Role of Stress in Development of Hypertension
The etiology of hypertension remains unknown, but one fact is clearly established: no single cause is responsible. High blood pressure is the end result of a number of factors, both genetic and environmental, that may be quantitatively and qualitatively different in different individuals. Furthermore, blood pressure is distributed continuously in the population, with no clear separation between normal and raised blood pressure. Any definition of hypertension is thus quite arbitrary.
For many years, it has been suspected that an adverse reaction between an individual and his or her environment may play a role in the development of hypertension, although conclusive evidence is still lacking. Four general approaches have been used to identify the potential role of such factors. The first is to study the effects of environmental stressors in epidemiologic studies, the second is to look for personality differences between normotensive and hypertensive individuals, and the third is to examine the relevance of individual differences in susceptibility or reactivity to standardized stressors in a laboratory setting. Fourth, conducting animal experiments of stress-induced hypertension also is informative.
There are three independent variables. First is the nature of the stressor, which is a characteristic of the environment. Second is the individual’s perception of the stressor. This will depend on both the individual’s personality and previous experience. What is stressful for one individual is not necessarily as stressful for another. The effects of the perceived stress on blood pressure will in turn depend on the third factor, which is the individual’s physiologic susceptibility. This may depend on genetic and environmental factors, e.g., a family history of hypertension, and the state of sodium balance. It seems reasonable to suppose that all three are important and that any effects on hypertension are likely to be interactive. This chapter discusses each of these three variables in turn, and also some of the possible physiologic mechanisms that might mediate their effects on blood pressure.
Environmental sources of psychologic stress
Individual psychologic factors and perception of stress
Two types of factors relevant to individual differences in the way in which potential stressors are perceived are the individual’s personality, which is relatively immutable, and the individual’s previous experience or learning, which obviously is not. Because there is no evidence that hypertension is a learned behavior pattern (although this remains an interesting possibility), my discussion is restricted to personality variables.
Personality Variables and Hypertension
The idea that there is a “hypertensive personality” has been mooted for many years, but is still unsettled. The concept originated with Alexander, who proposed that the hypertensive individual experiences repressed hostility, or “anger-in,” which is channeled into the autonomic nervous system, resulting in increased blood pressure. This theory has been reviewed by Shapiro. A potential problem with such studies is that characteristics such as anger and anxiety, which are frequently associated with hypertension, may be a consequence of making the diagnosis (a “labeling” phenomenon) rather than being etiologic factors, and there is always a problem of knowing what constitutes an appropriate control group. A more reliable method may be to study a randomly selected population. One such study was conducted by Harburg et al., who found that in men, anger-in was correlated with blood pressure. Two other personality variables that have been reported to be characteristic of hypertensive individuals are submissiveness and alexithymia, which has been defined as an inappropriate affect, difficulty in expressing emotions, and an absence of fantasies.
Type A Behavior Pattern, Anger, and Hostility
Several related personality characteristics have been investigated with regard to hypertension, which all relate to hostility and aggression. One of the earlier studies reporting this phenomenon was conducted by Wolf and Wolff, who evaluated personality measures using both interviews and questionnaires in 103 hypertensive patients, who were compared with 150 patients with allergies and 61 normotensive hospitalized patients. They concluded that the hypertensive patients had restrained aggression and excess inner tension. A more recent example of such a finding comes from a study by Perini et al., who compared young subjects with borderline hypertension with age-matched normotensive controls with and without a family history of hypertension. The hypertensive patients showed less externalized aggression, more internalized aggression, and more submissiveness. They also demonstrated evidence of increased sympathetic nervous system activity, such as faster heart rates and higher plasma catecholamines. Although many other studies have reported varying degrees of association between inhibited aggression and blood pressure (others have reported negative or inconsistent results).
The Type A behavior pattern, which is generally regarded as being at least in part a personality variable, has been most closely related to coronary heart disease. Most studies have not found any close relationship with hypertension, perhaps because most of them used only one or two blood pressure measurements. An example was the Western Collaborative Group Study, in which 3524 men were followed for 8 yr. In their study, Irvine et al. compared the prevalence of type A in 109 untreated hypertensive subjects and 109 demographically matched control subjects, whose blood pressure was measured five times over 5 mo. Type A behavior was assessed by the structured interview and was significantly more prevalent in the hypertensive subjects (78%) than in the normotensive subjects (60%). Hostility, which is now considered to be one of the most important components of coronary-prone behavior, was also higher in the hypertensive subjects.
Everson et al. found that anger (whether expressed or repressed) is associated with an increased risk of developing hypertension over a 4-yr follow-up period. Hostility has also been shown to have an interactive effect with occupational stress on blood pressure, at least over the short term. In a study of paramedics who wore an ambulatory blood pressure monitor during a workday, Jamner et al. found that subjects who scored high on hostility and defensiveness demonstrated higher diastolic pressures while in the hospital, but not while waiting for a call at the ambulance station.
Anxiety and Panic Disorder
Associations between anxiety and depression with hypertension and cardiovascular morbidity have been observed in cross-sectional studies. Panic disorders also occur more commonly in hypertensive than in normotensive individuals, although whether the panic disorder precedes or follows the hypertension is unknown. One prospective study has found that individuals who report high levels of symptoms of anxiety or depression are at increased risk of hypertension 9 yr later.
Individual differences in susceptibility to psychologic factors
Physiologic mechanisms that might mediate stress-induced hypertension
Conclusion
Attempts to find a single cause of hypertension have proven universally frustrating. This applies no less to psychologic than to physiologic causes. It thus seems plausible that hypertension may be the result of the interaction of a variety of factors, whose contribution may vary in different individuals. It is well established that blood pressure is socially and culturally determined, but the precise factors responsible for group and individual differences remain elusive. Although dietary habits are undoubtedly important, they cannot explain the observed differences, and there is a growing body of evidence to suggest that psychosocial stressors also play a role. Several different factors and models to explain their effects have been proposed, and a common feature to all is an element of discord between the individual and his or her social setting. In the job strain, or demand-control model, there is the conflict between demands and control. Thus, most of those models rely on an interaction of two or more factors to produce hypertension, rather than by any one factor acting in its own. Because human behavior is infinitely complex, these models are not mutually exclusive. For example, we should not necessarily expect a model that predicts hypertension in African Americans living in Detroit to work in Italian nuns living in a convent.
The role of individual factors must also be acknowledged to be important. Personality variables have been the subject of much attention. Repressed anger and submissiveness are the two that are most frequently invoked, but the findings are quite mixed. In part, this may be a measurement issue, because of the difficulties in quantifying something as nebulous as personality, but it may also mean that we should be focusing on subsets of patients (e.g., those with high renin levels) in whom personality variables contribute to blood pressure, perhaps in conjunction with environmental stressors.
The most extensively studied individual factor is blood pressure reactivity, but despite an enormous amount of research on the subject, the relevance of increased cardiovascular reactivity to the development and consequences of hypertension remains unclear. There is no consensus as to which test should be used to define reactivity, and the generalization of an individual’ s responses from one test to another cannot be assumed. Hypertensive subjects tend to show greater reactivity than normotensive subjects, particularly to behavioral challenges, but this may be a consequence rather than a cause of the hypertension. The reactivity hypothesis requires that reactivity predict the development of hypertension.
Attempts to demonstrate this have met with varying success, and in some of the reportedly positive studies, it has not been established that reactivity is independent of other predictors. These considerations lead us to conclude that on the basis of present evidence, increased blood pressure reactivity to behavioral stimuli is unlikely to play a primary role in the development of hypertension. The evidence that chronic exposure to environmental stressors can accelerate the development of hypertension, on the other hand, is quite encouraging. Whether or not such stressors produce a more pronounced effect in individuals who have a particular personality type, or who are hyperactive, remains unclear, but may prove to be a productive area for future research.
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