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Psychosomatic Medicine 70:7-12 (2008)
© 2008 American Psychosomatic Society

ORIGINAL ARTICLES

Effect of Social Support on Nocturnal Blood Pressure Dipping

Carlos J. Rodriguez, MD, MPH, Matthew M. Burg, PhD, Joyce Meng, MD, Thomas G. Pickering, MD, DPhil, Zhezhen Jin, PhD, Ralph L. Sacco, MD, MS, Bernadette Boden-Albala, DrPH, Shunichi Homma, MD and Marco R. Di Tullio, MD

From the Department of Medicine (C.J.R., M.M.B., J.M., T.G.P., S.H., M.R.D.T.), Columbia University, New York, New York; Department of Neurology (R.L.S., B.B.-A.), Columbia University, New York, New York; Department of Epidemiology (C.J.R., R.L.S.), Columbia University, Mailman School of Public Health, New York, New York; Department of Sociomedical Science (B.B.-A.), Columbia University, Mailman School of Public Health, New York, New York; Department of Biostatistics (Z.J.), Columbia University, Mailman School of Public Health, New York, New York; Department of Neurology (R.L.S.), University of Miami, Miami, Florida; and Section of Cardiovascular Medicine (M.W.B.), Yale University, New Haven, Connecticut.

Address correspondence and reprint requests to Carlos J. Rodriguez, Division of Cardiology, Columbia University, 622 W 168th street; PH 3-342, New York, NY 10032. E-mail: cjr10{at}columbia.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 NOTES
 REFERENCES
 
Objective: To determine if nocturnal blood pressure (BP) dipping among non-Hispanic blacks is influenced by social support. Non-Hispanic blacks have higher rates of cardiovascular morbidity and mortality from hypertension and are more likely to have ambulatory blood pressure (ABP) that remains high at night (nondipping).

Methods: A total of 68 non-Hispanic black normotensive and 13 untreated hypertensive participants (age 72 ± 10 years, 48% female) free of clinical cardiovascular disease completed 24-hour ABP monitoring and a questionnaire that included a modified version of the CARDIA Study Social Support Scale (CSSS). Nondipping was defined as a decrease of <10% in the ratio between average awake and average asleep systolic BP. Analyses were adjusted for age, gender, and systolic BP.

Results: The prevalence of nondipping was 26.8% in subjects in the highest CSSS tertile versus 41.1% in the lowest CSSS tertile (p = .009). On adjusted analysis, CSSS was analyzed as a continuous variable and remained independently and inversely associated with nondipping (odds ratio 0.27, 95% Confidence Interval 0.08–0.94, p = .04).

Conclusions: Social support may be an important predictor of BP dipping at night. These findings suggest that social support may have positive health affects through physiologic (autonomic) pathways.

Key Words: ambulatory blood pressure monitoring • social support • African-Americans • hypertension

Abbreviations: BP = blood pressure; ABP = ambulatory blood pressure; NOMASS = Northern Manhattan Stroke Study; CSSS = CARDIA Social Support Scale; BMI = body mass index; OR = odds ratio; SD = standard deviation.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 NOTES
 REFERENCES
 
Arterial blood pressure (BP) normally shows physiologic diurnal variation, with higher levels during the day and lower levels during sleep. Individuals with blunted circadian fluctuations—those showing <10% decrease in BP at night ("nondippers")—have a higher prevalence of target organ damage including left ventricular hypertrophy (1) and worsening renal function (2). Large cohort studies of both hypertensive and normotensive subjects have shown that "nondipping" of blood pressure at night confers higher cardiovascular morbidity and mortality risk (3–5). The mechanism underlying nondipping is not completely understood but evidence suggests that it reflects disrupted autonomic balance (6).

"Social support" is a construct that describes the structure of a person's social environment and the tangible, instrumental, and emotional resources the social environment provides. A wealth of data, particularly from large, long-term observational studies, has shown that higher levels of social support, whether measured by instrumental, tangible, or emotional indices, are associated with reduced cardiovascular morbidity and mortality (7–10). The mechanism(s) that underlie this apparent protective effect remain poorly understood. There is however considerable evidence that social support has a buffering effect that protects people from the adverse consequences of stress by exerting a positive influence in reducing neuroendocrine activity and sympathetic stimulation (11–14). Because changes in sympathetic activity during sleep modulate a range of cardiovascular processes including BP, social support may influence whether an individual's BP remains elevated during sleep—whether the individual is a nondipper.

Non-Hispanic blacks have a higher prevalence of nondipping status, (15,16) but studies of BP dipping have mainly focused on selected clinical samples. Hence, the prevalence of nondipping in a community-dwelling urban non-Hispanic black population remains to be fully described. A recent meta-analysis found that the prevalence of nondipping is substantially lower among non-Hispanic blacks studied outside the US compared with those within the US (17) This finding raises the possibility that the increased prevalence of nondipping among non-Hispanic blacks in the US may be at least partly a function of environmental factors rather than racial genetic make-up (18). One such factor could be social support (19). We therefore conducted a population-based cross-sectional investigation of 24-hour ambulatory BP (ABP) in non-Hispanic blacks, living in the northern Manhattan sections of New York City. We hypothesized that level of social support would be associated with dipping status in this population.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 NOTES
 REFERENCES
 
Participants were selected from the Northern Manhattan Study (NOMAS), a population-based, prospective, cohort study designed to investigate cardiovascular and stroke incidence, risk factors, and prognosis in a multiethnic randomly selected sample from northern Manhattan. The methods of subject recruitment and enrollment into NOMAS have been described elsewhere (20,21). Random digit dialing was performed and community participants were enrolled if they a) never had been diagnosed with stroke; b) were >40 years old; and c) resided in Northern Manhattan for ≥3 months in a household with a telephone.

The current report is of an ancillary study of NOMAS funded through the American Heart Association. Data were collected between April 26, 2004 and June 8, 2005. The sampling procedure is summarized in Figure 1. Participants were selected by a random recruitment scheme from among NOMAS participants if they were not taking any antihypertensive medicines and were self-identified as non-Hispanic black. Thus, the current study included normotensives and untreated hypertensives. Subjects were ineligible for the current study if they had a history of a previous cardiovascular event (congestive heart failure, stroke, significant coronary artery disease, or documented prior myocardial infarction), severe hypertension defined as an average resting systolic BP >220 mm Hg and/or average resting diastolic BP >110 mm Hg (average resting BP based on two measurements taken by a research assistant using a mercury sphygmomanometer and BP cuff appropriately sized to arm circumference), body mass index (BMI) >40kg/m2, or arm circumference >47 cm, which is too large to fit the largest sized ABP cuff. Eligible subjects were contacted by phone and offered participation in the current study. Those who indicated their interest were scheduled for an appointment. At this appointment, the study was explained, eligibility was confirmed, and informed consent was obtained. A total of 81 subjects were enrolled and underwent ABP and specific social support assessment not previously undertaken in NOMAS. The Institutional Review Board of Columbia University approved this study.


Figure 12
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  		Figure 1. Sampling scheme for this study. NOMAS = Northern Manhattan Stroke Study.

 

Diabetes mellitus was defined by the patient's self-report of such a history and/or current use of insulin or hypoglycemic agents, or a fasting glucose of >126 mg/dl. Smoking and alcohol history was defined as any history of alcohol use or current cigarette smoking. Height and weight were determined by the use of calibrated scales. The number of years of completed education was obtained from each study participant as an indicator of socioeconomic status.

Race/ethnicity was based on self-identification through a series of interview questions modeled after the 2000 US Census, and conformed to the standard definitions outlined by Directive 15.

Ambulatory Blood Pressure Assessment
An ABP monitor (SpaceLabs Ultralite 90217, Williams Medical Supplies Ltd., Rhymney, South Wales) was used to assess 24-hour BP as the subjects performed their normal activities. The accuracy and reliability of the device have been previously validated according to the Association for the Advancement of Medical Instrumentation criteria (22,23). ABP monitoring was performed according to previously published protocols with a BP cuff appropriately sized to arm circumference and placed on the subject's nondominant arm. The monitor was set to automatically record BP at 15-minute intervals during awake hours and 30-minute intervals during sleep hours. Before use, the device was calibrated against a reference mercury manometer; criterion for target agreement was within ±5 mm Hg.

Recordings were retrieved and analyzed with the aid of ABP report management system software (SpaceLabs Systems, 2004). The average systolic/diastolic BP and heart rate obtained by ABP assessment were calculated for a 24-hour period and separately for awake and sleep periods determined using subject-entered diary reports of actual sleep and awake times. On average, 77% of attempted readings over the 24-hour period were successful (range 28% to 99%). The average number of total readings over a 24-hour period was 53 (range 21–71). Over 90% of our participants had >50 readings over the 24-hour period. An average of 39 awake readings (range 16–56) and 14 readings during sleep (range 4–24) were recorded. The ratio of mean sleep systolic BP to mean awake systolic BP was computed (ABP night/day ratio). A nocturnal fall in BP of <10% was defined as nondipping (unless stated otherwise, systolic BP was used to define nondipping for all main analyses; separate additional analyses were performed using diastolic BP and mean arterial BP to define nondipping). Hypertension categories (normotensive, prehypertensive, and hypertensive) were based on ABP measures and defined according to Joint National Commission VII (24).

Social Support Assessment
The CARDIA Study Social Support Scale (CSSS) is based on a social support measure developed by Seeman and Syme (25). This scale has been extensively used in prior studies (26,27) and was modified for the purposes of this study to include 12-items related to emotional support (e.g., how often do you feel lonely, feel that others care, wish you had more close friends) and network adequacy (e.g., how often do friends and family let you down when you are counting on them). In data collection, we removed items that are not pertinent to Washington Heights area residents, such as "help with a ride...," because most NYC residents use public transportation. Questions on religion were not included in the data analysis because it was felt that these were stand-alone items not related to the social support indices we were trying to measure. The Cronbach's {alpha} of the test items used in our study is 0.74, which shows a good measurement for the underlying latent social supportive trait. Social support score was calculated using a weighted mean of emotional support and network adequacy. The intent of a total score was to assess the total amount of different types of support available to the individual.

Statistical Analyses
Mean ± standard deviation (SD) values were calculated for continuous variables, and proportions for categorical variables. Individuals were classified into dichotomous measures of dipping according to ABP night/day ratio (dippers: ratio ≤0.90; nondippers: ratio >0.90). The distribution of sociodemographic and cardiovascular risk factors was evaluated in the total cohort and among "dippers" versus "nondippers." Comparison between groups defined by dipping status was assessed using t tests for continuous variables and {chi}2 test for categorical variables. The mean ± SD values for systolic and diastolic, awake and sleep ABP were calculated for the total cohort and among the comparison groups. CSSS was analyzed both as a continuous variable and categorically according to tertiles defined as a) lower tertile: mean CSSS <2.6667; b) second tertile: mean CSSS 2.6667 to 3; and c) upper tertile: mean CSSS >3. Multiple linear regression was used to analyze the association between social support score and ABP night/day ratio. Association between the presence of nondipping and other clinical parameters was first analyzed by simple logistic regression and then by multivariate analysis adjusting for age, gender, level of social support, and systolic BP. Analysis of variance was used to examine the association between daytime BP and nighttime BP with category of social support. There is literature suggesting sociocultural factors, such as socioeconomic status, as potential moderators of the effect of social support. However, existing studies have been inconsistent (19,28). To determine whether the association between social support score and nondipping differed by education level as a measure of socioeconomic status, we used an "education-years x mean CSSS" interaction term. All tests are two-sided, with statistical significance set at the {alpha} = 0.05 level. Statistical analyses were conducted using SAS 9.1 computer software (SAS Institute, Cary, North Carolina).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 NOTES
 REFERENCES
 
The mean age of the 81 participants studied was 72.2 years ±9.8; 48.2% were women; 23.5% were active smokers; 19% were classified as diabetic. The average level of education was 13.1 year of schooling completed (some college). The cohort was predominantly normotensive as reflected by 24-hour ABP variables with 18.5% of subjects classified as prehypertensive and 70.4% classified as nondippers (Table 1). Dippers did not significantly differ from nondippers on any of the baseline characteristics.


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  		TABLE 1. Baseline Characteristics of Total Cohort and by Dipping Status

 

Analyses concerning the prediction of dipping status by traditional risk factors revealed that neither age, gender, diabetic status, BMI, smoking, level of education, alcohol use nor 24 hours systolic or diastolic BP were significant predictors of nondipping status (Table 2). Univariate predictors of dipping did not differ whether nondipping was defined by systolic BP, diastolic BP, or by mean arterial BP (data not shown). With increasing CSSS tertile, the prevalence of nondipping decreased (p trend = .009) with a 50% greater relative prevalence of nondipping in the lowest tertile (41.1%) versus the highest tertile (26.8%) of CSSS (Figure 2). Mean CSSS showed a protective effect against nondipping on univariate analysis whether nondipping was defined by systolic BP (odds ratio (OR) = 0.27, 95% Confidence Interval (CI) 0.08–0.94, p = .04), diastolic BP (OR = 0.31, 95% CI 0.12–0.81, p = .02), or by mean arterial BP (OR = 0.35, 95% CI 0.14–0.92, p = .03). Mean CSSS score was significantly lower in nondippers (dippers 3.0 ± 0.08, nondippers 2.8 ± 0.05; p = .04) adjusted for age, gender, and 24 hours systolic BP (Figure 3). On multivariate logistic regression analysis, the upper tertile of CSSS conferred significant and independent protection against nondipping compared with the lower tertile (OR = 0.22, 95% CI 0.07–0.72, p = .01). This independent protective effect remained with CSSS analyzed as a continuous variable (OR = 0.27, 95% CI 0.08–0.94, p = .04). CSSS score showed a nonsignificant trend for a continuous inverse association with ABP night/day ratio (β = –0.035 (SE = 0.019), p = .07).


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  		TABLE 2. Predictors of Nondipping

 

Figure 22
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  		Figure 2. Comparison of prevalence of nondipping among subjects according to the tertile level of social support. CSSS = CARDIA Social Support Scale.

 

Figure 32
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  		Figure 3. Median (horizontal line), 25th and 75th percentiles (box), and range (whiskers) of social support score for the total cohort and by nocturnal blood pressure dipping status of subjects.

 

Educational level was not related to CSSS score (p > .66) and the relationship between CSSS score and nondipping did not differ by education level (p > .66). There also was no relationship between daytime or nighttime BP and social support status (p > .27).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 NOTES
 REFERENCES
 
Nondipping prevalence has been reported as 69% in patients with autonomic system disorders (29) and as high as 82% in those with resistant hypertension (30) but in healthy populations, the rates have generally been much lower. In one study of younger normotensive and mildly hypertensive non-Hispanic blacks, the nondipping rate was 59% (16). We report that nondipping is a prevalent phenomenon in our elderly urban-dwelling inner city cohort of normotensive and untreated hypertensive non-Hispanic blacks which basically identifies this cohort as a high cardiovascular risk group. We also found that low social support was significantly associated with the lack of normal nocturnal BP reduction. Among our participants, we observed a graded and inverse influence of baseline social support on ABP night/day ratio. Increasing levels of social support were associated with a continuous decrease in ABP night/day ratio. High social support score was associated with a 69% lower likelihood of being a nondipper. Our results support previous findings in a cohort of 25- to 50-year-old African-Americans that suggested an inverse association between social support and BP (31).

Disease-related protective effects of social support were first described in the 1970s (32). From that time, there has been great interest in the relationship of social support to health, and in particular to cardiovascular disease (33). Several studies of initially healthy individuals (34–36) and of subjects with existing coronary artery disease (37–39) provided evidence that social support protects against the onset and progression of coronary artery disease. Few studies have examined the role of social support on ABP; however, Karlin et al. (40) assessed the role of workplace social support in 70 healthy New York traffic enforcement agents and found it to predict ABP during stressful work periods. The current study expands on this finding by utilizing 24-hour ABP and a more global measure of social support in an unselected community sample.

ABP monitoring provides information that is not obtained from conventional office-based BP measurements such as overall mean daily BP and night-day patterns. ABP measurements are known to be prognostically superior to office BP with nondipping being a particular high-risk feature linked to cardiovascular complications (3–5,41,42). In the PAMELA study, nocturnal BP was the single component of ABP most associated with cardiovascular events and mortality (43). Similarly, in the population-based Ohasama study, each 5% decrease in the decline of nocturnal systolic/diastolic BP was associated with an approximately 20% greater risk of cardiovascular mortality (5). Our result suggests the possibility that adequate social support can protect against the development of nondipping. A potential implication is that social support and psychosocial interventions may be important factors in the prevention and treatment of hypertension and ABP nondipping. Interventions that increase the level of social support in the individual or the community, such as starting patient-peer support groups and/or building senior centers, may have a large impact on cardiovascular health. This may be especially true among the vulnerable population of elderly non-Hispanic blacks considered in the present study, because it is well known that nondipping follows a race-ethnic pattern with a greater prevalence of nondipping in non-Hispanic blacks versus whites (17).

It is interesting to speculate about the potential mechanism by which high social support exerts its protective effects on BP reactivity and nondipping. One possible explanation is that people with better social support are less likely to engage in risky health behaviors. Nevertheless, alcohol consumption, smoking status, and BMI were not significantly associated with nondipping status in this study. Diet was not examined in the current study and the possibility that those with high social support consume low-salt diets remains. It may be more likely, however, that physiologic mechanisms are involved. Low social support may also be linked to autonomic dysregulation and cardiovascular reactivity (44). Uchino et al. (45) conducted a comprehensive review of the literature on the association between various domains of social support and the autonomic dysregulation, as indicated by cardiovascular reactivity and neuroendocrine measures. The authors cited a consistent cross-sectional correlation between social support and BP. A meta-analysis of experimental reactivity studies revealed a particularly robust trend, such that low social support was related to greater cardiovascular reactivity and individuals who were offered support showed comparatively less reactivity as measured by BP and heart rate (46). Sustained adrenergic activity during sleep is associated with blunted nocturnal BP reduction (47). Because blunted ABP variability reflects an altered sympathetic-parasympathetic balance, it is altogether consistent to hypothesize that high social support exerts some of its effects by altering the neurohormonal milieu, thereby counteracting the psychosocial and adrenergic stress in the individual (44,48). This hypothesis remains to be further tested with biomarkers in future studies.

Strengths and Limitations
Several other studies have investigated the prevalence of nondipping in hypertensive non-Hispanic blacks, but few have documented the prevalence of this phenomenon in a population-based, predominantly normotensive, elderly non-Hispanic black cohort. This is a population that is particularly vulnerable to cardiovascular complications; hence, their risk profile deserves careful characterization. In addition, few studies have found protective entities against nondipping.

First, because this is a cross-sectional study, we cannot determine the direction of the association (i.e., exclude the possibility that nondipping caused subjects to have less social support). This is unlikely, however, because nondipping is mostly an asymptomatic phenomenon and probably will not influence the social structure around an individual. It is possible that other processes could be related to both low social support and nondipping status, such as poor sleep quality/duration, depression, and/or sleep apnea. Elucidating the role of these potential confounders will require further investigation.

Second, a nonsignificant trend was apparent in the relationship between social support and dipping as continuous variables. This result was likely limited by our sample size to reach statistical significance and would be important to test in a larger cohort.

Third, the effect of socioeconomic status as a potential moderator of social support was nonsignificant. Prior studies examining the social support-socioeconomic status interaction have not been consistent (19,28,33). It is possible that education does not moderate the effect of social support in our cohort. However, it is also possible that education is not the best measure of socioeconomic status or that there was lack of statistical power given our small sample size for analyzing interaction terms.

Finally, although one study reports that nondipping is associated with higher cardiovascular mortality in the general population, the effect of nondipping in otherwise healthy, normotensive individuals has not been widely documented. It would be helpful to follow the individuals in this cohort to observe whether nondipping status predicts future end-organ damage.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
NOTES
 REFERENCES
 
In our community-based sample, we established that nondipping is a prevalent phenomenon among urban normotensive elderly non-Hispanic blacks and that a high level of social support had a significant protective effect against this high-risk feature. Further studies are needed to clarify the mechanism(s) that underlies this association as well as the impact of nondipping in this cohort. Interventions designed to foster social support in vulnerable populations may decrease the prevalence of this high-risk feature. This study links a measure of physiologic neurohormonal activity such as BP dipping to a social stressor such as lack of social support that may contribute to the increased prevalence of hypertension and left ventricular hypertrophy among non-Hispanic blacks.

Dr. Rodriguez had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. The authors wish to thank Mr. Devon Dyett for his assistance in the collection of the data and preparation of the manuscript.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 NOTES
REFERENCES
 
Received for publication January 24, 2007; revision received July 13, 2007.

This work was supported in part by Grants R01 NS-29993 (R.L.S.) and K24 NS02241 (M.R.D.T.) from the National Institute of Neurological Disorders and Stroke, as well as Grants PO1 HL 47540 and R24 HL76857 (T.G.P.) from the National Heart, Lung, and Blood Institute. Dr. Rodriguez has been supported by a Young Investigator award from the Association of Black Cardiologists, an American Heart Association Clinically Applied Research Grant, and Grant K23 HL079343–01A2 from the National Heart, Lung, and Blood Institute's Mentored Patient-Oriented Research Career Development Award.

This study was presented in part at the American Heart Association Conference on Cardiovascular Epidemiology and Prevention, March 3–5, 2006, Phoenix, Arizona.

DOI:10.1097/PSY.0b013e31815aab4e


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 NOTES
 REFERENCES
 

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D. C. Cooper, M. G. Ziegler, R. A. Nelesen, and J. E. Dimsdale
Racial Differences in the Impact of Social Support on Nocturnal Blood Pressure
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