 62 participants. They were divided by median split into high and low social support groups on the Interpersonal Support Evaluation List. After controlling for body mass and concomitant energy expenditure, high stress was associated with increased systolic blood pressure, diastolic blood pressure and heart rate. However, the impact of stress was buffered by social support, with no significant increase in blood pressure or heart rate with stress in the high support group. The accelerometers were also shown effectively to discriminate between blood pressure readings taken in the seated and standing positions in terms of energy expenditure. The results corroborate laboratory studies, in showing that social support buffers the impact of episodic stress on cardiovascular activity under naturalistic conditions during the working day. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Blood pressure; Heart rate; Stress; Social support; Posture; Ambulatory monitoring
1. Introduction There is a substantial literature relating stress and social support with cardiovascular disease, psychological well-being and other health outU
Tel.: q-44-208-725-5603; fax: q44-208-767-2741. E-mail address: [email protected] ŽA. Steptoe..
come ŽCoyne 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 ŽBurg 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
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viewpoints continues to emerge ŽLepore, 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 Žfor 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 Ž1990. 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 ŽRoy et al., 1998.. The relevance of acute psychophysiological responsivity to everyday life experience has been questioned by a number of authorities ŽPickering 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. Ž1987. 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 ŽSpitzer 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 ŽKamarck 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 Žtraffic agents or wardens. has shown a clear effect of social interaction on ambulatory cardiovascular activity ŽBrondolo et al., 1999.. Few studies have explored the influence of perceived social support on ambulatory cardiovascular activity. Linden et al. Ž1993. 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 ŽUnden 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 ŽSteptoe et al., 1996.. As noted by Carels et al. Ž1998., 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 ŽGell-
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man 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 ŽSherman 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 ŽLundin 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 withinsubjects between the two postures. These analyses were conducted on data from a larger study described elsewhere ŽSteptoe et al., 1999a,b..
2. Methods 2.1. Participants Full-time school teachers were selected from a larger survey of teachers in south London Žfor details, see Steptoe et al., 1999a.. After 12 months, participants were re-contacted and 137 Ž84.6%. 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
301
seven did not reply. Ambulatory blood pressure data were satisfactory in 133 participants. 2.2. Social support measure Social support was assessed using the Interpersonal Support Evaluation List ŽISEL, Cohen et al., 1985.. This questionnaire consists of 40 items designed to assess perceived availability of four separate types of support: tangible support or the perceived availability of practical aid, appraisal support or the availability of people to talk to and provide advice, self-esteem support measuring self-confidence and positive social comparison, and belonging support or the perceived availability of social contacts with whom one can do things. Responses to the 40 items Žscored yesrno. were summed to generate a total support score, with higher values reflecting greater perceived support. The internal consistency ŽCronbach ␣ . for the scale in this sample was 0.89. 2.3. Ambulatory monitoring procedures Ambulatory blood pressure monitoring was carried out using the SpaceLabs 90217 monitor ŽRedmond, WA, USA., an instrument that satisfies international instrumentation standards ŽBaumgart and Kamp, 1998.. The monitor was fitted between 08.00 and 09.00 h on a working day at each participating school. After confirmation of accurate functioning, the participant wore the monitor for the day and evening. Blood pressure was measured at 20-min intervals between 09.00 and 17.40 h, and subsequently at 30-min intervals. Participants were instructed to keep their arms still while the cuff inflated and deflated. Each reading was accompanied by an entry in a diary in which the participant recorded location, posture and activity. In addition, participants rated the extent to which they felt stressed at that moment, assessing the level of subjective pressure they were experiencing from 1 s extreme pressure to 7 s ¨ ery little pressure. The blood pressure readings were subsequently reviewed and outliers were excluded according to the methods described by Berardi et al. Ž1992.. At the end of the moni-
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toring day, participants were asked whether the day had been more stressful than average, average, or less stressful than average. Energy expenditure was recorded in 1-min epochs using a TriTrac-R3D research ergometer ŽReining International, Madison, WI, USA.. This solid state monitor contains accelerometers that assess movements in three planes ᎏ mediolateral, antero-posterior and vertical ᎏ and uses the integrated vector magnitude together with information concerning gender, age, weight and height to calculate estimated energy expenditure in kilocalories. Validation of the TriTrac and related triaxial accelerometers indicate that the instruments provide reliable and acceptable assessments of energy expenditure ŽBouten et al., 1994; Sherman et al., 1998.. Height, body weight, and body mass index were assessed using standard procedures. Smoking status was also recorded. So as to provide a measure of work status, participants were classified as class teachers, or teachers in supervisory grades who had additional administrative duties. 2.4. Data analysis Blood pressure and heart rate data recorded over the working day Ž9.00᎐17.40. were analysed in this report. The evening values were excluded since the physical environment and psychological pressures on individuals were very different from those experienced over the day. The total energy expenditure in kilocalories for the 5 min preceding each cuff inflation was computed. Average levels of blood pressure, heart rate and energy expenditure over the working day were calculated. For the analysis of cardiovascular activity and stress, the blood pressure, heart rate and energy expenditure values associated with each of the seven levels of subjective pressure rating were averaged. As not all participants had blood pressures associated with each of the stress ratings, data were subsequently grouped into low stress Žratings 6 and 7. and high stress Žratings 1᎐4. periods. Data were analysed using analysis of variance, with social support Žhighrlow. as the between-subject factor, and stress level Žhighrlow.
as the within-subject factor. Gender was included as a factor in preliminary analyses, but gender effects are not described unless they interacted with social support or stress level. In the analysis of posture, blood pressure, heart rate and energy expenditure were averaged for all readings when the participant was seated, and all readings during standing. These data were subsequently analysed with posture Žseatedrstanding. as the within-subject factor. Data are presented as means " S.D.
3. 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 Ž37 men, 67 women.. Comparison between these people and the remaining 29 revealed no significant differences in gender distribution, age, body mass index, smoking status, employment grade, social support, or in blood pressure and heart rate recorded during the day. Out of a maximum of 26 possible recordings, the mean number successfully obtained was 23.4" 6.2. Sixty-two individuals reported periods of both high and low stress associated with blood pressure readings over the working day. The remaining 42 did not have any high stress ratings over the day, so comparisons between high and low stress periods were not possible. Further comparison was made between the 62 participants in the final analysis, and the remaining 42 subjects. There were no significant differences in any of the background characteristics listed above, or in cardiovascular activity. However, only 10.3% of the 42 excluded individuals rated the work day as more stressful than average, compared with 27.3% of the 62 included in the main analyses Ž 2 s 4.10, P- 0.05.. In the 62 participants analysed in this study, high and low stress ratings were distributed evenly through the working day.
A. Steptoe r International Journal of Psychophysiology 37 (2000) 299᎐308 Table 1 Characteristics of the high and low social support groups
a
High social support Ž n s 30. Menrwomen Age Žyears. % Smokers % Supervisory grade Social support ŽISEL. Stressfulness of day Above average Average or less than average Body mass index Systolic blood pressure ŽmmHg. Diastolic blood pressure ŽmmHg. Heart rate Žb.p.m.. Energy expenditure Žkcalr5 min. a
303
Low social support Ž n s 32.
11r19 40.2 Ž8.7. 13.3% 53.3% 37.5 Žl.5.
11r21 37.3 Ž8.7. 18.8% 50.0% 28.7 Ž6.3.
22.2% 77.8% 26.7 Ž3.9. 124.6 Ž3.9. 81.0 Ž7.3. 79.9 Ž9.2. 8.60 Žl.9.
32.1% 67.9% 24.3 Ž2.9. 126.0 Ž2.9. 81.4 Ž2.9. 78.7 Ž9.0. 8.53 Ž2.3.
Means ŽS.D.. and percentages.
The 62 participants were divided by median split into high and low social support groups. The median criteria Ž33 for men, 35 for women. were based on gender, since scores on social support were on average higher in women Ž34.3" 5.9. than men Ž31.5" 6.9, F1,103 s 4.63, P- 0.05.. The characteristics of the high and low support groups are summarised in Table 1. The two groups did not differ in gender distribution, age, smoking status, or employment grade. Since the groups were defined in terms of social support, there was a substantial difference in social support scores Ž F1,60 s 55.6, P- 0.0001.. There was also a significant difference in body mass index, with the high support group being significantly heavier than the low support group Ž F1,60 s 7.34, P- 0.01.. As body mass is known to influence blood pressure, it was therefore included as a covariate in all cardiovascular analyses. As can be seen in Table 1, high and low support groups did not differ in average systolic blood pressure, diastolic blood pressure, heart rate or energy expenditure over the working day. The high and low support groups did not differ in the extent to which they rated the day as more stressful than average. 3.1. Stress, social support and blood pressure An average of 53.5" 24.3% of blood pressure
and heart rate readings were taken at low stress levels, and 25.3" 19.9 at high stress levels. This difference did not vary with social support group. The analyses of energy expenditure in the 5 min preceding high and low stress periods revealed a near significant main effect of stress level Ž F1,60 s 3.80, Ps 0.056.. Energy expenditure averaged 9.21" 4.2 kcalr5 min during the high stress, and 8.32" 1.9 kcalr5 min during low stress periods. The analyses of blood pressure and heart rate during high and low stress periods were therefore carried out with concomitant energy expenditure and body mass index as covariates. The main effect for stress level was significant in the analysis of systolic blood pressure Ž F1,59 s 3.89, P- 0.05., together with the support by stress level interaction Ž F1,59 s 6.91, P- 0.01.. These data are summarised in Fig. 1. It is evident that systolic blood pressure was raised during high compared with low stress periods, but only in the low social support group. Post hoc analyses of the social support groups separately indicated that the main effect for stress level was significant in the low support group Ž F1,30 s 9.55, P- 0.005., but not in the high stress group. In the analysis of diastolic blood pressure, the main effect of stress level was again significant Ž F1,59 s 4.48, P- 0.05., while the interaction of social support and stress level was not reliable Ž F1,59 s 2.29, Ps 0.14.. The
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Fig. 1. Mean levels of systolic blood pressure in mmHg Žadjusted for concurrent energy expenditure and body mass index. during periods of low and high stress over the working day, in the low and high social support groups. Error bars are S.E.M.
adjusted means in Fig. 2 suggest a similar but not so marked pattern as that found for systolic blood pressure. In separate post hoc analyses of the two groups, the main effect for stress level was significant in the low stress Ž F1,30 s 7.52, P- 0.01., but not the high stress group. The analyses of heart rate during the day again revealed a main effect of stress level Ž F1,59 s 7.67, P- 0.01., but no interaction of social support and stress Žsee Fig. 2.. The increase in heart rate between low and high stress levels was significant in the low support group Žaveraging 76.6" 10.5 vs. 82.1" 9.7 b.p.m., F1,30 s 8.97, P- 0.005., but not in the high support group Ž78.9" 9.6 vs. 81.1 " 11.3 b.p.m... The cardiovascular data are consistent with the notion that social support buffers the impact of perceived stress on ambulatory blood pressure and heart rate under naturalistic conditions. 3.2. Posture, energy expenditure and cardio¨ ascular acti¨ ity Data were available for 112 participants in the comparison of cardiovascular activity and energy expenditure for readings taken in the seated and standing positions. The results are summarised in Table 2. It can be seen that, as predicted, energy expenditure was significantly greater in the 5 min preceding blood pressure readings taken in the
standing than seated positions Ž F1,111 s 87.7, P0.0001.. Heart rate was also higher when participants were standing, with a mean increase from the seated position of 4.67 b.p.m. Ž F1,110 s 92.4, P- 0.0001.. There was no significant difference in systolic pressure between the two body positions, but diastolic blood pressure was higher when participants were standing Ž F1,111 s 18.5, P - 0.0001.. Despite the absence of differences between systolic blood pressures on sitting and standing, it should be noted that between-subjects, there was a significant positive correlation between systolic blood pressure and energy expenditure for readings taken when seated Ž r s 0.35, P- 0.001., and when standing Ž r s 0.28, P- 0.005.. 4. 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 ŽSteptoe et al., 1996., particularly in individuals who report more variable emotional states during ambulatory monitoring ŽCarels et al., 2000.. The results replicated and extended previous findings on the effects of stress on blood
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305
Fig. 2. Mean levels of diastolic blood pressure in mmHg Župper panel. and heart rate in b.p.m. Žlower panel., adjusted for concurrent energy expenditure and body mass index, during periods of low and high stress over the working day, in the low and high social support groups. Error bars are S.E.M.
pressure, in showing that pressor responses were confined to participants who reported low social support. These findings are consistent with perceived social support having a buffering effect, reducing the impact of naturally occurring stress on cardiovascular activity. The absolute effects are small, but this is consistent with other data on
psychosocial and mood correlates of ambulatory data ŽSchwartz et al., 1994.. The study was carried out with a sample of school teachers. This profession was selected because teaching is reported to be a stressful occupation, with multiple demands from pupils, parents, and administrators, coupled with limited fi-
Table 2 Effects of body position on cardiovascular activity and energy expenditure a Seated Energy expenditure Žkcalr5 min. Heart rate Žb.p.m..
7.47 Ž1.6.
Standing 8.71 Ž2.1.
75.6 Ž9.1.
80.3 Ž9.5.
Systolic blood pressure ŽmmHg.
123.4 Ž12.0.
123.7 Ž11.9.
Diastolic blood pressure ŽmmHg.
79.6 Ž6.9.
81.1 Ž7.1.
a
Means ŽS.D.. and statistical significance.
P 0.0001 0.0001 0.62 0.0001
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nancial rewards ŽTravers and Cooper, 1996.. In other analyses of the sample from which these data were derived, we have reported high levels of stress and increased risk of emotional disturbance ŽCropley et al., 1999.. Nevertheless, the proportion of participants who reported any episodes of stress during the day was only 60.4% Ž64r104. of the total population. Even among those individuals, the majority of blood pressure readings were obtained during periods of low stress, and only 25.3% at high stress levels. Little is known about how questionnaire ratings of work stress translate into moment-to-moment experiences, and it is possible that teachers might rate their jobs as generally stressful, even if they did not experience episodes of acute stress on a typical day. However, another possibility is that relatively few stressful episodes occurred over the day because participants limited their activities. Blanchard et al. Ž1990. compared self-reported activity ratings on a day of ambulatory blood pressure monitoring and a control day without monitoring. They showed that people restricted their activity levels during the ambulatory monitoring day, spending more time at home and less in miscellaneous settings than they did on the control day. We have recently reported in a subsample of the present cohort that energy expenditure was lower on the day of ambulatory monitoring than on an adjacent day ŽCosta et al., 1999.. It is possible therefore that teachers made choices about their activities during the day of blood pressure monitoring that reduced the probability of becoming stressed. There is some evidence to support this notion in the near significant difference between participants included and excluded from the high vs. low stress period analysis. Individuals who were excluded from the main analysis by virtue of the fact that none of their blood pressure readings were associated with significant stress were also more likely than others to rate the day average or less stressful than usual. The problem of controlling for concurrent physical activity during ambulatory blood pressure monitoring was addressed by having participants wear accelerometers. A number of measures of activity have been utilised in recent am-
bulatory psychophysiological studies, including actigraphs or accelerometers worn at the wrist or waist ŽFahrenberg, 1996; Shapiro and Goldstein, 1998; Kario et al., 1999., and muscle tension from the leg ŽJohnston et al., 1990.. The Tritrac accelerometer has been extensively employed in studies of sport, rehabilitation and ageing as an index of energy expenditure ŽBouten et al., 1994; Sherman et al., 1998; Welk et al., 1998.. There is no agreement about the period of activity measurement that should be taken into account when assessing ambulatory blood pressure and heart rate. In this study, the 5 min preceding cuff inflation were included, while Kario et al. Ž1999. used the 6 min before each measurement. The role of body position is important since it not only influences blood pressure and heart rate during ambulatory monitoring, but also affects stress responsivity ŽTurner and Sherwood, 1991.. As can be seen in Table 2, the Tritrac effectively discriminated between blood pressure readings obtained in the seated and standing positions, suggesting that it might be used to index body posture as well as concurrent activity. Across participants, we also found that systolic blood pressure was greater in those who were more physically active at the time. Interestingly, the main effect of stress level on energy expenditure approached statistical significance, suggesting a tendency towards participants being more physically active during periods of high than low stress. The finding confirms the importance of estimating energy expenditure in the interpretation of psychophysiological ambulatory monitoring studies. Nevertheless, the stressbuffering effects of social support were independent of concurrent energy expenditure. This implies that the differences in blood pressure and heart rate between low and high stress periods were stimulated by central nervous system activation through autonomic or neuroendocrine pathways. The limitations of this study should be noted. The investigation was carried out within a single working group, and it is not known whether findings were generalised to other populations. The haemodynamic mechanisms underlying the influence of stress on cardiovascular activity could not
A. Steptoe r International Journal of Psychophysiology 37 (2000) 299᎐308
be evaluated with the measures available. It is also not known what environmental events stimulated the reports of high stress over the working day. Nevertheless, this analysis corroborates the results of both laboratory and epidemiological studies in finding that stress provokes cardiovascular activation. High levels of perceived social support were protective, not by altering the appraisal of stressful events, but reducing the impact of these events on blood pressure in everyday life.
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