Blood pressure reactivity in children

Journal of Psychosomonc Pnnted in Great Britain.

Research,

Vol. 32. No.

I. pp. 1-12.

1988.

0022-3999188 $3 00 + .oO Pergamon Press plc

INVITED REVIEW BLOOD PRESSURE REACTIVITY JAMES F. SALLE,*

JOEL E. DIMSDALE

IN CHILDREN

and CHIPMAN CAINE

Abstract-This paper reviews the relevant pediatric literature about blood pressure (BP) hyperreactivity to stress as a possible precursor of hypertension and coronary heart disease. Two prospective studies of children indicate that BP hyperreactivity predicts later development of hypertension. Several studies have identified correlates of BP reactivity. Race, obesity, Type A and family history of hypertension appear to be associated with BP reactivity in children. There appear to be both genetic and environmental influences, but relatively few psychosocial variables have been studied. There are significant methodological concerns in terms of defining the characteristics of the stressors and the stability and generalizability of responses to laboratory stressors. BP reactivity in children is a potentially important area of inquiry that has been understudied.

INTRODUCTION

WHILE much has been written in the pediatric literature about cardiovascular disease (CVD) risk factors such as blood pressure, lipids, smoking, and obesity [l-4], relatively little attention has been paid to the reactivity of blood pressure (BP) to stress. This variable has been of increasing interest to investigators studying adult CVD because of growing evidence that blood pressure reactivity may be related to risk of CVD [5]. Child health researchers have become more active in studying BP reactivity, and it is timely to survey and integrate current knowledge in this field. This paper reviews the rationale for studying blood pressure reactivity. We briefly discuss adult reactivity studies and then examine in detail the association between BP reactivity in childhood and subsequent CVD risk. We also review correlates of hyperreactivity in children. We conclude with a discussion of methodological issues. It is our conviction that this is a fertile, understudied area of research with far-reaching implications. BLOOD

PRESSURE

HYPERTENSION

AND

REACTIVITY OVERVIEW

AS A PRECURSOR OF REACTIVITY

OF STUDIES

Clinical medicine relies heavily on a data base of observations and laboratory findings obtained during standardized conditions. The individual’s physiology is blood pressures are usually measured during a quiescent state. For example, routinely taken after the subject has rested briefly, and many laboratory chemistries are obtained preferentially as fasting morning samples. These controlled observations provide data about the individual’s undisturbed or baseline state.

*Address correspondence to: James F. Sallis, Child and Family Health Studies, M-031-F, of Pediatrics, University of California, San Diego, La Jolla, CA 92093, U.S.A.

Department

2

JAMES F. SALLISetal.

The emerging field of ambulatory blood pressure monitoring has demonstrated the relevance of obtaining physiological measures in other than the baseline state. We have learned that heart rate, blood pressure and plasma catecholamines, go through extraordinary variation in the context of daily routines [6]. Heretofore, we would have regarded such elevated levels as pathological; now we regard them as worrisome and puzzling. The crucial question is whether these elevations have clinical consequences. Recent ambulatory blood pressure studies suggest that indeed such elevations do have consequences. Ambulatory readings of blood pressure as compared to clinic blood pressures are better correlates of increased left ventricular mass [7] and are better predictors of subsequent cardiovascular morbidity and mortality [S]. The reaching of a diagnosis itself has been singularly influenced by such techniques. For instance, Pickering et al. [9] have demonstrated in adults that the correlation between an office blood pressure and an ambulatory (or behaviorally responsive) blood pressure is disappointingly low. It is awkward to obtain such physiological data in the field, so investigators have been attempting to devise laboratory techniques for measuring reactivity to standardized stimuli. The glucose tolerance test and the graded exercise test are two familiar examples of this approach in adult medicine. Curiously, blood pressure reactivity is not routinely examined, and casual resting blood pressures are relied upon, despite the fact that one experiences roughly 100 000 blood pressures per day. A body of literature is accumulating that examines the blood pressure reactivity to a host of tasks (e.g. cold pressor, behavioral stress, dynamic exercise). This area of research inquiry is entirely compatible with the tradition of careful observation of the patient both at baseline and in response to situations that mimic those that may be encountered in his or her day to day living. The following sections briefly review some of the blood pressure reactivity literature from adult medicine and then discuss more extensively the literature from pediatrics.

CORONARYHEARTDISEASE

ANDHYPERTENSION

Studies of adults

The BP reactivity of hypertensives and coronary heart disease patients is higher than that of normal controls [ 10-121, but longitudinal data are needed to determine whether hyperreactivity precedes pathological changes. Early reports that BP reactivity to cold pressor predicted future hypertension over a 6-yr period in a small sample of adults [13] stimulated a great deal of research. However, several long-term studies (up to 30 yr) of BP reactivity to cold pressor in adults revealed that reactive BP did not predict future hypertension [14-171. The reasons for these contradictory results are not apparent. More recent studies using exercise as the stressor have consistently found reactive BP’s to predict later levels of resting BP. Three investigations (total N studied = 8826) have shown that normotensive adults whose BP’s are hyper-reactive during exercise are more likely to become hypertensive in the subsequent three to six years [ 18-201. Several prospective studies have examined the relationship between BP reactivity and coronary heart disease (CHD) morbidity and mortality, and all of them have

Blood

pressure

reactivity

3

yielded positive results. Keys et al. [21] measured numerous variables and followed 279 men for 23 yr. BP reactivity to the cold pressor was a better predictor of CHD mortality than resting BP, cholesterol, and smoking. This study provides strong evidence that BP hyperreactivity is indicative of a pathological process. Other longitudinal studies of cold pressor [12] and postural change (lying to standing) [22] also found BP reactivity to be an independent predictor of CHD. These studies of adults are mixed in terms of predicting hypertension. On the other hand, they suggest strongly that BP reactivity predicts CHD. Longitudinal studies of children and adolescents have also been conducted that support the potential relevance of reactivity to subsequent CVD. Studies of children In 1937, Hines [23] began a study of BP reactivity in 300 school children aged 7-17. After 27 yr, 207 subjects had their blood pressures reassessed [24]. Of the 40 children who were initially hyperreactive to the cold pressor test (BP increase >25 mm Hg systolic or >20 mm Hg diastolic), four became hypertensive, while none of the 167 normal reactors did so. Blood pressure measurements were also collected on 142 of the subjects at a 45year follow-up [25]. Seventy-one percent of the original hyper-reactors were classified as hypertensive, as opposed to 19% of the normoreactors @ < 0.001). This landmark study indicates that BP reactivity in childhood is a strong predictor of hypertension, and it is virtually unprecedented in its length of follow-up. Another longitudinal study of subjects initially assessed during adolescence found that hyperreactivity predicted subsequent hypertension [26,27]. After 5 yr of followup, 54 of 80 borderline hypertensives (90-95th percentile) became hypertensive (>95th percentile). The eventual hypertensive group was characterized by family history of hypertension and by large BP responses to a mental arithmetic task. Those becoming hypertensive had greater systolic and diastolic BP responses to difficult subtraction problems than those remaining in the borderline range. A recent study of 3524 five to 14 yr old children from Bogalusa Heart Study [28] confirmed previous findings. Blood pressure response to cold pressor, handgrip, and orthostatic stressors all predicted systolic and diastolic BP levels four years later. The mean correlations between reactive BP and resting BP four years later were 0.57 for systolic and 0.42 for diastolic BP. Three prospective studies of children, using different stressors, found that BP reactivity predicts later BP levels. The Hines [23], Falkner [27], and Parker et al. [28], studies indicate that research on BP reactivity in children may add substantially to our knowledge of, and ability to predict, risk of future CVD in children. The results of adult studies of hypertension prediction produced mixed results, but the adult and child findings as a whole are impressive. There is a clear need for replication and further research. At this point the data are sufficiently intriguing to warrant inclusion of BP reactivity in prospective studies, to identify determinants of BP reactivity, and to further develop assessment methods. Correlates of blood pressure reactivity in children Given that BP reactivity appears to be a risk factor for the development hypertension and CHD, it is advisable to identify factors that may influence

of BP

4

JAMES F. SALLIS et al.

reactivity. This is especially important in childhood, since there is evidence of substantial tracking over time in BP and other CVD risk factors [29-311, so it is possible that BP reactivity is an enduring characteristic as well. Identification of correlates of childhood BP reactivity could promote knowledge of early factors in the etiology of CHD and hypertension and could guide the development of early interventions. The purpose of this section is to survey the literature of factors that may precede the onset of BP hyper-reactivity in children. There are numerous studies of both children [28, 32-341 and adults [lo, 12, 35, 361 demonstrating greater BP reactivity (especially systolic BP) in those with high resting BP. Table I summarizes 21 studies of potential correlates of BP reactivity to stress. In general, the studies examined children younger than 18, although some samples included a few older subjects. TABLE I. -CORRELATES

Study

n

Alpert etal. 1981[62]

221 white 184 black boys and girls 21 black 21 white all boys 25 black boys 21 black girls 20 white boys 11 white girls 37 boys 38 girls ? girls

Coates etal. 1982 [57] Ewart et al. [40]

Falkner et al. 1979 [26] Falkner etal. 1981[46]

Hohn et al. 1983 [42] James et al. 1980 [76] Lawler and Allen 1981[43] Lawler et al. 1981 [SS] Lundberg 1983 [56] Matthews and Jennings, 1984

Age

OF BP REACTIVITY IN CHILDREN

Stressor

Related to BP reactivity

cycle ergometer exercise

race (black white) body surface area

14-17

simultaneous video tasks

Type A body mass

x = 15

video game

family history

x= 15

mental arithmetic mental

resting BP family history salt loading increased reactivity in Ss with family history family history

615

; = 16

64 black 79 white both sexes 90 boys 59 girls 5% black 19 boys 20 girls 33% black 20 boys 21 girls 22% black 15 boys 11 girls

lo-17

treadmill exercise

r = 14

cycle ergometer exercise reaction time; anagram

resting BP body mass

reaction time; anagram

Type A

Type A

34 boys

x = 10

30 boys

x = 10

brief exercise; simulated anger computer game with and without opponent serial subtraction mirror-tracing reaction time

11-13

11-12

3-6

I541

resting BP family history

in boys Type A

Type A

Not related to BP reactivity

family history

Blood

pressure

TABLE

I. -

(continued) Related to BP reactivity

Not related to BP reactivity

5-minute speech at school;

anxiety: reactivity to subtraction and mirror-tracing

sex; Type A; trait anger; anger-out; family history reactivity handgrip.

ortostatic stress (lying to standing) videogame with challenge

resting BP (negative association) sex; race; race of experimenter -

Study

n

*gc

Stressor

Matthews et al., 1986 [60]

14 boys 11 girls

X = 15.4

McCrory et al. 1982 [73]

x= 15

Murphy et al. 1986 [66]

46 boys 17 girls 32% black 125 boys 88 girls

Murray et al., 1985 [61]

43 boys 44 girls

x = 12

Remington et al. 1960 [41]

65 boys 66 girls 38% black

Southard et al. 1986 [58] Spiga et al. 1986 [59] Strong etal. 1978 [63]

19 black boys 9 white boys 48 boys

Thomas et al. 1984 [641

30 boys 22 girls 40% black 183 boys 185 girls

Voors et al. 1980 [68]

618 x= 11

8-18

13-18 x = 16 10-12

170 black boys and girls x = 10

5-14

5

reactivity

mental arithmetic

orthostatic stress (lying to standing); cold pressor; breath-holding; deep knee-bends simultaneous video tasks competitive video game cycle ergometer exercise; handgrip reading aloud

Type A; clear/ ambiguous instructions anxiety

family history; resting BP

Type *

-

Type * hostility body surface area

impatience

obesity

race gender

orthostatic stress race; (lying to higher heart rates in whites; standing); lower plasma handgrip; cold pressor renin in blacks

gender

resting BP

The most frequently studied correlate of reactivity is family history of hypertension, which is a strong predictor of resting hypertension [37, 381. Seven of the studies listed in Table I included assessment of parental hypertension, and in five of them [39-43] a positive family history was associated with BP reactivity in children and adolescents. Rose et al. [44] found that identical twins had very similar levels of BP reactivity to a variety of tasks, while few response similarities were found in fraternal twins. These findings are consistent with a significant genetic determination of BP reactivity, but other factors may also be operating. Besides contributing genetic influences, families also shape environments. Baer et al. [45] observed that communication patterns differed in families with hypertensive and normotensive fathers. For the hypertensive group, the frequency of fathers’ negative nonverbal behavior in conflict situations was positively correlated with the child’s BP after a family discussion (r = 0.51; p < 0.05). There was a tendency

6

JAMES F. SALLE

etul.

for the BPS of children of hypertensive fathers to increase after the interaction and for BPS of children of normotensive fathers to decrease. Falkner et al. [46] suggest that genetic and environmental factors interact, based on findings that salt loading increased BP reactivity to mental arithmetic only in adolescents with a positive family history. It appears from these limited data that family influences are important determinants of BP reactivity and include both genetic and environmental components. Type A behavior has been associated with BP hyperreactivity in adults [47-49], even though its status as a CHD risk factor is in question due to contradictory findings [50, 511. it is possible to assess Type A-related behaviors in children by self-report, interview, or adult report [52-551. The relationship between Type A and BP reactivity in children has been evaluated in eight studies. In six of these studies Type A was associated with increased BP reactivity [54-591. This is an interesting finding considering the various measures of Type A, the different types of stressors, and the age range of the children studied (i.e. 3-18). However, in two well-conducted recent studies, Type A was not related to reactivity 160, 611, and the explanation of these discrepant findings is not clear. It would be useful to study the interaction of Type A and family history on BP reactivity in children, as has been done in adults [48]. Five studies have demonstrated an association between body size (i.e. body mass, body surface area) and BP reactivity to a variety of physical and psychological stressors in children [57, 62-651. There is strong and consistent evidence that body size is related both to resting BP and to BP reactivity. Three large studies found black children to have higher BP reactivity than whites to exercise [62], video games [66], and other stressors [68]. One small study found no racial differences when reading in class was the stressor [64]. These findings are consistent with the higher prevalence of hypertension in blacks [66]. Three studies suggested no gender differences in BP reactivity (60,63,64], but one study reported higher reactivity in boys [67]. A few studies have examined the influence of psychosocial variables on BP reactivity. Murphy et al. [67] found that race of experimenter interacted with race of subject such that higher BP reactivity was found in same-race pairings. Two studies have examined the effect of anxiety, with one study yielding a positive relationship [60] and another revealing no relationship to BP reactivity [61]. Overt hostility during a competitive task was associated with BP reactivity [59]. In general, the correlates of BP reactivity in children are the same as those for elevated resting BP and reactive BP in adults. Family history, body mass and race were fairly consistently associated with BP reactivity. The relationship between gender and reactivity BP is not clear. There is some indication that psychosocial variables may influence BP reactivity in children, but few studies have addressed these issues. Type A was a correlate of BP reactivity in children in several studies, but further study is required to understand this interesting relationship. Two correlates, body mass and Type A, are potentially modifiable, so these findings have implications for intervention. Methodological

Methodological

issues in blood pressure

concerns

reactivity assessment

have been discussed

at length

by Krantz

and Manuck

Blood pressure

reactivity

I

[69] in their excellent review. Many of the points they make also apply to children. However, there are additional difficulties in assessing BP in children that relate to cuff size and fourth vs fifth phase Korotkoff sounds as indicators of diastolic blood pressure [70, 711. Many types of stressors have been used to induce a pressor effect (see Table I). Goldstein [lo] reviewed the types of stressors employed in BP studies and found a great variety including video games, exercise, viewing films. interviews and psychological tests. The diversity of tasks makes selection of tasks for a particular study difficult, especially since different stressors have different effects and may tap different physiological mechanisms. It may be possible, however, to group tasks into meaningful categories. Light [71] has drawn a useful distinction between stressors that require effortful active coping and those in which passive coping is required. Active coping with stressors such as reaction time and possibly mental arithmetic have been linked with a pattern of responses suggestive of strong beta-adrenergic stimulation [72], and increased systolic BP and heart rate are typically seen. Stressors that are tolerated passively, such as cold pressor or films, tend to result in greater diastolic BP responses ]73]. These findings indicate that active and passive stressors may stimulate different hemodynamic mechanisms that may be of importance in assessing how reactivity is related to future development of CHD and hypertension. A number of other task dimensions have been proposed [69] but less extensively studied. It may be useful to evaluate the emotional content of tasks. Emotions such as anger and fear may play different roles in modifying hemodynamic responses. The dimension of physical activity may be important, in that some tasks require movement for writing or even actual exercise, while mental arithmetic and cold pressor require no movement. It is not reasonable to assume that all stressors produce similar results, but it is not known which classification schemes are most useful. More theoretical and empirical work is needed on this issue. Reliability and general&ability of BP reactivity in children A second major methodological issue concerns the reliability, or stability, of BP responses to stress. Data from adult studies generally indicate that BP responses are quite stable over a short time, but they are not so stable after one year [69]. Data on reliability of BP reactivity in children are limited. However. significant stability of reactivity to various stressors in children has been documented over one week [74], two weeks [75], two months [76], and six months [77]. Barnett et al. [24] provided data on the stability of the cold pressor test response after 27 yr. Of the 31 hyperreactors (>25/>20 mm Hg) at time 1, all were classified hyperreactors 27 yr later. Twenty-one of the original normoreactors (17.5%) became hyperreactive. Thus, of the 151 subjects followed, 86% stayed in the same category, suggesting substantial stability. Krantz and Manuck [69] concluded that generalizability of BP responses from task to task was stable in adults, but this question has been rarely addressed with children. Parker et al. [28] found highly significant intercorrelations among reactivity to cold pressor, orthostatic, and handgrip stressors (r = 0.53 to 0.84). Strong et al. [63] found that, in children, BP responses to submaximal and maximal endurance exercise were highly correlated (r = 0.70 to 0.84). There were, however, no signific-

8

JAMES

F. SALLIS et al

ant correlations between BP during isometric and during submaximal exercise, but there were modestly significant correlations between isometric and maximal exercise BP changes (r = 0.23 to 0.32). The generalizability from laboratory to naturalistic settings is a much more important issue. One study with adults [78] and three studies with adolescents [S7, 58, 601 suggest that BP responses to laboratory stressors are correlated with BP fluctuations in the natural environment. The study by Matthews and colleagues [60] is noteworthy. She assessed BP reactivity to several tasks in the laboratory and two weeks later assessed BP response to a speech given in a high school class. BP reactivity to two of the stressors (i.e. serial subtraction and mirror-tracing) was significantly related to BP before and during the school speech. Another study found that BP reactivity of adolescent boys to a laboratory task was significantly correlated with BP as assessed by ambulatory monitoring throughout the day [58]. This indication that laboratory reactivity generalizes to the natural environment suggests that BP reactivity assessment may have potential as a clinical assessment tool. In general, the studies carried out with children are inadequate in number and methodology to allow firm conclusions to be drawn regarding the reliability of BP reactivity across time or the generalizability across settings and tasks. Further examination of these basic issues is needed to clarify determinants of BP reactivity and to guide researchers in continued explorations of this potentially important phenomenon. GAPS IN THE LITERATURE

AND DIRECTIONS

FOR FUTURE

RESEARCH

There is some evidence from long-term prospective studies that BP reactivity to stress in children predicts later development of hypertension and CHD. Because the evidence is not unanimous and because the number of studies conducted with children is relatively small, definitive conclusions must be deferred. The data are sufficiently intriguing, however, to justify an active program of continuing research on BP reactivity in children. In this section, recommendations for promising research directions are presented. The most convincing (and costly) studies involve prospective epidemiologic studies of large representative groups. Therefore, measures of BP reactivity should be included in ongoing longitudinal studies of children. The most efficient method of investigating the health consequences of BP reactivity is to analyze existing data in which BP reactivity to exercise or postural change has been measured. For children of different ages, the stability of BP reactivity to standard tasks over the short term, and tracking over the long term, should be established. The field will be advanced more quickly by using a few standard stimuli whose characteristics and effects are well known. A reasonable amount of data is available on the cold pressor, mental arithmetic, reaction time, video games and exercise tests. Current evidence suggests that BP reactivity is associated with race, obesity, Type A and family history of hypertension. Data on the specific components of Type A that seem to influence BP reactivity would be useful. There is some indication that sodium intake is related to BP reactivity [43, 681, so dietary variables should be studied further, especially sodium, potassium and calcium. Since fitness

Blood

pressure

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9

and exercise appear to be related to BP reactivity in adults [79, 801, it is advisable to study their effects on children’s BP responses. Because the subject’s appraisal of the stressful stimulus is often a powerful mediator of the response [81], subjective evaluations of the stressfulness of the task should be included in assessment plans. Mechanisms by which BP reactivity may be related to hypertension and CHD have been investigated in adults [69], and similar studies in children may be very important. It has been suggested that cardiac output is elevated in the early stages of hypertension, while increases in vascular resistance are found in more chronic hypertension [82, 831. Light and colleagues [71] have demonstrated that tasks requiring effortful active coping produce repeatable increases in cardiac output, but not peripheral resistance. This finding may indicate that BP hyperreactivity is related to initiation but not the maintenance of hypertension. Confirmation of this finding in children would lend further support to the etiological significance of BP reactivity in children. CONCLUSION

There is a small but rapidly growing literature on BP reactivity in children. In several studies BP reactivity in children predicted resting BP levels up to 45 years later, and reactivity itself may be an enduring characteristic of the individual. Clearly this is a promising research area that is worth pursuing vigorously. Future work should consider the potential for using BP reactivity assessments in clinical medicine. Acknowledgements - This work was supported by NIH grants HL 30872 and HL 36005. Dr. Philip R. Nader provided thoughtful and valuable comments. Irma Valdez assisted in manuscript preparation.

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