Ambulatory Blood Pressure
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KeywordsAmbulatory BP Vascular Unloading Technique Laboratory Stress Challenge Appraisal Ambulatory Measurements
Ambulatory blood pressure is arterial blood pressure measured in real-life settings by an automatic device.
Blood pressure (BP) was first measured in the eighteenth century by Halles following Harvey’s work on the circulation of blood. It has been measured in the clinic and operating theater since the early part of the twentieth century and its utility as a predictor of cardiovascular disease established in the second half of the century. The ambulatory measurement of BP (ABPM) outside the clinic or laboratory is a development of the later part of the century. Despite its comparatively recent origin, it is now regarded as the measure of choice clinically since it provides a more reliable and valid measure of an individual’s BP and is a better predictor of later disease, perhaps because it reduces “white coat” hypertension, the elevation of BP produced in some individuals when BP is measured in a medical setting. The majority of ambulatory BP monitors are automatic miniaturized versions of the standard sphygmomanometer and can measure systolic and diastolic BP at predetermined times, often every 20 or 30 min during the day, less frequently when the participant is asleep. There are also devices that measure BP continuously, the most successful of which use the vascular unloading technique first described by Penaz (1973). Continuous measurement is not widely used but has considerable advantages for psychophysiological studies since it greatly improves the power of studies to detect the relationship between BP and environmental events or psychological phenomena that might be transitory, as well as providing repeated measurement during more enduring events. The Penaz-derived devices also enable one to determine the mechanisms, vascular or cardiac, that underlie the elevations in blood pressure. Alternatively, the underlying mechanisms can be determined by combining intermittent ABPM with cardiac output measured by ambulatory measures of cardiac impedance. Whether infrequent or continuous, ambulatory blood pressure is affected by many factors that are often noise with respect to the question under study. The most important of these are movement and posture, and they are usually controlled either through questionnaires completed at the time of measurement or, more satisfactorily, through direct measurement and recording with accelerometer-based devices.
While ABPM was developed to deal with clinical issues, it has great relevance for behavioral medicine. Most psychophysiological studies of the cardiovascular system are conducted in the laboratory for reasons of convenience, control, and the accuracy of measurement. ABPM allows the study of the psychological processes that in part determine BP to be extended to real life with obvious benefits in ecological validity (although at the cost of loss of control) and a potential increase in understanding of the role of stress in hypertension and cardiovascular disease. ABPM can also provide insight into psychological process that have effects on the cardiovascular (CV) system by providing sophisticated measures of autonomic arousal that illuminate or index processes that cannot be studied by observation or self-report.
BP is elevated in many situations that are conventionally seen as stressful, such as public speaking or examination. This was originally shown in laboratory studies but has been confirmed in field studies using ABPM when it is often found that the responses are considerably larger than in the laboratory simulations of these situations. Interpersonal conflict, often difficult to study meaningfully in the laboratory, is also associated with elevated BP in field settings. Perhaps unsurprisingly, it has also been shown that heightened subjective feelings of anxiety or arousal are associated with increased BP, as are variations in the demand that people feel they are under or their perception of the control that they feel they have over the situation since high demand and low control has been widely shown to be associated with increased strain. Such effects are moderated by personality with, for example, some studies showing that highly hostile people had high BP whatever their mood, while the less hostile had high BP only when in a negative mood. The highly hostile are also less likely to show a reduction in stress-related BP with social support. There is an additional evidence of gender moderating the effects of stress on BP with women benefiting more than men from social support during stressful situations. The effects of social interaction are subtle with interactions with a person with whom one has an ambivalent relationship leading, in one study, to greater BP elevations than interactions with people that were more clearly either liked or disliked. Such studies have also shown that it can be the nature of the relationship rather than the nature of a specific interaction that relates to BP. This information on complex and subtle social situations could only be obtained in real life using ABPM.
BP is determined by the interplay of cardiac output and vascular resistance. Laboratory studies suggest that objectively different situations and subjectively different appraisal of these situations can affect the determinants of BP. Tasks that involve active coping or the related appraisal of situations as challenging are associated with cardiac effects, while tasks involving passive coping or the appraisal of a situation as threatening, leading to a vascular response. While this has not been studied extensively in real life, in one study, challenge appraisals were related to cardiac effects in people making an academic presentation and threat with a more vascular response. More vascular responses have been reported in lonely people who are hypothesized to adopt passive coping strategies.
An enduring issue in laboratory and ambulatory studies of the effects of stress has been the extent to which responses seen in the laboratory generalize to real life. The response to laboratory stressors is a poor candidate as an index of the risk of disease if it relates only weakly, or not at all, to the response in real life. The issue is controversial, but the most recent studies using the best available measurement and analytic techniques suggested that reliable CV (heart rate and BP) responses to laboratory stressors, obtained by combining the responses to several stressors, do relate to the CV response to objective stressful environments, negative emotions, and perceptions of the situation as stressful.
The reactivity hypothesis has been the dominant theory in cardiovascular behavior medicine since 1980. In its simplest form, this theory states that individuals who show an excessive CV response to stress, the hyperreactive, are at increased risk of CV disease. Recently it has been proposed that hyporeactivity is also a risk factor for CV disease (the “blunting hypothesis”). Prospective studies using the CV response to laboratory stressors to predict cardiovascular disease endpoint have had mixed findings at best. However, hyperreactivity is not enough since a hyperreactive person has to be reacting to something. A vulnerability factor, like hyperreactivity, needs an appropriate environment to actually become a risk factor. Laboratory studies only establish the vulnerability. Ambulatory studies can go some way to establishing if the appropriate environment also exists and there is evidence that increased BP during periods of high strain (high demand and low control) in real life is associated with subclinical arterial disease.
References and Further Readings
- Penaz, J. (1973). Photoelectric measurement of blood pressure volume and flow in the finger. In Digest of the international conference on medicine and biological engineering (pp. 104–104). Dresden.Google Scholar
- Steptoe, A. (2001). Ambulatory monitoring of blood pressure in daily life: A tool for investigating psychosocial processes. In J. Fahrenberg & M. Myrtek (Eds.), Progress in ambulatory monitoring (pp. 257–269). Seattle: Hogrefe & Huber.Google Scholar