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Longitudinal Association of Cardiovascular Reactivity and Blood Pressure in Samoan Adolescents

From the Department of Medicine (J.D.N., S.T.McG.), The Miriam Hospital, Brown University School of Medicine, Providence Rhode Island; and Department of Anthropology (M.S.S.), University of Washington, Seattle, Washington.

Address reprint requests to: Stephen T. McGarvey, PhD, MPH, Department of Medicine, The Miriam Hospital, 164 Summit Ave., Providence, RI 02906. E:mail: Stephen_McGarvey{at}brown.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Methods RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
OBJECTIVE: The longitudinal association between blood pressure (BP) reactivity to a video game and resting BP 3 to 4 years later was investigated in 83 Samoan adolescents from American and (Western) Samoa as part of a multidisciplinary study of cardiovascular disease (CVD) risk in modern Samoans.

METHODS: Participants ranged in age from 11 to 14 years at baseline, in 1992 to 1993, and 14 to 18 years at follow-up in 1996. Video game BP reactivity was defined as the residual score of the regression of the maximum BP during the video game on the minimum resting BP before the video game. The predictive effect of baseline video game BP reactivity on follow-up resting BP was tested using regression models with baseline resting BP, baseline body mass index (BMI), and age as covariates.

RESULTS: Systolic BP reactivity to the video game at baseline was significantly, p = .04, and independently associated with resting systolic BP 3 to 4 years later. Samoan adolescents who had higher systolic BP reactivity scores at baseline had significantly higher resting systolic BP at follow-up after adjustment for the significant effects of baseline resting systolic BP, age, and BMI. There were no interactions between sex and reactivity or between residence, American Samoa or (Western) Samoa, and reactivity in the models, indicating that the effects of systolic BP reactivity in early adolescence on later adolescent resting systolic BP were similar in the entire study sample.

CONCLUSIONS: Video game cardiovascular reactivity seems to assess aspects of psychophysiological arousal and prospective CVD risk in Samoan adolescents of both sexes residing in both Samoas, and may be useful for understanding the role of psychosocial stress and health in modernizing societies.

Key Words: cardiovascular reactivity, • blood pressure, • adolescents, • Samoans, • stress, • modernization.

Abbreviations: BP = blood pressure;; SBP = systolic blood pressure;; DBP = diastolic blood pressure;; BMI = body mass index;; CVR = cardiovascular reactivity;; CVD = cardiovascular disease;; HR = heart rate.

	INTRODUCTION

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Cardiovascular reactivity (CVR) is measured by cardiovascular responses to laboratory stressors such as hand-grip tests, posture change, video game challenges, public speaking, or mental arithmetic (1–5). These responses include blood pressure, heart rate, stroke volume, and peripheral resistance, among others. BP and HR responses have been studied most often and are generally characterized by increased levels during the standardized challenge, but with substantial heterogeneity among individuals. CVR is presumed to elicit responses similar to those evoked by chronic and acute psychosocial stressors of daily life and thus provide insights about the relation of stress and cardiovascular health (1–5). Hypertensive individuals exhibit greater CVR to laboratory stressors than normal individuals (6, 7). CVR has a predictive relationship to BP later in life in some studies but not all (3, 8–10). Reactivity to standardized physical stressors have shown better predictive ability to later BP than reactivity to psychological stressors (3, 4).

CVR in children and adolescents has been associated with BP 5 years later (8), but overall few studies have investigated the longitudinal relationships during childhood, adolescence, and early adulthood of CVR and blood pressure (6, 10–14). Adolescence and young adulthood are periods of rapid physical growth, development, and cardiovascular changes. Behavioral and psychosocial adjustments to adult roles and expectations are also necessary during these periods of maturation. It may be difficult to detect the effects of CVR on later BP in the midst of these dynamic biopsychosocial processes. However, it may be an ideal time to determine how CVR patterns become established and whether they are causally related to BP increases in adult life.

Because of the rapid social, economic, and cultural changes of modernization, adolescents are exposed to a higher level of biobehavioral influences on BP than adolescents in either more traditional or more developed societies. Increased dietary intake of calories, sodium, and fats, decreased physical activity, and the consequent adiposity together lead to increases in adolescent BP (15). Increased education and wage occupations and increased expectations for living standards also influence the context of adolescence. For example, in Samoan adolescents, increasing education and exposure to modern mass media increase acculturation to nontraditional attitudes and aspirations about adolescence and adulthood (16, 17). In Samoa, exposure to modern youth culture’s emphasis on material possessions, individual expression, and behavioral autonomy conflicts with both the traditional obedience among youth to family and village authority, and with the financial abilities of families (17). Problematic adolescent behaviors, such as cigarette smoking and alcohol and illegal drug use, are also now being documented in both Samoan polities (18, 19). These broad changes in the context and experiences of adolescence and the resultant stressors are occurring throughout the world’s developing regions, of course, not just in Samoa. Thus, longitudinal studies of cardiovascular risk, including CVR, in modernizing adolescents and young adults may provide unique information about the antecedents of adult cardiovascular disease.

We conducted cross-sectional CVR studies in American Samoa and (Western) Samoa in 1992 and 1993 in a sample of 142 adolescents 11 to 14 years of age and found substantial heterogeneity in reactivity to a video game (20, 21). Although the focus of the prior study was differences in BP and heart rate responses to a video game stressor between the two locales, increased resting BP and heart rate values among the (Western) Samoan sample indicated that the novelty of the procedure probably confounded our ability to conclude about broad modernization effects. The objective of the present study was to test the predictive relationship between BP video game reactivity and resting BP 3 to 4 years later among the adolescent Samoans studied in 1992 and 1993. For this study, we considered the adolescents seen in 1992 and 1993 and in 1996 as one cohort with substantial variation in body size and general exposure to modernization.

	Methods

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Study Population
The Samoan archipelago is a relatively isolated chain of nine islands in the South Pacific Ocean, which has undergone significant economic modernization through the increased contact with Western nations since the end of World War II. This contact exposed the archipelago to western social, economic, cultural, and dietary patterns. American Samoa is a United States territory with a 1990 population of 46,773, 91% of whom are Samoan ethnicity (22). American Samoa receives annual external funding from the U.S. Department of the Interior and is able to receive specific grants for health, transportation, and other infrastructural improvements. Many American Samoans are returning after residing in Hawaii and the western United States, and visits by American Samoans to families in the United States and elsewhere have increased knowledge about contemporary non-Samoan ways of life (17). Samoa (formerly Western Samoa) had a population in 1991 of 161,298, almost 98% of whom are Samoans (23). Samoa became an independent country in 1962, but maintains close ties with New Zealand. Samoa is considered a developing nation, still with a large subsistence agriculture and fishing economy in rural areas. Emigrants from Samoa send substantial amounts of money home to families, with estimates that more than half of the nation’s income ultimately derives from these remittances. Educational levels in both Samoas have increased more than wage labor opportunities, leading to underemployment especially in young adults (22, 23).

Previous studies described in detail the broad effects of modernization on the prevalence and incidence of adult hypertension, obesity, and on levels of other CVD risk factors such as blood lipids and diet in the Samoan archipelago (24–32).

Research Design and Study Sample
This is a longitudinal study of the relationship between CVR to a video game at baseline and follow-up resting BP with an average follow-up period of 3.5 years. Details of the baseline sampling have been described fully (20, 21). The longitudinal sample consists of 34 American Samoans and 49 (Western) Samoans ranging in age from 14 to 18 years in 1996 (Table 1) who were seen in 1992 to 1993 in a study of video game CVR in modernizing adolescents. The follow-up study participants, consisting of both sexes from two locations, are treated as one study sample to maximize the power to detect the effects of baseline reactivity on later BP. We also adjusted for variation in body size and BP over time and tested for interactions (see below) to control for the potential confounding due to sex and residence in the study sample.

The follow-up study sample had an approximately equal sex distribution, 50.6% males and 49.4% females. There was a higher proportion of (Western) Samoans in the follow-up relative to the baseline samples, 68% (49/72) versus 49% (34/70) of American Samoans. This was due to the younger age at baseline of the Western Samoan youth (Table 1), and the reduced opportunity for emigration and jobs in less modernized (Western) Samoa (23). Signed parental consent forms translated into Samoan and verbal consent from each participant were obtained. The protocol was approved by the institutional review board of The Miriam Hospital.

Procedure
Data collection procedures for video game reactivity are fully described in earlier reports (8, 11), including the baseline findings among Samoan adolescents (20, 21). The Breakout video game (Atari, Inc., Sunnyvale, CA) was used with a demonstration and practice period, followed by three games: game 1 was played and point total recorded; before game 2, participants were urged to try hard to beat the score of game 1; and before game 3, a T-shirt was offered if more points were scored than in either games 1 or 2. BP was recorded with a Dinamap monitor (Critikon, Inc., Tampa, FL) four times at rest before the video game, with the first measurement discarded due to the novelty of the procedure for Samoan adolescents. BP was measured once during each of the three video games, after 15 to 30 seconds of playing. In the original study we assessed reactivity as the difference between the mean of the three video game BPs and the mean of the three resting BPs (20). As described below, in the present study we use a residual score to assess the longitudinal association of reactivity to later BP.

Stature and weight were measured using standard protocols with subjects wearing light tropical clothing and shoes removed at both baseline and follow-up. Follow-up BP was measured in the seated position after a minimum 5-minute rest with a mercury sphygmomanometer and proper cuff sizes based on upper arm circumference. The first and fifth Korotkoff sounds were used to determine systolic and diastolic BP, respectively. Three BP readings, each 1 minute apart, were taken by a single trained observer at follow-up and averaged.

Statistical Analysis
A residual BP reactivity score was computed based on the regression of the maximum video game BP on the minimum resting BP before the video game. This measure of the highest residual change in BP, adjusted for resting BP value, maximizes the interindividual differences in BP reactivity within the sample. Residual change scores during experimental procedures are thought to be preferable to difference scores or ratios, in asmuch as they adjust for level of baseline and regression to the mean (33). Pearson correlations were estimated among baseline, follow-up, and their difference for various anthropometric and cardiovascular factors and with the residual reactivity measure.

Regression models were used to test for a longitudinal relation between baseline residual reactivity and follow-up BP 3 to 4 years later. The models included baseline BP reactivity as the main predictor and follow-up mean resting BP as the outcome, with baseline mean resting BP as the main covariate to adjust for baseline BP level. Other covariates of follow-up BP were included as additional predictors, including baseline age and BMI. The aim of these models was to detect the independent effect of video game reactivity on follow-up resting BP after adjustment for the effects of baseline BP and other covariates. Interactions between residence (American Samoa vs. [Western] Samoa) and reactivity, and between sex and reactivity, were also tested in the regression models of follow-up BP.

	RESULTS

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At follow-up the American Samoan males and females were significantly, p < .05, older, taller, and heavier, and had a greater BMI than their (Western) Samoan counterparts (Table 1). American Samoan males and females had significantly, p <.05, greater longitudinal change in SBP than (Western) Samoans, and American Samoan males had greater, p < .05, increase in DBP than (Western) Samoan males.

SBP and DBP responses during the video game indicated a substantial BP reactivity (Table 2). The differences between the maximum video game BP and minimum resting BP were very similar for males and females from the two locations: SBP/DBP reactivity for American Samoan males was 23.1/19.9, for American Samoan females 26.2/22.7, for Western Samoan males 27.5/30.2, and for Western Samoan females 28.0/24.6 mm Hg, respectively. The Samoan responses were greater overall than in the Tennessee European-American and African-American youths studied by Murphy et al (8, 20).

There were several significant bivariate correlations between baseline age, BMI, SBP, and DBP and follow-up SBP and DBP in the combined follow-up study sample (Table 3). For example, the correlations of baseline BMI with follow-up SBP and DBP were 0.32 and 0.46, respectively, both significant at p < .01. The longitudinal correlations of BP at baseline with BP at follow-up ranged from 0.32 to 0.36 over the 3- to 4-year period. As expected there were inverse associations between baseline BP and the difference between follow-up and baseline BP. The residual SBP reactivity measure had a marginal bivariate association with follow-up SBP. The bivariate correlations between residual SBP reactivity and follow-up resting SBP were similar in the two residence subsamples, eg, 0.27 and 0.20, in American and Western Samoa, respectively. Neither was significant at p < .05, due to low sample sizes.

	DISCUSSION

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Samoan adolescents characterized by greater SBP reactivity at 11 to 14 years of age had significantly increased follow-up resting SBP at age 14 to 18 years. This predictive relationship was independent of the separate influences on follow-up resting SBP level of baseline resting BP, BMI, and age, and was present in the entire study sample of males and females from both Samoas. This study supports other findings of associations between BP reactivity and levels of BP (6, 10–14), and replicates, in a smaller but unique sample of modernizing adolescents, the 5-year longitudinal relationship of video game BP responses and resting BP levels found among Tennessee white and black school children (8).

There are no data on the mediating mechanisms underlying this longitudinal association of SBP reactivity and later SBP levels in modernizing Samoan adolescents. Much of the interindividual differences in BP reactivity is likely due to the genetic and biological factors underlying cardiovascular function. However, we can speculate about some potential stressors for contemporary modernizing Samoan adolescents from ethnographic, local government census and social survey information, and our two-decade research experience. Ethnographic descriptions of Samoan culture emphasize the critical importance of maintaining a balance among the needs for individual adult development, extended family competition for social prestige and access to material resources as adults, and the requirement for cooperation in small village and circumscribed island communities (16–19, 34–38). These traditional ecological, demographic, and social constraints have produced individual psychological adaptations characterized by group orientation, obedience or even submissiveness to authority, especially among youth, and suppressed emotional expression (34–40). Traditionally, youth and young adults work for their family in subsistence labor or return a substantial portion of earnings to the family. In the contemporary Samoan islands, youth and young adults are increasingly expected to work for wages and give their earnings to the family. The family uses the money for improvements in the standard of living, purchases to enhance prestige, and off-island opportunities for family members (16–19, 37–40).

The sociocultural context of adolescence throughout the Samoan islands is more similar through exposure to education and global youth culture (from television, popular music, films, printed media, and visitors) than the variation in occupation, household income, and standard of living between American Samoa and independent Samoa. We believe that for adolescents, family demands may conflict with individualistic attitudes and goals that are responses to increased educational and occupational opportunities and to perceptions of a possibly more autonomous way of life. Additionally, in contemporary Samoa there seem to be increases in the usual adolescent risk behaviors of more developed nations, such as frequent alcohol consumption, illegal drugs use, incidents of violence, and reported stress in youth (18, 19). In summary, contemporary modernizing Samoan adolescents may be facing increased levels of psychosocial stressors as they mature to adulthood. These potential life stressors may increase BP reactivity because of a chronic level of arousal, and may interact with underlying biological susceptibility to increased responses.

The video game procedure that rewards individual achievement against one’s own prior performance, but without shaming others or losing face, may permit Samoan adolescents to actively strive to do well, get engaged in the experimental challenge, and become psychophysiologically aroused. More generally, for studies of cardiovascular reactivity in Samoans, additional research is needed to establish: a) whether different inducements during the video game would increase game involvement and BP responses; and b) whether other procedures, such as public speaking or mental arithmetic, may elicit reliable and predictive BP and heart rate responses. We cannot exclude the possibility that the novelty of the baseline video game challenge elicited BP responses unrelated to normal life exposures, especially in the Western Samoans (20). However, the significant association shown here indicates that those with greater CVR had higher BP values 3 to 4 years later, whatever the source of perceived stressors, eg, including the normal demands of contemporary Samoan adolescence, specific psychosocial exposures to modern life, or the experiment itself. Moreover, the follow-up BP levels in the Western Samoan subsample are similar to BP levels for age measured in a larger rural and urban survey of body size, fatness, and BP conducted there from 1991 to 1995 (unpublished data). This suggests that the predictive findings here were not affected by the perhaps artificially increased resting BP measured as part of the reactivity study in 1993.

Several methodological decisions and techniques provide confidence in our findings. First, we used a residual reactivity measure that may be better than difference scores or ratios for controlling the effects of prechallenge resting levels on the response (26). We used a maximum video game response reactivity residual based on the maximum BP during the video game and the minimum pregame resting BP. This was done to increase interindividual variation and the chance to detect longitudinal effects. We believed from our earlier studies (20) that participants may have been stimulated by the setting and novelty of video game procedure. Thus, we felt justified in selecting the single lowest resting BP to use in our calculation of reactivity. When we calculated a residual reactivity from the mean of video game BPs and the mean resting BP, the longitudinal regression results were very similar, except for an attenuated association between the reactivity measure and resting follow-up systolic BP. Thus, we think our residual reactivity measure is a more efficient measure for detecting the hypothesized association. Second, we adjusted resting follow-up BP for the resting level at baseline as well as baseline BMI and age. Thus the reactivity measure had to explain significant variation in follow-up BP after the powerful influence of prior resting BP was taken into account, and the effects of adiposity and age were considered. Thus, we conclude that CVR 3 to 4 years earlier had an independent and additional influence on follow-up BP. Third, we combined the participants into one study sample, despite variation in age and residence, as well as body size, BP, and their longitudinal changes (Table 1). We did this to increase our power to detect associations. We explicitly tested for the potential effect modification of sex and residence on the reactivity and follow-up BP relationship. When we did this there were no significant interactions, indicating the longitudinal relationship was present throughout the entire study sample. We showed the bivariate correlations by residence which also indicated that the longitudinal independent relationship is present throughout the sample. Fourth, we modeled change in BMI from baseline to follow-up in our longitudinal models and found virtually identical findings to those shown. This indicates that the longitudinal BP reactivity and follow-up BP relationship is not confounded by changes in adiposity.

One weakness in the study is that most of the adolescent participants derived from the peri-urban areas of the Samoan archipelago. The findings may not be generalized to all Samoan adolescents, especially rural residents. Because of the school-based sampling, we believe the participants are an adequate cross-section of the general peri-urban adolescent population of both American and Western Samoa. Additional work on CVR is needed among adolescents in rural districts of both countries. Secondly, several factors that influence BP in adolescence were not measured, including pubertal status, cigarette smoking, over-the-counter medications, physical activity, and alcohol and caffeine consumption. Some of these may be associated with reactivity, and thus confound the predictive relations shown here (1–3). Future studies should include measurements of these potential confounders. Finally, although we did not find significant interactions between reactivity and sex or residence, future studies with larger sample sizes may reveal effect modification of the relationship between reactivity and follow-up BP by sex, age, degree of acculturation, lifestyle incongruity, or anger expression (38–40).

In summary, the longitudinal findings support the study of cardiovascular reactivity to a psychological stressor and CVD risk outcomes in modernizing Samoans. These associations may be unique to adolescents and require additional studies in adults. These findings must be compared with other studies on ambulatory BP or life stress interviews. Future studies of video-game reactivity in adolescents and adults are planned with other variables, such as self-reported stressors, emotion coping, and lifestyle incongruity. These studies may permit us to determine whether video-game and other standard reactivity measures simulate genuine psychosocial challenges in modernizing Samoa.

	ACKNOWLEDGMENTS

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This work was supported by National Institutes of Health Grants AG09375 and HL52611.

We acknowledge the original contribution of Dr. Joseph Murphy to the baseline study of reactivity in Samoan adolescents and his influence on many of the ideas discussed here. We thank the American Samoa Department of Health, Division of Public Health, especially Dr. Joseph Tufa, Director, Diana Tuinei, RN, Head of Public Health Nurses, Heather Lindstrom-Ufuti, and Tu’u Maiava. We also thank the Director of the Department of Education of American Samoa, Dr. Tautalasi Tuato’o, for permission to work in the schools, and the administration and teachers at Matafao and Lupelele Schools, Samoana High School, Nu’uuli High School, and Tafuna Vocational & Technical High School. We thank the Department of Health of Samoa, especially, the Honorable Misa Telefoni, Minister of Health, Dr. Eti Enosa, Director General of Health, Dr. Nu’ualofa Tu’uau-Potoi, Head of Public Health, and Christine Quested, Chief of Nutrition Center. We also thank Superintendent Elder Harold Jensen, and the administration and teachers of Pesega School in Apia. Finally, we thank the student participants and their parents for their permission and cooperation.

Received for publication November 20, 1997.

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